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CAPEC-105: HTTP Request Splitting

Attack Pattern ID: 105
Abstraction: Detailed
Status: Stable
Presentation Filter:
+ Description

An adversary abuses the flexibility and discrepancies in the parsing and interpretation of HTTP Request messages by different intermediary HTTP agents (e.g., load balancer, reverse proxy, web caching proxies, application firewalls, etc.) to split a single HTTP request into multiple unauthorized and malicious HTTP requests to a back-end HTTP agent (e.g., web server).

See CanPrecede relationships for possible consequences.

+ Extended Description

This entails the adversary injecting malicious user input into various standard and/or user defined HTTP headers within a HTTP Request through user input of Carriage Return (CR), Line Feed (LF), Horizontal Tab (HT), Space (SP) characters as well as other valid/RFC compliant special characters and unique character encoding. This malicious user input allows for web script to be injected in HTTP headers as well as into browser cookies or Ajax web/browser object parameters like XMLHttpRequest during implementation of asynchronous requests.

This attack is usually the result of the usage of outdated or incompatible HTTP protocol versions as well as lack of syntax checking and filtering of user input in the HTTP agents receiving HTTP messages in the path.

This differs from CAPEC-34 HTTP Response Splitting, 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 Splitting is an attempt to compromise a back-end HTTP agent via HTTP Request messages.

HTTP Smuggling (CAPEC-33 and CAPEC-273) is different from HTTP Splitting due to the fact it relies upon discrepancies in the interpretation of various HTTP Headers and message sizes and not solely user input of special characters and character encoding. HTTP Smuggling was established to circumvent mitigations against HTTP Request Splitting techniques.

+ Likelihood Of Attack

Medium

+ Typical Severity

High

+ Relationships
Section HelpThis table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
NatureTypeIDName
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.220Client-Server Protocol Manipulation
PeerOfDetailed Attack PatternDetailed Attack Pattern - A detailed level attack pattern in CAPEC provides a low level of detail, typically leveraging a specific technique and targeting a specific technology, and expresses a complete execution flow. Detailed attack patterns are more specific than meta attack patterns and standard attack patterns and often require a specific protection mechanism to mitigate actual attacks. A detailed level attack pattern often will leverage a number of different standard level attack patterns chained together to accomplish a goal.34HTTP Response Splitting
CanPrecedeStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.63Cross-Site Scripting (XSS)
CanPrecedeMeta Attack PatternMeta Attack Pattern - A meta level attack pattern in CAPEC is a decidedly abstract characterization of a specific methodology or technique used in an attack. A meta attack pattern is often void of a specific technology or implementation and is meant to provide an understanding of a high level approach. A meta level attack pattern is a generalization of related group of standard level attack patterns. Meta level attack patterns are particularly useful for architecture and design level threat modeling exercises.115Authentication Bypass
CanPrecedeStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.141Cache Poisoning
CanPrecedeMeta Attack PatternMeta Attack Pattern - A meta level attack pattern in CAPEC is a decidedly abstract characterization of a specific methodology or technique used in an attack. A meta attack pattern is often void of a specific technology or implementation and is meant to provide an understanding of a high level approach. A meta level attack pattern is a generalization of related group of standard level attack patterns. Meta level attack patterns are particularly useful for architecture and design level threat modeling exercises.148Content Spoofing
CanPrecedeMeta Attack PatternMeta Attack Pattern - A meta level attack pattern in CAPEC is a decidedly abstract characterization of a specific methodology or technique used in an attack. A meta attack pattern is often void of a specific technology or implementation and is meant to provide an understanding of a high level approach. A meta level attack pattern is a generalization of related group of standard level attack patterns. Meta level attack patterns are particularly useful for architecture and design level threat modeling exercises.154Resource Location Spoofing
CanPrecedeStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.593Session Hijacking
Section HelpThis table shows the views that this attack pattern belongs to and top level categories within that view.
+ Execution Flow
Explore
  1. 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.

    Techniques
    Scan networks to fingerprint HTTP infrastructure and monitor HTTP traffic to identify HTTP network path with a tool such as a Network Protocol Analyzer.
Experiment
  1. 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, HTTP headers, syntax checking and input filtering.

  2. Cause differential HTTP responses by experimenting with identified HTTP Request vulnerabilities: The adversary sends maliciously crafted HTTP requests with custom strings and embedded web scripts and objects in HTTP headers 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.

