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

ID: 113Date: (C)2012-05-14   (M)2018-12-28
Type: weaknessStatus: INCOMPLETE
Abstraction Type: Base


The software receives data from an upstream component, but does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers.

Extended Description

Including unvalidated data in an HTTP header allows an attacker to specify the entirety of the HTTP response rendered by the browser. When an HTTP request contains unexpected CR (carriage return, also given by %0d or \r) and LF (line feed, also given by %0a or \n) characters the server may respond with an output stream that is interpreted as two different HTTP responses (instead of one). An attacker can control the second response and mount attacks such as 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 being validated for malicious characters.

Applicable Platforms
Language Class: All

Time Of Introduction

  • Implementation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
Modify application data
Gain privileges / assume identity
CR and LF characters in an HTTP header may give attackers control of the remaining headers and body of the response the application intends to send, as well as allowing them to create additional responses entirely under their control.

Detection Methods

Potential Mitigations

Input Validation
Construct HTTP headers very carefully, avoiding the use of non-validated input data.
Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Output Encoding
Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.
Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass whitelist validation schemes by introducing dangerous inputs after they have been checked.


Related CWETypeViewChain
CWE-113 ChildOf CWE-896 Category CWE-888  

Demonstrative Examples   (Details)

  1. 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.

Observed Examples

  1. CVE-2004-2146 : Application accepts CRLF in an object ID, allowing HTTP response splitting.
  2. CVE-2004-1620 : HTTP response splitting via CRLF in parameter related to URL.
  3. CVE-2004-1656 : HTTP response splitting via CRLF in parameter related to URL.
  4. CVE-2005-2060 : Bulletin board allows response splitting via CRLF in parameter.
  5. CVE-2005-2065 : Bulletin board allows response splitting via CRLF in parameter.
  6. CVE-2004-2512 : Response splitting via CRLF in PHPSESSID.
  7. CVE-2005-1951 : Chain: Application accepts CRLF in an object ID, allowing HTTP response splitting.
  8. CVE-2004-1687 : Chain: HTTP response splitting via CRLF in parameter related to URL.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions

Black Box Definitions

Taxynomy Mappings

PLOVER  HTTP response splitting
7 Pernicious Kingdoms  HTTP Response Splitting
HTTP Response Splitting


  2. Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 2: Web-Server Related Vulnerabilities (XSS, XSRF, and Response Splitting)." Page 31'. Published on 2010.

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