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HTTP request smuggling

(also HRS)

HTTP request smuggling definition

HTTP request smuggling (HRS) is a high-level security vulnerability that manipulates how internet servers handle HTTP requests. By exploiting discrepancies in how servers interpret HTTP request headers, an attacker can smuggle a malicious HTTP request inside another legitimate request. This obfuscation technique bypasses security measures and can lead to various harmful actions, like cache poisoning, session hijacking, or bypassing security controls.

See also: SSL encryption, secure cookie

HTTP request smuggling examples

  • Cache poisoning: Attackers may use HRS to smuggle a malicious response into a server's cache, causing unsuspecting users who request the cached page to receive and execute the malicious code.
  • Session hijacking: Smuggled requests can also be used to hijack a user's session, granting unauthorized access to sensitive information or controls.

Advantages and disadvantages of HTTP request smuggling


  • Evasion: HRS can be used to bypass security measures, including firewalls and intrusion detection systems, making it difficult for administrators to detect.


  • Security threat: HRS poses a significant risk to web security because it can bypass security measures and lead to harmful actions like cache poisoning or session hijacking.
  • Complexity: Executing an HRS attack requires significant technical knowledge and an understanding of HTTP protocols and server behavior.

Preventing HTTP request smuggling

  • Consistency in processing: Ensuring that all your proxies and servers interpret HTTP requests in the same way can help prevent HRS.
  • Use HTTPS: While not a complete solution, using HTTPS instead of HTTP can provide an additional layer of protection.
  • Regular updates: Keeping your server software updated can prevent potential HRS exploits.