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1 Post authored by: Sampson Chandler Employee

Cross-site request forgery (CSRF) is an attack which forces an end user to execute unwanted actions on a web application to which they are currently authenticated.

 

CSRF vulnerabilities may arise when applications rely solely on HTTP cookies to identify the user that has issued a particular request. Because browsers automatically add cookies to requests regardless of the request's origin, it may be possible for an attacker to create a malicious web site that forges a cross-domain request to the vulnerable application.

 

CSRF is an attack which forces an end user to execute unwanted actions on a web application in which he/she is currently authenticated. With a little help of social engineering (like sending a link via email or chat), an attacker may force the users of a web application to execute actions of the attacker’s choosing. A successful CSRF exploit can compromise end user data and operation, when it targets a normal user. If the targeted end user is the administrator account, a CSRF attack can compromise the entire web application.

 

 

CSRF relies on the following:

[1] Web browser behavior regarding the handling of session-re-lated information such as cookies and http authentication information;

 

[2] Knowledge by the attacker of valid web application URLs;

 

[3] Application session management relying only on information which is known by the browser;

 

[4] Existence of HTML tags whose presence cause immediate access to an http[s] resource; for example the image tag img.

 

Points 1, 2, and 3 are essential for the vulnerability to be present, while point 4 is accessory and facilitates the actual exploitation, but is not strictly required.

 

Point 1)

Browsers automatically send information which is used to identify a user session. Suppose site is a site hosting a web application, and the user victim has just authenticated himself to site. In response, site sends victim a cookie which identifies requests sent by victim as belonging to victim’s authenticated session. Basically, once the browser receives the cookie set by site, it will automatically send it along with any further requests directed to site.

 

Point 2)

If the application does not make use of session-related information in URLs, then it means that the application URLs, their parameters, and legitimate values may be identified (either by code analysis or by accessing the application and taking note of forms and URLs embedded in the HTML/JavaScript).

 

Point 3) ”Known by the browser” refers to information such as cookies, or http-based authentication information (such as Basic Authentication; and not form-based authentication), which are stored by the browser and subsequently resent at each request directed towards an application area requesting that authentication.

 

 

The vulnerabilities discussed next apply to applications which rely entirely on this kind of information to identify a user session.

Suppose, for simplicity’s sake, to refer to GET-accessible URLs (though the discussion applies as well to POST requests). If victim has already authenticated himself, submitting another request causes the cookie to be automatically sent with it (see picture, where the user accesses an application on www.example.com).

The GET request could be originated in several different ways:

  • by the user, who is using the actual web application;
  • by the user, who types the URL directly in the browser;
  • by the user, who follows a link (external to the application)pointing to the URL.

 

These invocations are indistinguishable by the application. In particular, the third may be quite dangerous. There are a number of techniques (and of vulnerabilities) which can disguise the real properties of a link. The link can be embedded in an email message, or appear in a malicious web site where the user is lured, i.e., the link appears in content hosted elsewhere (another web site, an HTML email message, etc.) and points to a resource of the application.

 

If the user clicks on the link, since it was already authenticated by the web application on site, the browser will issue a GET request to the web application, accompanied by authentication information (the session id cookie). This results in a valid operation performed on the web application and probably not what the user expects to happen. Think of a malicious link causing a fund transfer on a web banking application to appreciate the implications.

 

By using a tag such as img, as specified in point 4 above, it is not even necessary that the user follows a particular link.

 

 

Suppose the attacker sends the user an email inducing him to visit an URL referring to a page containing the following (oversimplified) HTML:

 

 

What the browser will do when it displays this page is that it will try to display the specified zero-width (i.e., invisible) image as well.

This results in a request being automatically sent to the web application hosted on site. It is not important that the image URL does not refer to a proper image, its presence will trigger the request specified in the src field anyway. This happens provided that image download is not disabled in the browsers, which is a typical

configuration since disabling images would cripple most web applications beyond usability.

 

The problem here is a consequence of the following facts:

  • there are HTML tags whose appearance in a page result in automatic http request execution (img being one of those);
  • the browser has no way to tell that the resource referenced by img is not actually an image and is in fact not legitimate;
  • image loading happens regardless of the location of the alleged image, i.e., the form and the image itself need not be located in the same host, not even in the same domain. While this is a very handy feature, it makes difficult to compartmentalize applications.

