Software Engineering Institute

HTTP request headers contain parameters to describe an HTTP request, such as a request's size, type, source, and destination. Entities that handle HTTP data, such as web servers, web caches, and proxy servers, may not process HTTP requests in a consistent manner. A remote attacker may be able to leverage this inconsistency to force incorrect and possibly malicious data to be returned in response to a valid request.

By including multiple Content-length headers along with crafted, embedded carriage return-line feed (CRLF) pairs within the request data, the attacker may be able to send multiple requests through the web cache or browser cache in between the user and web server. The attacker is then able to control the content of the second response from the target in question, and can now perform the following attacks:

Cross-Site Scripting (XSS): The XSS attack can now be attempted even without complete control of the Location header.
Web Cache Poisoning: A web cache may be poisoned into accepting data supplied by the attacker and indexing it as the true data for a given page.
Cross User Attacks: Multiple successive users may be served data supplied by the attacker, allowing the attacker to set or read session state and perform other tasks.
Page Hijacking: Some leakage of confidential user information may occur.
Browser Cache Poisoning: Similar to the web cache poisoning scenario, a user's web browser may cache attacker-controlled data.

In some cases, this may also lead to a reversal of the attack scenario if the user downloads content which, when loaded or executed, carries out the attack in a manner that delivers protected or otherwise inaccessible content to the attacker.

HTTP Response Splitting is outlined in depth in the Watchfire HTTP Response Splitting whitepaper.

A remote unauthenticated attacker may be able to inject malicious content into a web or browser cache, to perform cross-site scripting attacks, to hijack user and session data, or to bypass content protection mechanisms. These flaws are platform independent.

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Do not follow unsolicited links

Do not click on unsolicited links received in email, instant messages, web forums, or internet relay chat (IRC) channels. Type URLs directly into the browser to avoid these misleading links. While these are generally good security practices, following these behaviors will not prevent exploitation of this vulnerability in all cases, particularly if a trusted site has been compromised or allows cross-site scripting.

Check Certificates

US-CERT recommends that prior to providing any sensitive information over a secure (HTTPS) connection, you check the name recorded in the certificate to be sure that it matches the name of the site to which you think you are connecting.