    Techniques
    Continue the monitoring of HTTP traffic.

    Utilize different sequences of special characters (CR - Carriage Return, LF - Line Feed, HT - Horizontal Tab, SP - Space and etc.) to bypass filtering and back-end encoding and to embed:

    • additional HTTP Requests with their own headers
    • malicious web scripts into parameters of HTTP Request headers (e.g., browser cookies like Set-Cookie or Ajax web/browser object parameters like XMLHttpRequest)
    • adversary chosen encoding (e.g., UTF-7)

    to utilize additional special characters (e.g., > and <) filtered by the target HTTP agent.

    Note that certain special characters and character encoding may be applicable only to intermediary and front-end agents with rare configurations or that are not RFC compliant.

    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.
Exploit
  1. Perform HTTP Request Splitting attack: Using knowledge discovered in the experiment section above, smuggle a message to cause one of the consequences.

    Techniques
    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 headers capable of being user-manipulated.
HTTP agents running on HTTP/1.0 or HTTP/1.1 that allow for Keep Alive mode, Pipelined queries, and Chunked queries and responses.
+ Skills Required
[Level: Medium]
Detailed knowledge on HTTP protocol: request and response messages structure and usage of specific headers.
[Level: Medium]
Detailed knowledge on how specific HTTP agents receive, send, process, interpret, and parse a variety of HTTP messages and headers.
[Level: Medium]
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 messages responses.
+ Indicators
Differences in requests processed by the two agents. This requires careful monitoring or a capable log analysis tool.
+ Consequences
Section HelpThis table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Confidentiality
Integrity
Availability
Execute Unauthorized Commands
Confidentiality
Access Control
Authorization
Gain Privileges
Confidentiality
Read Data
Integrity
Modify Data
+ 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: 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.
+ Example Instances

Microsoft Internet Explorer versions 5.01 SP4 and prior, 6.0 SP2 and prior, and 7.0 contain a vulnerability that could allow an unauthenticated, remote adversary to conduct HTTP request splitting and smuggling attacks. The vulnerability is due to an input validation error in the browser that allows adversaries to manipulate certain headers to expose the browser to HTTP request splitting and smuggling attacks. Attacks may include cross-site scripting, proxy cache poisoning, and session fixation. In certain instances, an exploit could allow the adversary to bypass web application firewalls or other filtering devices. Microsoft has confirmed the vulnerability and released software updates.

+ Notes

Relationship

HTTP Smuggling is an evolution of previous HTTP Splitting techniques which are commonly remediated against.

Terminology

HTTP Splitting – "the act of forcing a sender of (HTTP) messages to emit data stream consisting of more messages than the sender’s intension. The messages sent are 100% valid and RFC compliant" [REF-117].

Terminology

HTTP Smuggling – "the act of forcing a sender of (HTTP) messages to emit data stream which may be parsed as a different set of messages (i.e. dislocated message boundaries) than the sender’s intention. This is done by virtue of forcing the sender to emit non-standard messages which can be interpreted in more than one way" [REF-117].
+ Taxonomy Mappings
Relevant to the WASC taxonomy mapping
Entry IDEntry Name
24HTTP Request Splitting
+ References
[REF-117] "HTTP Response Smuggling". Beyond Security. <http://www.securiteam.com/securityreviews/5CP0L0AHPC.html>.
[REF-679] Robert Auger. "HTTP Request Splitting". The Web Application Security Consortium. 2011. <http://projects.webappsec.org/w/page/13246929/HTTP%20Request%20Splitting>. URL validated: 2021-10-14.
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2017-08-04CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns, Resources_Required
2019-04-04CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns
2020-07-30CAPEC Content TeamThe MITRE Corporation
Updated Related_Weaknesses
2020-12-17CAPEC Content TeamThe MITRE Corporation
Updated @Abstraction, References, Taxonomy_Mappings
2021-10-21CAPEC Content TeamThe MITRE Corporation
Updated @Status, Consequences, Description, Example_Instances, Execution_Flow, Extended_Description, Indicators, Mitigations, Notes, Prerequisites, References, Related_Attack_Patterns, Related_Weaknesses, Resources_Required, Skills_Required
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Page Last Updated or Reviewed: October 21, 2021