Understanding and Preventing HTTP Response Splitting Attacks

What is HTTP Response Splitting?

HTTP Response Splitting is a web application vulnerability that arises when an application incorporates untrusted input into HTTP response headers without proper validation or encoding. Attackers exploit this flaw by injecting carriage return (CR) and line feed (LF) characters into the input, causing the server to interpret the input as separate HTTP responses. This manipulation can lead to various malicious activities, including cross-site scripting (XSS), web cache poisoning, and unauthorized header injection. 

How Does HTTP Response Splitting Work?

In an HTTP Response Splitting attack, an attacker crafts input containing CR (%0D) and LF (%0A) sequences, which are used to terminate HTTP headers. When the server processes this input without proper sanitization, it interprets the injected CRLF sequences as the end of one response and the beginning of another. This allows the attacker to create a malicious response that can be sent to users or cached by intermediary proxies.​

Example:

Consider a web application that reflects user input within an HTTP header:​imperva.com

HTTP/1.1 200 OK
Content-Type: text/html
Set-Cookie: sessionId=abc123; Path=/; User=attackerInput
...

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If an attacker submits input like:

attacker%0D%0AContent-Length:%200%0D%0A%0D%0AHTTP/1.1%20200%20OK%0D%0AContent-Type:%20text/html%0D%0AContent-Length:%2025%0D%0A%0D%0A<script>alert(1)</script>

The server's response may be split into two:​

HTTP/1.1 200 OK
Content-Type: text/html
Set-Cookie: sessionId=abc123; Path=/; User=attacker
Content-Length: 0

HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 25

<script>alert(1)</script>
...

This results in the user's browser executing the injected script, leading to potential XSS attacks.

Potential Impacts of HTTP Response Splitting

• Cross-Site Scripting (XSS): Attackers can inject malicious scripts that execute in users' browsers, stealing sensitive information or performing unauthorized actions.​
• Web Cache Poisoning: Malicious responses can be cached by intermediary proxies, causing subsequent users to receive the attacker's content instead of legitimate content.​
• Session Hijacking: Manipulated responses can set unauthorized cookies or headers, leading to user session compromise.​
• Security Policy Bypass: Attackers can inject headers that alter security policies, such as Content Security Policy (CSP), weakening the application's defenses.​

Prevention Strategies

To protect your applications from HTTP Response Splitting attacks, consider implementing the following measures:

1. Input Validation and Sanitization:
   
   • Ensure that all user-supplied input is validated and sanitized before inclusion in HTTP headers. Remove or encode CR (%0D) and LF (%0A) characters to prevent header injection. ​
2. Use Frameworks with Built-in Protections:
   
   • Leverage modern web frameworks that automatically handle input sanitization and encoding, reducing the risk of HTTP Response Splitting vulnerabilities.
3. Encode Output Appropriately:
   
   • When including user input in HTTP headers, apply proper encoding to ensure special characters are treated as data, not control characters.​
4. Avoid Directly Including User Input in Headers:
   
   • Design applications to avoid directly incorporating user input into HTTP headers. Instead, use predefined values or thoroughly sanitized input.​
5. Conduct Regular Security Testing:
   
   • Perform regular security assessments, including code reviews and penetration testing, to identify and remediate HTTP Response Splitting vulnerabilities.​

Conclusion

HTTP Response Splitting is a critical vulnerability that can lead to severe security issues, including XSS attacks and web cache poisoning. By implementing robust input validation, leveraging secure frameworks, and adhering to best practices in output encoding, developers can effectively mitigate the risks associated with HTTP Response Splitting and enhance the overall security of their web applications.​