Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')

The product receives data from an HTTP agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers.


HTTP agents or components may include a web server, load balancer, reverse proxy, web caching proxy, application firewall, web browser, etc. Regardless of the role, they are expected to maintain coherent, consistent HTTP communication state across all components. However, including unexpected data in an HTTP header allows an attacker to specify the entirety of the HTTP message that is rendered by the client HTTP agent (e.g., web browser) or back-end HTTP agent (e.g., web server), whether the message is part of a request or a response.

When an HTTP request contains unexpected CR and LF characters, the server may respond with an output stream that is interpreted as "splitting" the stream into two different HTTP messages instead of one. CR is carriage return, also given by %0d or \r, and LF is line feed, also given by %0a or \n.

In addition to CR and LF characters, other valid/RFC compliant special characters and unique character encodings can be utilized, such as HT (horizontal tab, also given by %09 or \t) and SP (space, also given as + sign or %20).

These types of unvalidated and unexpected data in HTTP message headers allow an attacker to control the second "split" message to mount attacks such as server-side request forgery, cross-site scripting, and cache poisoning attacks.

HTTP response splitting weaknesses may be present when:

Data enters a web application through an untrusted source, most frequently an HTTP request.

The data is included in an HTTP response header sent to a web user without neutralizing malicious characters that can be interpreted as separator characters for headers.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code segment reads the name of the author of a weblog entry, author, from an HTTP request and sets it in a cookie header of an HTTP response.

String author = request.getParameter(AUTHOR_PARAM);
Cookie cookie = new Cookie("author", author);

Assuming a string consisting of standard alpha-numeric characters, such as "Jane Smith", is submitted in the request the HTTP response including this cookie might take the following form:

HTTP/1.1 200 OK
Set-Cookie: author=Jane Smith

However, because the value of the cookie is composed of unvalidated user input, the response will only maintain this form if the value submitted for AUTHOR_PARAM does not contain any CR and LF characters. If an attacker submits a malicious string, such as

Wiley Hacker\r\nHTTP/1.1 200 OK\r\n

then the HTTP response would be split into two responses of the following form:

HTTP/1.1 200 OK
Set-Cookie: author=Wiley Hacker
HTTP/1.1 200 OK

The second response is completely controlled by the attacker and can be constructed with any header and body content desired. The ability to construct arbitrary HTTP responses permits a variety of resulting attacks, including:

cross-user defacement

web and browser cache poisoning

cross-site scripting

page hijacking

Example Two

An attacker can make a single request to a vulnerable server that will cause the server to create two responses, the second of which may be misinterpreted as a response to a different request, possibly one made by another user sharing the same TCP connection with the server.

Cross-User Defacement can be accomplished by convincing the user to submit the malicious request themselves, or remotely in situations where the attacker and the user share a common TCP connection to the server, such as a shared proxy server.

In the best case, an attacker can leverage this ability to convince users that the application has been hacked, causing users to lose confidence in the security of the application.

In the worst case, an attacker may provide specially crafted content designed to mimic the behavior of the application but redirect private information, such as account numbers and passwords, back to the attacker.

Example Three

The impact of a maliciously constructed response can be magnified if it is cached, either by a web cache used by multiple users or even the browser cache of a single user.

Cache Poisoning: if a response is cached in a shared web cache, such as those commonly found in proxy servers, then all users of that cache will continue receive the malicious content until the cache entry is purged. Similarly, if the response is cached in the browser of an individual user, then that user will continue to receive the malicious content until the cache entry is purged, although the user of the local browser instance will be affected.

Example Four

Once attackers have control of the responses sent by an application, they have a choice of a variety of malicious content to provide users.

Cross-Site Scripting: cross-site scripting is common form of attack where malicious JavaScript or other code included in a response is executed in the user's browser.

The variety of attacks based on XSS is almost limitless, but they commonly include transmitting private data like cookies or other session information to the attacker, redirecting the victim to web content controlled by the attacker, or performing other malicious operations on the user's machine under the guise of the vulnerable site.

The most common and dangerous attack vector against users of a vulnerable application uses JavaScript to transmit session and authentication information back to the attacker who can then take complete control of the victim's account.

Example Five

In addition to using a vulnerable application to send malicious content to a user, the same weakness can also be leveraged to redirect sensitive content generated by the server to the attacker instead of the intended user.

Page Hijacking: by submitting a request that results in two responses, the intended response from the server and the response generated by the attacker, an attacker can cause an intermediate node, such as a shared proxy server, to misdirect a response generated by the server to the attacker instead of the intended user.

Because the request made by the attacker generates two responses, the first is interpreted as a response to the attacker's request, while the second remains in limbo. When the user makes a legitimate request through the same TCP connection, the attacker's request is already waiting and is interpreted as a response to the victim's request. The attacker then sends a second request to the server, to which the proxy server responds with the server generated request intended for the victim, thereby compromising any sensitive information in the headers or body of the response intended for the victim.

See Also

Comprehensive Categorization: Injection

Weaknesses in this category are related to injection.

OWASP Top Ten 2021 Category A03:2021 - Injection

Weaknesses in this category are related to the A03 category "Injection" in the OWASP Top Ten 2021.

SFP Secondary Cluster: Tainted Input to Command

This category identifies Software Fault Patterns (SFPs) within the Tainted Input to Command cluster (SFP24).

Comprehensive CWE Dictionary

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CWE Cross-section

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Weaknesses Introduced During Implementation

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