HTTP Response Splitting [CWE-113]

HTTP Response Splitting weakness describes improper neutralization of CRLF sequences in HTTP headers.

Created: September 11, 2012
Latest Update: December 28, 2020

Table of Content

1. Description
2. Potential impact
3. Attack patterns
4. Affected software
5. Severity and CVSS Scoring
6. Mitigations
7. Vulnerability Remediation Techniques and Examples
8. Common Fix Errors and Bypasses
9. References

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1. Description

This weakness occurs when software accepts data from an upstream provider, but does not neutralize or incorrectly neutralizes CR and LF characters before including data into HTTP response headers. This provides an attacker with ability to inject arbitrary headers into the HTTP response, which is sent to a client. As a result an attacker might be able to modify contents of the HTTP response by means of unexpected CR (carriage return - %0d or \r) and LF (line feed - %0a or \n) characters and send to the browser two different HTTP responses instead of one. The attacker, who controls the second HTTP response, can perform different attacks such as cache poisoning and cross-site scripting.

HTTP response splitting might occur when an application receives data from an untrusted source, such as HTTP request, or when data is not validated before sending it back to the client in HTTP response. Modern programming languages have built-in protection mechanisms against HTTP response splitting that do not allow sending more than one response header to client browser, so the HTTP response splitting attack is possible only when application uses untrusted input to construct headers.

2. Potential impact

An attacker might be able to perform cross-site scripting, phishing and cache poisoning attacks. This weakness is a significant threat for high load servers that use caching proxies to deliver content to the end-users. One request sent by an attacker can be cached and displayed to all visitors of a webpage. As a result, an attacker might be able to gain access to potentially sensitive data and perform different attacks against website users.

3. Attack patterns

CAPEC has the following patterns for this weakness:

• CAPEC-31: Accessing/Intercepting/Modifying HTTP Cookies
• CAPEC-34: HTTP Response Splitting
• CAPEC-63: Simple Script Injection
• CAPEC-85: Client Network Footprinting (using AJAX/XSS)

This weakness is described in WASC classification as four separate techniques:

• WASC-27: HTTP Response Smuggling
• WASC-25: HTTP Response Splitting
• WASC-26: HTTP Request Smuggling
• WASC-24: HTTP Request Splitting

4. Affected software

Any software that uses input data to construct headers is potentially vulnerable to this weakness. In most cases these are web applications, web servers, caching proxies.

5. Severity and CVSS Scoring

Depending on potential damage this weakness could impact integrity of the application and is usually scored as:
5.3 [CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N] – Medium severity.

6. Mitigations

All input passed to the application must be considered untrustworthy and should be handled as such. Use a good known input validation strategy based on whitelists to construct headers and do not rely exclusively on looking for malicious or malformed inputs. All input should be decoded and canonicalized to the application's current internal representation.

7. Vulnerability Remediation Techniques and Examples

PHP

Usage of the header() function for HTTP headers output is safe in modern PHP versions.

PERL

ASP.NET

With ASP.NET 2.0 this attack is impossible, the "\r\n" sequences are disallowed by default.

Python

JAVA/JSP

NodeJS

The ‘http’ module containing functions to deal with HTTP header output contains fixes for previously discovered HTTP response splitting vulnerabilities.

These are general recommendations. Every case must be treated separately.

Caution: do not blindly copy-paste the above-mentioned solutions into your application code. In some cases this may result in incorrect behavior of the application or inconsistent patch. Carefully read the References or consult security specialists in case you are not sure how to patch a vulnerability.

8. Common Fix Errors and Bypasses

Attackers can use different character encoding to bypass certain filters:

Double URL Encoding	%250d%250a
UTF-8	%E5%98%8A%E5%98%8D
UTF-16	0x000A 0x000D
Various URL Character Codes	/%2f..%0d%0a

9. References

1. CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting') [cwe.mitre.org]
2. HTTP Response Splitting [www.owasp.org]
3. HTTP Response Splitting Attack [resources.infosecinstitute.com]
4. Introduction to HTTP Response Splitting [www.securiteam.com]

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