Lee Kirkpatrick

Around the Fire With Old Friends (CVE-2019–0604, and CVE-2017-0144)

Blog Post created by Lee Kirkpatrick Employee on Feb 14, 2020

In this post we will cover CVE-2019-0604 (https://nvd.nist.gov/vuln/detail/CVE-2019-0604), albeit a somehwhat older vulnerability, it is one that is still being exploited. This post will also go a little further than just the initial exploitation of the Sharepoint server, and use EternalBlue to create a user on a remote endpoint to allow lateral movement with PAExec; again, this is an old, well-known vulnerability, but something still being used. We will then utilise Dumpert to dump the memory of LSASS (https://github.com/outflanknl/Dumpert) and obtain credentials, and then employ atexec from Impacket (https://github.com/SecureAuthCorp/impacket) to further laterally move.

 

The Attack

The initial foothold on the network is obtained is via the Sharepoint vulnerability (CVE-2019-0604). We will use the PoC code developed by Voulnet (https://github.com/Voulnet/desharialize) to drop a web shell:

 

In the above command we are targetting the vulnerable Picker.aspx page, and using cmd to echo a Base64 encoded web shell to a file in C:\ProgramData\ - we then use certutil to Base64 decode the file into a publicly accessible directory on the Sharepoint server and name it, bitreview.aspx.

 

To access the web shell we just dropped onto the Sharepoint server, we are going to use the administrative tool, AntSword (https://github.com/AntSwordProject/antSword). Here we add the URL to the web shell we dropped, and supply the associated password:

 

Now we can open a terminal and begin to execute commands on the server to get a lay of the land and find other endpoints to laterally move to:

 

The AntSword tool has a nice explorer view which allows us to easily upload additional tools to the server. In this instance we upload a scanner to check if an endpoint is vulnerable to EternalBlue:

 

Now we can iterate through some of the endpoints we uncovered earlier to see if any of them are vulerable to EternalBlue:

 

Now that we have uncovered a vulnerable endpoint, we can use this to create a user that will allow us to laterally move to it. Using the PoC code created by Worawit (https://github.com/worawit/MS17-010), we can exploit the endpoint and execute custom shellcode of our choosing. For this I compiled some shellcode to create a local administrative user called, helpdesk. I uploaded my shellcode and EternalBlue exploit executable and ran it against the vulnerable machine:

 

Now we have created a local administrative user on the endpoint, we can laterally move to it using those credentials. In this instance, we upload PAExec, and Dumpert, so we can laterally move to the endpoint and dump the memory of LSASS. The following command copies the and executes the Outflank-Dumpert.exe using PAExec and the helpdesk user we created via EternalBlue:

 

This tool will locally dump the memory of LSASS to C:\Windows\Temp - so we will mount one of the administrative shares on the endpoint, and confirm if our dump was successful:

 

Using AntSwords explorer, we can easily navigate to the file and download it locally:

 

We can then use Mimikatz on the attackers' local machine to dump the credentials which may help us laterally move to other endpoints:

 

We decide to upload the atexec tool from Impacket to execute commands on the remote endpoint to see if there are other machines we can laterally move to. Using some reconaissance commands, we find an RDP session using the username we pulled from the LSASS dump:

 

From here, we could continue to laterally move, dump credentials, and further own the network.

 

Detection using NetWitness Network

NetWitness doesn't always have to be used for threat hunting, it can also be used to search for things you know about, or have been currently researching. Taking the Sharepoint RCE as an example, we can easily search using NetWitness to see if any exploits have taken place. Given that this is a well documented CVE, we can start our searching by looking for requests to picker.aspx (filename = 'picker.aspx'), which is the vulnerable page - from the below we can see two GET requests, and a POST for Picker.aspx (inbound requests directly to this page are uncommon):

Next we can reconstruct the events to see if there is any useful information we can ascertain. Looking into the HTTP POST, we can see the URI matches what we would expect for this vulnerability, we also see that the POST parameter, ctl00$PlaceHolderDialogBodySection$ctl05$hiddenSpanData, contains the hex encoded and serialized .Net XML payload. The payload parameter also starts with two underscores which will ensure the payload reaches the XML deserialization function as is documented:

 

Seeing this would already warrant investigation on the Sharepoint server, but lets use some Python so we can take the hex encoded payload and deserialize it to see what was executed:

 

From this, we have the exact command that was run on the Sharepoint server that dropped a web shell. This also means we now know the name of the web shell and where it is located, making the next steps of investigation easier:

As analysts, it sometimes pays to do these things to find additional breadcrumbs to pull from, although be careful, as this can be time consuming and other methods can make it a lot easier, like MFT analysis that is described later on in the blog post.

