CAPEC-273 Detail

An adversary manipulates and injects malicious content in the form of secret unauthorized HTTP responses, into a single HTTP response from a vulnerable or compromised back-end HTTP agent (e.g., 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 to intended targets 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 Response vulnerabilities] The adversary sends maliciously crafted HTTP request to back-end HTTP infrastructure to inject adversary data into HTTP responses (intended for intermediary and/or front-end client/victim HTTP agents communicating with back-end HTTP infrastructure) for the purpose of interfering with the parsing of HTTP response. The intended consequences of the malicious HTTP request and the subsequent adversary injection and manipulation of HTTP responses will be observed to confirm applicability of identified vulnerabilities in the adversary's plan of attack.

• Continue the monitoring of HTTP traffic.

• Inject additional HTTP headers to utilize various combinations of HTTP Headers within a single HTTP message such as: Content-Length & Transfer-Encoding (CL;TE), Transfer-Encoding & Content-Length (TE;CL), or double Transfer-Encoding (TE;TE), so that additional embedded message or data in the body of the original message are unprocessed and treated as part of subsequent messages 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 headers combination, the first HTTP agent, in the HTTP message path that receives the HTTP message, takes precedence or only processes the one header but not the other, while the second/final HTTP agent processes the opposite header allowing for embedded HTTP message 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 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 messages to be smuggled. See Mitigations for details.

• Construct a very large HTTP message via multiple Content-Length headers of various data lengths that can potentially cause subsequent messages to be ignored by an intermediary HTTP agent (e.g., firewall) and/or eventually parsed separately by the target HTTP agent.

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

• Monitor HTTP traffic using a tool such as a Network Protocol Analyzer.

[Perform HTTP Response 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

A vulnerable or compromised server or domain/site capable of allowing adversary to insert/inject malicious content that will appear in the server's response to target HTTP agents (e.g., proxies and users' web browsers).
Differences in the way the two HTTP agents parse and interpret HTTP responses 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 monitoring HTTP messages, and crafting malicious HTTP messages and/or injecting malicious content into HTTP messages.

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-675

HTTP Response Smuggling
Robert Auger.
http://projects.webappsec.org/w/page/13246930/HTTP%20Response%20Smuggling

REF-676

Mozilla Foundation Security Advisory 2006-33 HTTP response smuggling
Kazuho Oku.
https://www.mozilla.org/en-US/security/advisories/mfsa2006-33/

REF-677

Testing for HTTP Splitting Smuggling
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-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/

Submission

Modifications