 

It is the fact that HTML content unrelated to the web application may refer components in the application, and the fact that the browser automatically composes a valid request towards the application, that allows such kind of attacks. As no standards are defined right now, there is no way to prohibit this behavior unless it is made impossible for the attacker to specify valid application URLs. This means that valid URLs must contain information related to the user session, which is supposedly not known to the attacker and therefore make the identification of such URLs impossible.

 

The problem might be even worse, since in integrated mail/browser environments simply displaying an email message containing the image would result in the execution of the request to the web application with the associated browser cookie.

 

Things may be obfuscated further, by referencing seemingly valid image URLs such as

where [attacker] is a site controlled by the attacker, and by utilizing a redirect mechanism on

 

Cookies are not the only example involved in this kind of vulnerability. Web applications whose session information is entirely supplied by the browser are vulnerable too. This includes applications relying on HTTP authentication mechanisms alone, since the authentication information is known by the browser and is sent

automatically upon each request. This DOES NOT include form based authentication, which occurs just once and generates some form of session-related information (of course, in this case, such information is expressed simply as a cookie and can we fall back to one of the previous cases).

 

 

Sample scenario

Let’s suppose that the victim is logged on to a firewall web managementapplication. To log in, a user has to authenticate himself and session information is stored in a cookie.

Let’s suppose the firewall web management application has a function that allows an authenticated user to delete a rule specified by its positional number, or all the rules of the configuration if the user enters ‘*’ (quite a dangerous feature, but it will make the example more interesting). The delete page is shown next. Let’s

suppose that the form – for the sake of simplicity – issues a GET request, which will be of the form (to delete rule number one)(to delete all rules).

The example is purposely quite naive, but shows in a simple way the dangers of CSRF.

 

 

Now, this is not the only possible scenario. The user might have accomplished the same results by manually submitting the URL or by following a link pointing, directly or via a redirection, to the above URL. Or, again, by accessing an HTML page with an embedded img tag pointing to the same URL.

 

In all of these cases, if the user is currently logged in the firewall management application, the request will succeed and will modify the configuration of the firewall. One can imagine attacks targeting sensitive applications and making automatic auction bids, money transfers, orders, changing the configuration of critical

software components, etc.

 

An interesting thing is that these vulnerabilities may be exercisedbehind a firewall; i.e., it is sufficient that the link being attacked be reachable by the victim (not directly by the attacker). In particular, it can be any Intranet web server; for example, the firewall management station mentioned before, which is unlikely to be exposed to the Internet.

 

Self-vulnerable applications, i.e., applications that are used both as attack vector and target (such as web mail applications), make things worse.

If such an application is vulnerable, the user is obviously logged in when he reads a message containing a CSRF attack, that can target the web mail application and have it perform actions such as deleting messages, sending messages appearing as sent by the user, etc.

 

 

 

How to Test:

Black Box Testing

For a black box test the tester must know URLs in the restricted

(authenticated) area. If they possess valid credentials, they

can assume both roles – the attacker and the victim. In this case,

testers know the URLs to be tested just by browsing around the

application.

Otherwise, if testers don’t have valid credentials available, they

have to organize a real attack, and so induce a legitimate, logged

in user into following an appropriate link. This may involve a substantial

level of social engineering.

Either way, a test case can be constructed as follows:

  • let u the URL being tested; for example, u =

http://www.example.com/action

  • build an html page containing the http request referencing URL

u (specifying all relevant parameters; in the case of http GET this

is straightforward, while to a POST request you need to resort to

some Javascript);

  • make sure that the valid user is logged on the application;
  • induce him into following the link pointing to the URL to be

tested (social engineering involved if you cannot impersonate

the user yourself);

  • observe the result, i.e. check if the web server executed the

request.

 

 

Gray Box Testing

Audit the application to ascertain if its session management is

vulnerable. If session management relies only on client side values

(information available to the browser), then the application is

vulnerable. “Client side values” mean cookies and HTTP authentication

credentials (Basic Authentication and other forms of HTTP

authentication; not form-based authentication, which is an application-

level authentication). For an application to not be vulnerable,

it must include session-related information in the URL, in a

form of unidentifiable or unpredictable by the user ([3] uses the

term secret to refer to this piece of information).