 

This means we could search for this filename in NetWitness to see if we have any hits (filename = 'bitreview.aspx'):

 

As you can see from the above highlighted indicators, even without knowing the name of the web shell we would have still uncovered it as NetWitness created numerous meta values surrounding its usage. A fairly recent addition to the Lua parsers available on RSA Live is, fingerprint_minidump.lua - this parser creates the meta value, minidump, under the Filetype meta key, and also creates the meta value, lsass minidump, under the Indicators of Compromise meta key. This parser is a fantastic addition as it tags LSASS memory dumps traversing the network, which is uncommon behaviour.

 

Reconstructing the events, we can see the web shell traffic which looks strikingly similar to China Chopper. The User-Agent is also a great indicator for this traffic, which is the name of the tool used to connect to the web shell:

 

We can Base64 decode the commands from the HTTP POST's and get an insight into what was being executed. The below shows the initial POST which returns the current path, operating system, and username:

 

The following shows a directory listing of C:\ProgramData\ being executed, which is where the initial Base64 encoded bitreview.aspx web shell was dropped:

 

We should continue to Base64 decode all of these commands to gain a better understanding of what the attacker did, but for this blog post I will focus on the important pieces of the traffic. Pivoting into the Events view for the meta value, hex encoded executable, we can see the magic bytes for an executable that have been hex encoded:

 

Extracting all the hex starting from 4D5A (MZ header in hexadecimal) and decoding it with CyberChef, we can clearly see this is an executable. From here, we could save this file and perform further analysis on it:

 

Continuing on with Base64 decoding the commands we come across something interesting, it is the usage of a tool called eternalblue_exploit7.exe against an endpoint in the same network. This gives the defender additional information surrounding other endpoints of interest to the attacker and endpoint to focus on:

If you only have packet visibility, you should always decode every command. This will help you as a defender better understand the attackers actions and uncover additional breadcrumbs. But if you have NetWitness Endpoint, it may be easier to see the commands there, as we will see later.

 

Knowing EternalBlue uses SMB, we can pivot on all SMB traffic from the Sharepoint server to the targetted endpoint. Opening the Enablers of Compromise meta key we can see two meta values indicating the use of SMBv1; this is required for EternalBlue to work. There is also an meta value of not implemented under the Error meta key this is fairly uncommon and can help detect potential EternalBlue exploitation:

 

Reconstructing the events for the SMBv1 traffic, we come across a session that contains a large sequence of NULLs, this is the beginning of the EternalBlue exploit and these NULLs essentially move the SMB server state to a point where the vulnerability exists:

 

With most intrusions, there is typically some form of lateral movement that takes place using SMB. As a defender we should iterate through all possible lateral movement techniques, but for this example I want to see if PAExec has been used. To do this I use the following query (service = 139 && filename = 'svcctl' && filename contains 'paexe'). From the below, we can see that there is indeed some PAExec activity. By default, PAExec also includes the hostname of where the activity occured from, so from the below filenames, we can tell that this activity came from SP2016, the Sharepoint server. We can also see that a file was transferred, as is indicated by the paexec_move0.dat meta value - this is the Outflank-Dumpert.exe tool:

 

Back in the Investigation view, under the Indicators of Compromise meta key, we see a meta value of lsass minidump. Pivoting on this value, we see the dumpert.dmp file in the temp\ directory for the endpoint that was accessed over the ADMIN$ share - this is our LSASS minidump created using the Outflank-Dumpert.exe tool:

 

Navigating back to view all the SMB traffic, and focusing on named pipes (service = 139 && analysis.service = 'named pipe'), we can see a named pipe being used called atsvc. This named pipe gives access to the AT-Scheduler Service on an endpoint and can be used to scheduled tasks remotely. We can also see some .tmp files being created in the temp\ directory on this endpoint with what look like randomly generated names, and window cli admin commands associated with one of them:

 

