Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-113 Variant Incomplete
Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
This vulnerability occurs when an application accepts user-supplied data and includes it directly in HTTP headers without properly filtering out carriage return (CR) and line feed (LF) characters.…
Definition
What is CWE-113?
This vulnerability occurs when an application accepts user-supplied data and includes it directly in HTTP headers without properly filtering out carriage return (CR) and line feed (LF) characters. This allows an attacker to inject new headers or split a single HTTP response into two separate responses, corrupting the intended communication flow.
HTTP request/response splitting happens when untrusted data, often from user input in an HTTP request, flows into an outgoing HTTP response header without validation. Attackers can inject CR (%0d or \r) and LF (%0a or \n) characters—or other separators like tabs and spaces—to break the header structure. The receiving agent (like a browser or proxy) then interprets the injected characters as the end of one header and the start of a new one, effectively splitting a single response into two distinct, attacker-influenced messages.
This manipulation of the HTTP stream enables several serious attacks. By controlling the second, forged response, an attacker can perform cross-site scripting (XSS), poison web caches to serve malicious content to other users, or trick the server into making unauthorized internal requests (SSRF). The core issue is a failure to sanitize separator characters before data is placed in headers, breaking the trust between interconnected HTTP components like servers, proxies, and browsers.
Real-world impact
Real-world CVEs caused by CWE-113
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Chain: Proxy uses a substring search instead of parsing the Transfer-Encoding header (CWE-697), allowing request splitting (CWE-113) and cache poisoning
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Scala-based HTTP interface allows request splitting and response splitting through header names, header values, status reasons, and URIs
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Javascript-based framework allows request splitting through a path option of an HTTP request
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Application accepts CRLF in an object ID, allowing HTTP response splitting.
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Shopping cart allows HTTP response splitting to perform HTML injection via CRLF in a parameter for a url
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Bulletin board allows response splitting via CRLF in parameter.
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Response splitting via CRLF in PHPSESSID.
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e-commerce app allows HTTP response splitting using CRLF in object id parameters
How attackers exploit it
Step-by-step attacker path
- 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.
- 2
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:
- 3
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
- 4
then the HTTP response would be split into two responses of the following form:
- 5
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:
Vulnerable code example
Vulnerable Java
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);
cookie.setMaxAge(cookieExpiration);
response.addCookie(cookie); Attacker payload
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 Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-113
- Implementation Construct HTTP headers very carefully, avoiding the use of non-validated input data.
- Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. If an input does not strictly conform to specifications, reject it or transform it into something that conforms. 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. This 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, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Implementation 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.
- Implementation 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 allowlist validation schemes by introducing dangerous inputs after they have been checked.
Detection signals
How to detect CWE-113
Plexicus auto-detects CWE-113 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-113?
How serious is CWE-113?
What languages or platforms are affected by CWE-113?
How can I prevent CWE-113?
How does Plexicus detect and fix CWE-113?
Where can I learn more about CWE-113?
Frequently asked questions
What is CWE-113?
This vulnerability occurs when an application accepts user-supplied data and includes it directly in HTTP headers without properly filtering out carriage return (CR) and line feed (LF) characters. This allows an attacker to inject new headers or split a single HTTP response into two separate responses, corrupting the intended communication flow.
How serious is CWE-113?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-113?
MITRE lists the following affected platforms: Web Based.
How can I prevent CWE-113?
Construct HTTP headers very carefully, avoiding the use of non-validated input data. Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. If an input does not strictly conform to specifications, reject it or transform it into something that conforms. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of…
How does Plexicus detect and fix CWE-113?
Plexicus's SAST engine matches the data-flow signature for CWE-113 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-113?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/113.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-113
CWE-93 Parent
Improper Neutralization of CRLF Sequences ('CRLF Injection')
This vulnerability occurs when an application uses carriage return and line feed characters (CRLF) to structure data, like separating…
CWE-79 Can precede
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…
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