CAPEC Definitions / CAPEC-33
CAPEC-33: HTTP Request Smuggling
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.
Extended Description
A maliciously crafted HTTP request, which contains a second secretly embedded HTTP request is interpreted by an intermediary web proxy as single benign HTTP request, is forwarded to a back-end server, that interprets and parses the HTTP request as two authorized benign HTTP requests bypassing security controls. This attack usually involves the misuse of the HTTP headers: Content-Length and Transfer-Encoding. These abuses are discussed in RFC 2616 #4.4.3 and section #4.2 and are related to ordering and precedence of these headers. [REF-38] Additionally this attack can be performed through modification and/or fuzzing of parameters composing the request-line of HTTP messages. This attack is usually the result of the usage of outdated or incompatible HTTP protocol versions in the HTTP agents. This differs from CAPEC-273 HTTP Response Smuggling, which is usually an attempt to compromise a client agent (e.g., web browser) by sending malicious content in HTTP responses from back-end HTTP infrastructure. HTTP Request Smuggling is an attempt to compromise a back-end HTTP agent via HTTP Request messages. HTTP Splitting (CAPEC-105 and CAPEC-34) is different from HTTP Smuggling due to the fact that during implementation of asynchronous requests, HTTP Splitting requires the embedding/injection of arbitrary HTML headers and content through user input into browser cookies or Ajax web/browser object parameters like XMLHttpRequest.
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.
Relationships with other CAPECs
CAPEC-220: Client-Server Protocol Manipulation
CAPEC-273: HTTP Response Smuggling
CAPEC-115: Authentication Bypass
CAPEC-141: Cache Poisoning
CAPEC-63: Cross-Site Scripting (XSS)
CAPEC-593: Session Hijacking
CAPEC-148: Content Spoofing
CAPEC-154: Resource Location Spoofing
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.
Related Weaknesses
CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