Resources accessible via HTTP GET requests are easily vulnerable,

though POST requests can be automated via Javascript and are

vulnerable as well; therefore, the use of POST alone is not enough

to correct the occurrence of CSRF vulnerabilities.

 

 

Tools

Category:OWASP_WebScarab_Project

Category:OWASP_CSRFTester_Project

site_request_forgery.php (via img)

site_framing.php (via iframe)

Remediation

 

 

 

 

The following countermeasures are divided among recommendations to users and to developers.

 

Users

Since CSRF vulnerabilities are reportedly widespread, it is recommended

to follow best practices to mitigate risk. Some mitigating

actions are:

  • Logoff immediately after using a web application
  • Do not allow the browser to save username/passwords, and do

not allow sites to “remember” the log in details.

  • Do not use the same browser to access sensitive applications

and to surf freely the Internet; if it is necessary to do both things

at the same machine, do them with separate browsers.

Integrated HTML-enabled mail/browser, newsreader/browser

environments pose additional risks since simply viewing a mail

message or a news message might lead to the execution of an

attack.

 

 

Developers

Add session-related information to the URL. What makes the

attack possible is the fact that the session is uniquely identified

by the cookie, which is automatically sent by the browser. Having

other session-specific information being generated at the URL

level makes it difficult to the attacker to know the structure of

URLs to attack.

Other countermeasures, while they do not resolve the issue, contribute

to make it harder to exploit:

  • Use POST instead of GET. While POST requests may be simulated

by means of JavaScript, they make it more complex to mount an

attack.

  • The same is true with intermediate confirmation pages (such as:

“Are you sure you really want to do this?” type of pages).

They can be bypassed by an attacker, although they will make

their work a bit more complex. Therefore, do not rely solely on

these measures to protect your application.

  • Automatic log out mechanisms somewhat mitigate the

exposure to these vulnerabilities, though it ultimately depends

on the context (a user who works all day long on a vulnerable

web banking application is obviously more at risk than a user

who uses the same application occasionally).

 

 

 

Description of CSRF Vulnerabilities -

See the OWASP article on CSRF Vulnerabilities.

How to Avoid CSRF Vulnerabilities -

See the OWASP Development Guide article on how to Avoid

CSRF Vulnerabilities.

How to Review Code for CSRF Vulnerabilities -

See the OWASP Code Review Guide article on how to Review

Code for CSRF Vulnerabilities.

How to Prevent CSRF Vulnerabilites -

See the OWASP CSRF Prevention Cheat Sheet for prevention

measures.

 

 

 

 

 

 

In this example we will be using Burp’s CSRF PoC generator to help us hijack a user's account by changing their details (the email address associated with the account) on an old, vulnerable version of “GETBOO”.

The version of “GETBOO” we are using is taken from OWASP’s Broken Web Application Project. Find out how to download, install and use this project.

 

 

 

Burp Scanner is able to locate potential CSRF issues.

The Scanner identifies a number of conditions, including when an application relies solely on HTTP cookies to identify the user, that result in a request being vulnerable to CSRF.

 

 

To manually test for CSRF vulnerabilities, first, ensure that Burp is correctly configured with your browser.

 

In the Burp Proxy "Intercept" tab, ensure "Intercept is off".

Visit the web application you are testing in your browser.

 

Ensure you are authenticated to the web application you are testing.

 

In this example by logging in to the application.

You can log in using the credentials user:user.

 

Access the page you are testing.

Alter the value in the field/s you wish to change, in this case “Email”.

 

In this example we will add a number to the email.

 

Return to Burp.

 

In the Proxy "Intercept" tab, ensure "Intercept is on".

Submit the request so that it is captured by Burp.

 

In the "Proxy" tab, right click on the raw request to bring up the context menu.

 

Go to the “Engagement tools” options and click “Generate CSRF PoC”.

 

Note: You can also generate CSRF PoC's via the context menu in any location where HTTP requests are shown, such as the site map or Proxy history.

 

 

In the "CSRF PoC generator" window you should alter the value of the user supplied input.

