PT-2026-39727 · Ninenines · Cowlib

Peter Ullrich · Published 2026-05-11 · Updated 2026-05-11 · CVE-2026-43969

NVD

CVSS v4.0

2.1

Low

Base vector

AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

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.

Fix

Weakness Enumeration

CWE-93

Related Identifiers

CVE-2026-43969

Affected Products

Cowlib

References · 4

• https://github.com/erlef/cowlib/commit/177953dd51540da11090666c1f007214127a1144 · Patch
• https://nvd.nist.gov/vuln/detail/CVE-2026-43969 · Security Note
• https://osv.dev/vulnerability/EEF-CVE-2026-43969 · Note
• https://cna.erlef.org/cves/CVE-2026-43969.html · Note