CVE-2026-43969: Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1
Cookie Request Header Injection via Unvalidated Encoder in cowcookie:cookie/1
Other sources
Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.
cowcookie:cookie/1 in cowlib builds a client-side Cookie: request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject ;, ,, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting ; admin=1 to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (parsecookiename/1, parsecookievalue/1) and setcookie/3 already validate and reject these characters; the encoder alone is missing the check.
This issue affects cowlib: from 2.9.0 onward.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
cowlibto a version that resolves this vulnerability.Fixed in 2.9.0 - Configuration
Update cowlib so the encoder in cow_cookie:cookie/1 validates and rejects untrusted cookie name and value fields containing ';', ',', CR, LF, or TAB (the decoder side already validates via parse_cookie_name/1 and parse_cookie_value/1, but the encoder was missing the check).
cow_cookie:cookie/1 (cowlib) encoder Validate/Reject cookie name and value characters = Reject characters: ';', ',', CR, LF, TAB
Event History
Frequently Asked Questions
What is the severity of CVE-2026-43969?
CVE-2026-43969 has been classified as a critical vulnerability due to its potential for HTTP request splitting and cookie smuggling.
How do I fix CVE-2026-43969?
To mitigate CVE-2026-43969, ensure that input validation is enforced on the cookie name and value fields to properly neutralize any CRLF sequences.
What software versions are affected by CVE-2026-43969?
CVE-2026-43969 affects cowlib versions 2.9.0 and above.
What are the potential impacts of CVE-2026-43969?
Exploitation of CVE-2026-43969 can lead to session hijacking through cookie smuggling and the ability to execute arbitrary requests on behalf of users.
Is CVE-2026-43969 related to other vulnerabilities?
CVE-2026-43969 is related to other CRLF injection vulnerabilities that also involve improper handling of HTTP headers.