Reconstructing the events for this traffic, we can see the scheduled tasks being created. In the below screenshot, we can see the XML and the associated parameters passed, in this instance using CMD to run netstat looking for RDP connections and output the results to %windir%\Temp\rWLePJvp.tmp:

 

This is lateral movement behaviour via Impackets tool, atexec. It writes the output of the command to a file so it can read it and display back to the attacker - further analysing the payload, we can see the output that was read from the file and gain insight into what the attacker was after, and subsequently endpoints to investigate:

 

 

Detection using NetWitness Endpoint

As always with NetWitness Endpoint, I like to start my hunting by opening the three compromise keys (IOC, BOC, and EOC). In this case, I only had meta values under the Behaviours of Compromise meta key. I have highlighted a few I deem more interesting with regards to this blog, but you should really investigate all of them:

 

Let's start with the runs certutil with decode arguments meta value. Opening this in the Events view, we can see the parameter that was executed, which is a Base64 encoded value being echo'ed to the C:\ProgramData\ directory, and then certutil being used to decode it and push it to a directory on the server:

 

From here, we could download the MFT of the endpoint:

 

Locate the file that was decoded, download it locally, and see what the contents are:

 

From the contents of the file, we can see that this is a web shell:

 

We also observed the attacker initially drop a file in the C:\ProgramData\ directory, so this is also a directory of interest and somewhere we should browse to within the MFT - here we uncover the attackers tools which we could download and analyse:

 

Navigating back to the Investigate view and opening the meta value http daemon runs command prompt in the Events view, we can see the HTTP daemon, w3wp.exe, executing reconaissance commands on the Sharepoint server:

This is a classic indicator for a web shell, whereby we have a HTTP daemon spawning CMD to execute commands.

 

Further analysis of the commands executed by the attacker shows EternalBlue executables being run against an endpoint, after this, the attacker uses PAExec with a user called helpdesk to connect to the endpoint - implying that the EternalBlue exploit created a user called helpdesk that allowed them to laterally move (NOTE: we will see how the user creation via this exploit looks a little later on):

 

Navigating back to Investigate, and this time opening the Events view for creates local user account, we see lsass.exe running net.exe to create a user account called, helpdesk; this is the EternalBlue exploit. LSASS should never create a user, this is a high fidelity indicator of malicious activity:

 

Another common attacker action is to dump credentials. Due to the popularity of Mimikatz, attackers are looking for other methods of dumping credentials, this typically involves creating a memory dump of the LSASS process. We can therefore use the following query (action = 'openosprocess' && filename.dst='lsass.exe'), and open the Filename Source meta key to look for something opening LSASS that stands out as anomalous. Here we can see a suspect executable opening LSASS named, Outflank-Dumpert.exe:

 

As defenders, we should continue to triage all of the meta values observed. But for this blog post, I feel we have demonstrated NetWitness' ability to detect these threats.

 

Detection Rules

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

Appliance

Description

Logic

Fiedlity

Packet Decoder

Requests of interest to Picker.aspx

service = 80 && action = ‘post’ && filename = 'picker.aspx' && directory contains '_layout'

Medium

Packet Decoder

AntSword tool usage

client begins 'antsword'

High

Packet Decoder

Possible Impacket atexec usage

service = 139 && analysis.service = 'named pipe' && filename = 'atsvc' && filename ends 'tmp'

Medium

Packet Decoder

Dumpert LSASS dump

fiilename = 'dumpert.dmp'

High

Packet Decoder

Possible EternalBlue exploit

service = 139 && error = 'not implemented' && eoc = 'smb v1 response'

Low

Packet Decoder

PAExec activity

service = 139 && filename = 'svcctl' && filename contains 'paexe'

 

High

Endpoint Log Hybrid

Opens OS process LSASS

action = 'openosprocess' && filename.dst='lsass.exe'

Low

Endpoint Log Hybrid

LSASS creates user

filename.src = ‘lsass.exe’ && filename’dst = ‘net.exe’ && param.dst contains’ /add’

High

 

Conclusion

Threat actors are consistently evolving and developing new attack methods, but they are also using tried and tested methods as well - there is no need for them to use a new exploit when a well-known one works just fine on an unpatched system. Defenders should not only be keeping up to date with the new, but also retroactively searching their data sets for the old.

Outcomes