CWE-444 is a weakness class for HTTP response splitting, where untrusted input is inserted into response headers or status lines. It can let an attacker inject extra headers or body content, which may enable cache poisoning, cross-site scripting, or request smuggling depending on the application and intermediaries.
What HTTP response splitting means
HTTP response splitting is a header-injection weakness, not just a formatting bug. When untrusted data reaches a response header or status line, the application can be tricked into emitting extra headers or even a crafted response body, creating a boundary break between application output and HTTP framing.
This matters because the browser, proxy, or cache may interpret the resulting bytes differently from the origin server. In practice, that disagreement can let an attacker influence how downstream systems store, forward, or render the response.
Why CWE-444 is dangerous
CWE-444 becomes dangerous when response construction is influenced by user-controlled input and the application fails to neutralize carriage return and line feed characters or other framing delimiters. The flaw can be especially severe in stacked environments where reverse proxies, caches, load balancers, or application servers do not parse the response stream in exactly the same way.
The security impact often extends beyond a single malformed reply. Depending on how the response is consumed, the weakness can contribute to cache poisoning, cross-site scripting, or request smuggling-like desynchronization effects. NHIMG’s HTTP Request Smuggling Guide is useful background because the same class of parsing disagreement and message framing confusion often underpins these failures.
Common places the flaw appears
Response splitting usually appears in code that builds redirects, sets cookies, reflects user input in headers, or copies request values into status messages. Any path that writes directly to Location, Set-Cookie, custom headers, or the status line deserves close review, especially when input passes through multiple encoding or normalization steps.
Intermediaries can make the issue harder to spot. A response that looks harmless in one layer may be split, cached, or reassembled differently by another, so the effective attack surface includes the origin application and every HTTP component that handles the response on the way out.
How to reduce exposure and confirm remediation
Prevention starts with strict output handling: treat header values as structured fields, reject control characters, and use framework-supported APIs instead of manual string concatenation. Review every code path that can place user input into headers or status metadata, and test the full delivery chain, not just the origin server.
After a fix, validate that the application no longer emits multiple responses, malformed headers, or unexpected cacheable variants when given hostile input. If the flaw may have been exploited, review logs, cache entries, and proxy behavior for signs that a poisoned or split response was stored or served. For broader context on weakness classification and root-cause mapping, see the CWE Top 25 Guide and the CVE, CWE and CPE Guide.
Risk and Threat Considerations
HTTP response splitting is risky because it turns response metadata into an attacker-controlled channel. Even when the origin application only intends to reflect a small piece of text, the injected delimiters can change how downstream infrastructure interprets the full message, which creates a reliable path to poisoning or response manipulation.
Failure mechanism: The application fails to separate data from HTTP framing, so crafted input breaks the header boundary and alters the response stream seen by clients or intermediaries.
Impact: Attackers can redirect traffic, poison shared caches, inject script content, or help create desynchronization conditions that undermine trust in the response path.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP ASVS, NIST SP 800-53 Rev 5 and CIS Controls v8 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP ASVS | V12 — Secure Communication | Response splitting is a wire-format integrity failure in HTTP output handling. |
| V16 — Security Logging and Error Handling | Malformed responses and exploitation traces often appear through logging and error paths. | |
| Recommendation — Validate header and response construction so untrusted input cannot alter HTTP framing. Log anomalous response construction and review errors that indicate header injection attempts. | ||
| NIST SP 800-53 Rev 5 | SI-10 — Information Input Validation | CWE-444 is triggered when control characters and delimiters are accepted into response output. |
| SC-8 — Transmission Confidentiality and Integrity | The weakness compromises integrity of the HTTP message as it traverses intermediaries and clients. | |
| Recommendation — Enforce input validation that blocks response-framing characters before data reaches headers. Protect the HTTP response path so intermediaries cannot rewrite or misinterpret message framing. | ||
| CIS Controls v8 | 16 — Application Software Security | Application output handling and secure coding practices directly address response splitting flaws. |
| Recommendation — Review application output handling to prevent header injection and response manipulation. | ||
Practitioner Guidance
What to watch for: Prioritise any endpoint that reflects user input into headers, redirects, cookies, or status fields, especially if it sits behind a cache or reverse proxy. The most important judgement is usually not whether the input is “sanitised somewhere,” but whether the final byte stream is guaranteed to remain a single, valid HTTP response.
Practitioner takeaway: If the response path includes multiple parsers, validate the exact wire output under hostile input, because response splitting is often a systems problem as much as an application bug.
Related resources from NHI Mgmt Group
- What do teams get wrong when they rely on top CWE rankings alone?
- Why do repeat CWE classes keep causing breaches even when teams know about them?
- How should security teams use the CWE Top 25 and OWASP Top 10 together without duplicating work?
- Why do CWE-level findings provide stronger audit evidence than OWASP category labels?
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Reviewed and updated by the NHIMG editorial team on November 2, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org