HTTP Response Splitting is a web application vulnerability that arises when an application includes untrusted data in an HTTP response header without proper validation. This vulnerability allows an attacker to inject malicious characters, such as carriage return (CR) and line feed (LF), into the header. By doing so, the attacker can manipulate the structure of the HTTP response.

The primary goal of HTTP Response Splitting is to control the remaining headers and body of the response or to create additional responses. This manipulation can lead to various malicious outcomes, making it a significant concern in web security. Understanding this vulnerability is crucial for developers and security professionals to ensure robust application security.

How does HTTP Response Splitting Work?

HTTP Response Splitting works by exploiting the way web servers handle HTTP headers. Attackers inject malicious characters, specifically carriage return (CR) and line feed (LF), into the input fields of a vulnerable application. These characters are used to terminate one header and start a new one, effectively splitting the HTTP response into multiple parts.

When the server processes these injected CRLF sequences, it interprets them as legitimate delimiters for headers and the body of the HTTP response. This allows the attacker to manipulate the structure of the response, creating additional headers or even entirely new responses. The server, failing to sanitize these inputs, inadvertently sends multiple HTTP responses from a single request.

This manipulation can lead to various exploits, as the server's response is altered in a way that can confuse clients and intermediaries. By controlling the headers and body of the response, attackers can inject arbitrary content, leading to significant security vulnerabilities.

What are Examples of HTTP Response Splitting?

Examples of HTTP Response Splitting can be found in various programming environments and applications. One notable example involves a Java application where an attacker injects CR and LF characters into an HTTP header. For instance, if a user input is directly included in a cookie without proper validation, an attacker could submit a string like "Wiley Hacker\r\nContent-Length:999\r\n\r\n…". This would split the HTTP response, allowing the attacker to inject malicious content into the response body.

Another example is seen in the PHP 5 session extension mechanism. Here, a user-supplied session ID is sent back within the Set-Cookie HTTP header. If the session ID is not properly validated, it can lead to HTTP Response Splitting. An attacker could inject arbitrary HTTP headers into the response body, exploiting the vulnerability to manipulate the server's response and potentially execute unauthorized commands.

What are the Potential Risks of HTTP Response Splitting?

The potential risks of HTTP Response Splitting are significant and can lead to various security vulnerabilities. Here are some of the key risks associated with this type of attack:

• Cross-Site Scripting (XSS) Attacks: Attackers can inject malicious scripts into the HTTP response, which can then be executed in the context of the user's browser, leading to unauthorized actions and data theft.

• Web Cache Poisoning: By manipulating the HTTP response, attackers can poison web caches, causing users to receive malicious content instead of the legitimate web page.

• Session Hijacking: Attackers can manipulate session cookies or other session-related data, potentially taking over user sessions and gaining unauthorized access to sensitive information.

• Phishing Attacks: HTTP Response Splitting can be used to create convincing phishing pages by injecting malicious content into legitimate web pages, tricking users into divulging personal information.

• Website Defacement: Attackers can alter the content of web pages, leading to defacement and damage to the website's reputation.

How Can You Protect Against HTTP Response Splitting?.

Protecting against HTTP Response Splitting requires a multi-faceted approach to ensure that web applications are secure from this vulnerability. Here are some key strategies:

• Validate User Input: Always validate and sanitize user inputs to ensure they do not contain malicious characters like CR and LF.

• Use Secure Libraries and Frameworks: Utilize modern libraries and frameworks that have built-in protections against HTTP Response Splitting.

• Encode Output: URL-encode strings before including them in HTTP headers to prevent injection of malicious characters.

• Regular Security Audits: Conduct regular security audits and code reviews to identify and fix potential vulnerabilities.

• Keep Software Updated: Ensure that all software and dependencies are up to date with the latest security patches.