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:
-
CL-TE (Content-Length & Transfer-Encoding) Smuggling:
- This occurs when one server uses the
Content-Length
header and another server uses theTransfer-Encoding: chunked
header to determine the end of a request.
- This occurs when one server uses the
-
TE-CL Smuggling:
- The opposite of CL-TE, where one server uses the
Transfer-Encoding
header, and another server uses theContent-Length
header.
- The opposite of CL-TE, where one server uses the
Examples of Vulnerable Requests and Responses:
-
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 theContent-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:
-
Consistent Parsing:
- Ensure all intermediary and backend systems (like caches, firewalls, and web servers) parse HTTP requests consistently.
-
Input Validation:
- Implement strict input validation to reject ambiguous requests with multiple content-length headers or conflicting transfer-encoding and content-length headers.
-
Regular Updates and Testing:
- Keep all web servers, proxies, and other network components updated. Regularly test for smuggling vulnerabilities.
-
Web Application Firewalls (WAF):
- Deploy a WAF that can recognize and block smuggling attempts.
-
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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