Understanding HTTP Request Smuggling: Types, Examples, and Prevention

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HTTP Request Smuggling is a complex web application attack technique that exploits inconsistencies in the way a website processes sequences of HTTP requests. It can lead to various security issues, including web cache poisoning, bypassing security controls, and cross-site scripting (XSS). Let’s explore its types, examples, and how to prevent them.

Types of HTTP Request Smuggling:

  1. CL-TE (Content-Length & Transfer-Encoding) Smuggling:
    • This occurs when one server uses the Content-Length header and another server uses the Transfer-Encoding: chunked header to determine the end of a request.
  2. TE-CL Smuggling:
    • The opposite of CL-TE, where one server uses the Transfer-Encoding header, and another server uses the Content-Length header.

Examples of Vulnerable Requests and Responses:

  1. CL-TE Smuggling Example:
    • Request:
POST / HTTP/1.1
Host: vulnerable-website.com
Content-Length: 13
Transfer-Encoding: chunked

0

GPOST / HTTP/1.1
Host: vulnerable-website.com
Content-Length: 6

foo

Server Response:

HTTP/1.1 200 OK
...
  • Explanation: The server that prioritizes Content-Length processes the request according to the Content-Length header, treating the rest of the data as a new request.

TE-CL Smuggling Example:

  • Request:
POST / HTTP/1.1
Host: vulnerable-website.com
Transfer-Encoding: chunked
Content-Length: 6

5
GPOST
0

Server Response:

HTTP/1.1 200 OK
...

Explanation: The server that prioritizes Content-Length processes the request according to the Content-Length header, treating the rest of the data as a new request.

Prevention Methods:

  1. Consistent Parsing:
    • Ensure all intermediary and backend systems (like caches, firewalls, and web servers) parse HTTP requests consistently.
  2. Input Validation:
    • Implement strict input validation to reject ambiguous requests with multiple content-length headers or conflicting transfer-encoding and content-length headers.
  3. Regular Updates and Testing:
    • Keep all web servers, proxies, and other network components updated. Regularly test for smuggling vulnerabilities.
  4. Web Application Firewalls (WAF):
    • Deploy a WAF that can recognize and block smuggling attempts.
  5. Unified Processing:
    • Avoid using multiple, disparate systems to process parts of HTTP requests. A unified approach reduces discrepancies in request processing.

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