CVE-2026-43969Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1

Official description

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. cow_cookie: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 (parse_cookie_name/1, parse_cookie_value/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 before 2.21.0.

Affected products (as reported)

References

• GitHub Advisory patch
• EEF CNA record for CVE-2026-43969 related, third-party-advisory
• OSV record EEF-CVE-2026-43969 related
• Introducing commit f017f8a in ninenines/cowlib related
• Fix commit c17fd73 in ninenines/cowlib patch