Cross-Site Request Forgery (CSRF)
Cross-Site Request Forgery (CSRF) is an attack that tricks the victim into loading a page that contains a malicious request. It is malicious in the sense that it inherits the identity and privileges of the victim to perform an undesired function on the victim's behalf, like change the victim's e-mail address, home address, or password, or purchase something. CSRF attacks generally target functions that cause a state change on the server but can also be used to access sensitive data.
For most sites, browsers will automatically include with such requests any credentials associated with the site, such as the user's session cookie, basic auth credentials, IP address, Windows domain credentials, etc. Therefore, if the user is currently authenticated to the site, the site will have no way to distinguish this from a legitimate user request.
In this way, the attacker can make the victim perform actions that they didn't intend to, such as logout, purchase item, change account information, retrieve account information, or any other function provided by the vulnerable website.
Sometimes, it is possible to store the CSRF attack on the vulnerable site itself. Such vulnerabilities are called Stored CSRF flaws. This can be accomplished by simply storing an IMG or IFRAME tag in a field that accepts HTML, or by a more complex cross-site scripting attack. If the attack can store a CSRF attack in the site, the severity of the attack is amplified. In particular, the likelihood is increased because the victim is more likely to view the page containing the attack than some random page on the Internet. The likelihood is also increased because the victim is sure to be authenticated to the site already.
Synonyms: CSRF attacks are also known by a number of other names, including XSRF, "Sea Surf", Session Riding, Cross-Site Reference Forgery, Hostile Linking. Microsoft refers to this type of attack as a One-Click attack in their threat modeling process and many places in their online documentation.
How does the attack work?
There are numerous ways in which an end-user can be tricked into loading information from or submitting information to a web application. In order to execute an attack, we must first understand how to generate a malicious request for our victim to execute. Let us consider the following example: Alice wishes to transfer $100 to Bob using bank.com. The request generated by Alice will look similar to the following:
POST http://bank.com/transfer.do HTTP/1.1 ... ... ... Content-Length: 19; acct=BOB&amount=100
However, Maria notices that the same web application will execute the same transfer using URL parameters as follows:
GET http://bank.com/transfer.do?acct=BOB&amount=100 HTTP/1.1
Maria now decides to exploit this web application vulnerability using Alice as her victim. Maria first constructs the following URL which will transfer $100,000 from Alice's account to her account:
Now that her malicious request is generated, Maria must trick Alice into submitting the request. The most basic method is to send Alice an HTML email containing the following:
<a href="http://bank.com/transfer.do?acct=MARIA&amount=100000">View my Pictures!</a>
Assuming Alice is authenticated with the application when she clicks the link, the transfer of $100,000 to Maria's account will occur. However, Maria realizes that if Alice clicks the link, then Alice will notice that a transfer has occurred. Therefore, Maria decides to hide the attack in a zero-byte image:
<img src="http://bank.com/transfer.do?acct=MARIA&amount=100000" width="1" height="1" border="0">
If this image tag were included in the email, Alice would only see a little box indicating that the browser could not render the image. However, the browser will still submit the request to bank.com without any visual indication that the transfer has taken place.
Prevention measures that do NOT work
- Using a secret cookie
- Remember that all cookies, even the secret ones, will be submitted with every request. All authentication tokens will be submitted regardless of whether or not the end-user was tricked into submitting the request. Furthermore, session identifiers are simply used by the application container to associate the request with a specific session object. The session identifier does not verify that the end-user intended to submit the request.
- Only accepting POST requests
- Applications can be developed to only accept POST requests for the execution of business logic. The misconception is that since the attacker cannot construct a malicious link, a CSRF attack cannot be executed. Unfortunately, this logic is incorrect. There are numerous methods in which an attacker can trick a victim into submitting a forged POST request. The two most common methods are through the use of phishing sites (sites which appear to look like another valid site) and through the use of XMLHTTPRequest in a Cross-Site Scripting attack.
- Add a per-request nonce to URL and all forms in addition to the standard session. This is also referred to as "form keys". Many frameworks (ex, Drupal.org 4.7.4+) either have or are starting to include this type of protection "built-in" to every form so the programmer does not need to code this protection manually.
- TBD: Add a per-session nonce to URL and all forms
- TBD: Add a hash(session id, function name, server-side secret) to URL and all forms
- TBD: .NET - add session identifier to ViewState with MAC
- Checking HTTP referrer details can help mitigate the attack but does certainly not provide a bullet proof solution. By ensuring the HTTP posts have come from the original site means that the attacks from other sites will not function. However, if the CSRF attack was used in combination with XSS on the original site then this mechanism will not provide any protection.
- "Although cross-site request forgery is fundamentally a problem with the web application, not the user, users can help protect their accounts at poorly designed sites by logging off the site before visiting another, or clearing their browser's cookies at the end of each browser session." -http://en.wikipedia.org/wiki/Cross-site_request_forgery#_note-1
- The Cross-Site Request Forgery (CSRF/XSRF) FAQ
- quote: "This paper serves as a living document for Cross-Site Request Forgery issues. This document will serve as a repository of information from existing papers, talks, and mailing list postings and will be updated as new information is discovered."
- Testing for CSRF
- CSRF (aka Session riding) paper from the OWASP Testing Guide project (need to integrate)
- CSRF Vulnerability: A 'Sleeping Giant'
- Overview Paper
- RequestRodeo: Client Side Protection against Session Riding
- Martin Johns and Justus Winter's interesting paper and presentation for the 4th OWASP AppSec Conference which described potential techniques that browsers could adopt to automatically provide CSRF protection - PDF paper
- CSRF Guard
- A J2EE Filter which appends a unique request token to each form and link in the HTML response