 

In this example we will change to "newemail@malicious.com".

 

In the same window, click “Copy HTML”.

 

Open a text editor and paste the copied HTML.

 

Save the file as a HTML file.

 

In the Proxy "Intercept" tab, ensure "Intercept is off".

 

If necessary, log back in to the application.

 

Initially we will test the attack on the same account.

 

Open the HTML file in the same browser.

 

Dependent on the CSRF PoC options you may need to submit the request or it may be submitted automatically.

 

 

In this case we are submitting the request manually.

 

If the attack has been successful and the account information has been successfully changed, this serves as an initial check to verify whether the attack is plausible.

Now login to the application using a different account (in this example the admin account for the application).

 

Once you are logged in, perform the attack again by opening the file in the same browser.

The attack is successful if the account information in the web application has been altered.

 

A successful attack shows that the web application is vulnerable to CSRF.

For the attack to fire in a real world environment, the victim needs to access a page under the attacker's control while authenticated.

 

In our example web application, a new password can be set for the account using the email address. In this way an attacker could gain full ownership 

 

 

 

 

Generate CSRF PoC:

This function can be used to generate a proof-of-concept (PoC) cross-site request forgery (CSRF) attack for a given request.

To access this function, select a URL or HTTP request anywhere within Burp, and choose "Generate CSRF PoC" within "Engagement tools" in the context menu.

When you execute this function, Burp shows the full request you selected in the top panel, and the generated CSRF HTML in the lower panel. The HTML uses a form and/or JavaScript to generate the required request in the browser.

You can edit the request manually, and click the "Regenerate" button to regenerate the CSRF HTML based on the updated request.

You can test the effectiveness of the generated PoC in your browser, using the "Test in browser" button. When you select this option, Burp gives you a unique URL that you can paste into your browser (configured to use the current instance of Burp as its proxy). The resulting browser request is served by Burp with the currently displayed HTML, and you can then determine whether the PoC is effective by monitoring the resulting request(s) that are made through the Proxy.

Some points should be noted regarding CSRF techniques:

  • The cross-domain XmlHttpRequest (XHR) technique only works on modern HTML5-capable browsers that support cross-origin resource sharing (CORS). The technique has been tested on current versions of Firefox, Internet Explorer and Chrome. The browser must have JavaScript enabled. Note that with this technique, the application's response is not processed by the browser in the normal way, so it is not suitable for making cross-domain requests to deliver reflected cross-site scripting (XSS) attacks. Cross-domain XHR is subject to various restrictions which may prevent it from working with some request features. Burp will display a warning in the CSRF PoC generator if this is liable to occur.
  • Some requests have bodies (e.g. XML or JSON) that can only be generated using either a form with plain text encoding, or a cross-domain XHR. In the former case, the resulting request will include the header "Content-Type: text/plain". In the latter case, the request can include any Content-Type header, but will only qualify as a "simple" cross-domain request (and so avoid the need for a pre-flight request which typically breaks the attack) if the Content-Type header has one of the standard values that may be specified for normal HTML forms. In some cases, although the message body exactly matches that required for the attack request, the application may reject the request due to an unexpected Content-Type header. Such CSRF-like conditions might not be practically exploitable. Burp will display a warning in the CSRF PoC generator if this is liable to occur.
  • If you manually select a CSRF technique that cannot be used to produce the required request, Burp will generate a best effort at a PoC and will display a warning.
  • If the CSRF PoC generator is using plain text encoding, then the request body must contain an equals character in order for Burp to generate an HTML form which results in that exact body. If the original request does not contain an equals character, then you may be able to introduce one into a suitable position in the request, without affecting the server's processing of it.

CSRF PoC options

The following options are available:

  • CSRF technique - This option lets you specify the type of CSRF technique to use in the HTML that generates the CSRF request. The "Auto" option is generally preferred, and causes Burp to select the most appropriate technique capable of generating the required request.
  • Include auto-submit script - Using this option causes Burp to include a script in the HTML that causes a JavaScript-enabled browser to automatically issue the CSRF request when the page is loaded.

 

From <https://portswigger.net/burp/documentation/desktop/functions/generate-csrf-poc>

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