What is CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')?+
CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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.
What are the security consequences of Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')?+
If exploited, CWE-113 (Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')) it can compromise Integrity and Access Control, leading to outcomes such as Modify Application Data and Gain Privileges or Assume Identity.
How do you prevent or mitigate Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')?+
Recommended mitigations for CWE-113 include: Construct HTTP headers very carefully, avoiding the use of non-validated input data.
Which programming languages are affected by Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')?+
CWE-113 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.
What are real-world examples of Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')?+
MITRE documents real CVEs mapped to CWE-113, including CVE-2020-15811, CVE-2021-41084, CVE-2018-12116, CVE-2004-2146 and CVE-2004-1656. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.
What is the difference between a CWE and a CVE?+
A CWE (Common Weakness Enumeration) like CWE-113 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.