Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1

Summary

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.

Timeline

1. May 11, 2026, 6:06 PM UTCCVE published
2. May 11, 2026, 6:55 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Oct 9, 2026, 6:57 AM UTCRecord last updated

Affected versions

• cowlib

  From 2.9.0 · fixed in 2.21.0

• ninenines/cowlib

  From f017f8a0ecbffd5033d9ab49bf180186f7a523a7 · fixed in c17fd73945ef536cccecc492d286fb6b6d1c3307

• cowlibHex

  From 2.9.0 · fixed in 2.21.0

• cowlibHex

  From 2.9.0 · through 2.16.1

References 5