Identifying Source Origin
To identify the source origin, we recommend using one of these two standard headers that almost all requests include one or both of:
- Origin Header
- Referer Header
Synchronizer (CSRF) Tokens –
The synchronizer token pattern requires the generation of random “challenge” tokens (anti-CSRF tokens) that are associated with the user’s current session. These challenge tokens are then inserted within the HTML forms and links associated with sensitive server-side operations. When users submit the form or make a request to the links, the anti-CSRF token should be included in the request. Then, the server application will verify the existence and correctness of this token before processing the request. If the token is missing or incorrect, the request will be rejected.
- Any state changing operation requires a secure random token (e.g., CSRF token) to prevent CSRF attacks
- Characteristics of a CSRF Token
- Unique per user session
- Large random value
- Generated by a cryptographically secure random number generator
- The CSRF token is added as a hidden field for forms or within the URL if the state changing operation occurs via a GET
- The server rejects the requested action if the CSRF token fails validation
In order to facilitate a “transparent but visible” CSRF solution, developers are encouraged to adopt the Synchronizer Token Pattern (http://www.corej2eepatterns.com/Design/PresoDesign.htm). The synchronizer token pattern requires the generating of random “challenge” tokens that are associated with the user’s current session. These challenge tokens are then inserted within the HTML forms and links associated with sensitive server-side operations. When the user wishes to invoke these sensitive operations, the HTTP request should include this challenge token. It is then the responsibility of the server application to verify the existence and correctness of this token. By including a challenge token with each request, the developer has a strong control to verify that the user actually intended to submit the desired requests. Inclusion of a required security token in HTTP requests associated with sensitive business functions helps mitigate CSRF attacks as successful exploitation assumes the attacker knows the randomly generated token for the target victim’s session. This is analogous to the attacker being able to guess the target victim’s session identifier. The following synopsis describes a general approach to incorporate challenge tokens within the request.
When a Web application formulates a request (by generating a link or form that causes a request when submitted or clicked by the user), the application should include a hidden input parameter with a common name such as “CSRFToken”. The value of this token must be randomly generated such that it cannot be guessed by an attacker. Consider leveraging the java.security.SecureRandom class for Java applications to generate a sufficiently long random token. Alternative generation algorithms include the use of 256-bit BASE64 encoded hashes. Developers that choose this generation algorithm must make sure that there is randomness and uniqueness utilized in the data that is hashed to generate the random token.
<form action="/transfer.do" method="post"> <input type="hidden" name="CSRFToken" value="OWY4NmQwODE4ODRjN2Q2NTlhMmZlYWE... wYzU1YWQwMTVhM2JmNGYxYjJiMGI4MjJjZDE1ZDZ... MGYwMGEwOA=="> … </form>
In general, developers need only generate this token once for the current session. After initial generation of this token, the value is stored in the session and is utilized for each subsequent request until the session expires. When a request is issued by the end-user, the server-side component must verify the existence and validity of the token in the request as compared to the token found in the session. If the token was not found within the request or the value provided does not match the value within the session, then the request should be aborted, token should be reset and the event logged as a potential CSRF attack in progress.
To further enhance the security of this proposed design, consider randomizing the CSRF token parameter name and/or value for each request. Implementing this approach results in the generation of per-request tokens as opposed to per-session tokens. Note, however, that this may result in usability concerns. For example, the “Back” button browser capability is often hindered as the previous page may contain a token that is no longer valid. Interaction with this previous page will result in a CSRF false positive security event at the server. Regardless of the approach taken, developers are encouraged to protect the CSRF token the same way they protect authenticated session identifiers, such as the use of TLS.
Double Submit Cookie
If storing the CSRF token in session is problematic, an alternative defense is use of a double submit cookie. A double submit cookie is defined as sending a random value in both a cookie and as a request parameter, with the server verifying if the cookie value and request value match.
When a user authenticates to a site, the site should generate a (cryptographically strong) pseudorandom value and set it as a cookie on the user’s machine separate from the session id. The site does not have to save this value in any way, thus avoiding server side state. The site then requires that every transaction request include this random value as a hidden form value (or other request parameter). A cross origin attacker cannot read any data sent from the server or modify cookie values, per the same-origin policy. This means that while an attacker can force a victim to send any value he wants with a malicious CSRF request, the attacker will be unable to modify or read the value stored in the cookie. Since the cookie value and the request parameter or form value must be the same, the attacker will be unable to successfully force the submission of a request with the random CSRF value.
As an example, the Direct Web Remoting (DWR) Java library version 2.0 has CSRF protection built in that implements the double cookie submission transparently.
Encrypted Token Pattern
The Encrypted Token Pattern leverages an encryption, rather than comparison, method of Token-validation. After successful authentication, the server generates a unique Token comprised of the user’s ID, a timestamp value and a nonce, using a unique key available only on the server. This Token is returned to the client and embedded in a hidden field. Subsequent AJAX requests include this Token in the request-header, in a similar manner to the Double-Submit pattern. Non-AJAX form-based requests will implicitly persist the Token in its hidden field. On receipt of this request, the server reads and decrypts the Token value with the same key used to create the Token. Inability to correctly decrypt suggest an intrusion attempt. Once decrypted, the UserId and timestamp contained within the token are validated to ensure validity; the UserId is compared against the currently logged in user, and the timestamp is compared against the current time.
On successful Token-decryption, the server has access to parsed values, ideally in the form of claims. These claims are processed by comparing the UserId claim to any potentially stored UserId (in a Cookie or Session variable, if the site already contains a means of authentication). The Timestamp is validated against the current time, preventing replay attacks. Alternatively, in the case of a CSRF attack, the server will be unable to decrypt the poisoned Token, and can block and log the attack.
This pattern exists primarily to allow developers and architects protect against CSRF without session-dependency. It also addresses some of the shortfalls in other stateless approaches, such as the need to store data in a Cookie, circumnavigating the Cookie-subdomain and HTTPONLY issues.