CAPEC-33 Detail

An adversary abuses the flexibility and discrepancies in the parsing and interpretation of HTTP Request messages using various HTTP headers, request-line and body parameters as well as message sizes (denoted by the end of message signaled by a given HTTP header) by different intermediary HTTP agents (e.g., load balancer, reverse proxy, web caching proxies, application firewalls, etc.) to secretly send unauthorized and malicious HTTP requests to a back-end HTTP agent (e.g., web server).

See CanPrecede relationships for possible consequences.

Execution Flow

[Survey network to identify target] The adversary performs network reconnaissance by monitoring relevant traffic to identify the network path and parsing of the HTTP messages with the goal of identifying potential targets.

• Scan networks to fingerprint HTTP infrastructure and monitor HTTP traffic to identify HTTP network path with a tool such as a Network Protocol Analyzer.

[Identify vulnerabilities in targeted HTTP infrastructure and technologies] The adversary sends a variety of benign/ambiguous HTTP requests to observe responses from HTTP infrastructure in order to identify differences/discrepancies in the interpretation and parsing of HTTP requests by examining supported HTTP protocol versions, message sizes, and HTTP headers.

[Cause differential HTTP responses by experimenting with identified HTTP Request vulnerabilities] The adversary sends maliciously crafted HTTP requests to interfere with the parsing of intermediary and back-end HTTP infrastructure, followed by normal/benign HTTP request from the adversary or a random user. The intended consequences of the malicious HTTP requests will be observed in the HTTP infrastructure response to the normal/benign HTTP request to confirm applicability of identified vulnerabilities in the adversary's plan of attack.

• Continue the monitoring of HTTP traffic.

• Utilize various combinations of HTTP Headers within a single HTTP Request such as: Content-Length & Transfer-Encoding (CL;TE), Transfer-Encoding & Content-Length (TE;CL), or double Transfer-Encoding (TE;TE), so that additional embedded requests or data in the body of the original request are unprocessed and treated as part of subsequent requests by the intended target HTTP agent.

  From these HTTP Header combinations the adversary observes any timing delays (usually in the form of HTTP 404 Error response) or any other unintended consequences.

  • For CL;TE and TE;CL HTTP header combinations, the first HTTP agent, in the HTTP message path that receives the HTTP request, takes precedence or only processes one header but not the other, while the second/final HTTP agent processes the opposite header, allowing for embedded HTTP requests to be ignored and smuggled to the intended target HTTP agent.
  • For TE;TE HTTP headers combination, all HTTP agents in HTTP message path process Transfer-Encoding header, however, adversary obfuscation (see Mitigations for details) of one of the Transfer-Encoding headers, by not adhering strictly to the protocol specification, can cause it to be unprocessed/ignored by a designated HTTP agent, hence allowing embedded HTTP requests to be smuggled. .

• Construct a very large HTTP request using multiple Content-Length headers of various data lengths that can potentially cause subsequent requests to be ignored by an intermediary HTTP agent (firewall) and/or eventually parsed separately by the target HTTP agent (web server).

  Note that most modern HTTP infrastructure reject HTTP requests with multiple Content-Length headers.

• Follow an unrecognized (sometimes a RFC compliant) HTTP header with a subsequent HTTP request to potentially cause the HTTP request to be ignored and interpreted as part of the preceding HTTP request.

[Perform HTTP Request Smuggling attack] Using knowledge discovered in the experiment section above, smuggle a message to cause one of the consequences.

• Leverage techniques identified in the Experiment Phase.

Prerequisites

An additional intermediary HTTP agent such as an application firewall or a web caching proxy between the adversary and the second agent such as a web server, that sends multiple HTTP messages over same network connection.
Differences in the way the two HTTP agents parse and interpret HTTP requests and its headers.
HTTP agents running on HTTP/1.1 that allow for Keep Alive mode, Pipelined queries, and Chunked queries and responses.

Skills Required

Detailed knowledge on HTTP protocol: request and response messages structure and usage of specific headers.
Detailed knowledge on how specific HTTP agents receive, send, process, interpret, and parse a variety of HTTP messages and headers.
Possess knowledge on the exact details in the discrepancies between several targeted HTTP agents in path of an HTTP message in parsing its message structure and individual headers.

Resources Required

Tools capable of crafting malicious HTTP messages and monitoring HTTP message responses.

Mitigations

Design: evaluate HTTP agents prior to deployment for parsing/interpretation discrepancies.
Configuration: front-end HTTP agents notice ambiguous requests.
Configuration: back-end HTTP agents reject ambiguous requests and close the network connection.
Configuration: Disable reuse of back-end connections.
Configuration: Use HTTP/2 for back-end connections.
Configuration: Use the same web server software for front-end and back-end server.
Implementation: Utilize a Web Application Firewall (WAF) that has built-in mitigation to detect abnormal requests/responses.
Configuration: Prioritize Transfer-Encoding header over Content-Length, whenever an HTTP message contains both.
Configuration: Disallow HTTP messages with both Transfer-Encoding and Content-Length or Double Content-Length Headers.
Configuration: Disallow Malformed/Invalid Transfer-Encoding Headers used in obfuscation, such as:

• Headers with no space before the value “chunked”
• Headers with extra spaces
• Headers beginning with trailing characters
• Headers providing a value “chunk” instead of “chunked” (the server normalizes this as chunked encoding)
• Headers with multiple spaces before the value “chunked”
• Headers with quoted values (whether single or double quotations)
• Headers with CRLF characters before the value “chunked”
• Values with invalid characters

Configuration: Install latest vendor security patches available for both intermediary and back-end HTTP infrastructure (i.e. proxies and web servers)
Configuration: Ensure that HTTP infrastructure in the chain or network path utilize a strict uniform parsing process.
Implementation: Utilize intermediary HTTP infrastructure capable of filtering and/or sanitizing user-input.

Related Weaknesses

References

REF-38

HTTP 1.1 Specification (RFC 2616)
http://www.ietf.org/rfc/rfc2616.txt

REF-117

HTTP Response Smuggling
http://www.securiteam.com/securityreviews/5CP0L0AHPC.html

REF-617

OWASP Web Security Testing Guide
https://owasp.org/www-project-web-security-testing-guide/latest/4-Web_Application_Security_Testing/07-Input_Validation_Testing/15-Testing_for_HTTP_Splitting_Smuggling.html

REF-672

HTTP Request Smuggling
Robert Auger.
http://projects.webappsec.org/w/page/13246928/HTTP%20Request%20Smuggling

REF-673

HTTP Request Smuggling: Complete Guide to Attack Types and Prevention
Dzevad Alibegovic.
https://www.neuralegion.com/blog/http-request-smuggling-hrs/

REF-674

A Pentester’s Guide to HTTP Request Smuggling
Busra Demir.
https://cobalt.io/blog/a-pentesters-guide-to-http-request-smuggling

REF-678

HTTP Desync Attacks in the Wild and How to Defend Against Them
Edi Kogan, Daniel Kerman.
https://www.imperva.com/blog/http-desync-attacks-and-defence-methods/

REF-681

HTTP Desync Attacks: Request Smuggling Reborn
James Kettle.
https://portswigger.net/research/http-desync-attacks-request-smuggling-reborn

REF-682

HTTP request smuggling
https://portswigger.net/web-security/request-smuggling

REF-683

Finding HTTP request smuggling vulnerabilities
https://portswigger.net/web-security/request-smuggling/finding

REF-684

Exploiting HTTP request smuggling vulnerabilities
https://portswigger.net/web-security/request-smuggling/exploiting

Submission

Modifications