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.

Workarounds

Validate inputs into cow_cookie:cookie/1 to only include valid cookie name and value characters as defined in RFC 6265 Section 4.1.1 before passing them to the function.

Configurations

The application must pass attacker-controlled bytes as cookie names or values to cow_cookie:cookie/1. Applications that construct cookie lists exclusively from trusted, application-controlled values are not affected.