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🛡️ CVE-2026-43969 — cowlib

🟢 CVSS 2.0 — Low ✅ No Known Exploit CWE-93 OSV
2.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

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 onward.

Workaround

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.

Configuration

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.

How this vulnerability can be exploited

This issue can be reached with local access to the system, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. Rated impact: confidentiality none, integrity low, availability none.

CVSS metrics in full

The score comes from this 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

  • Attack vector: Local — a local account, shell or session on the host is needed.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Attack requirements: Present — the target has to be in a particular state for the attack to work.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Confidentiality impact: None.
  • Integrity impact: Low — limited, and the attacker does not choose what is affected.
  • Availability impact: None.

Weakness class

CVE-2026-43969 is classified as CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection'). The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.

Affected software

CVE-2026-43969 is recorded against 1 package.

  • cowlib

Timeline and source

Published on 11 May 2026 and last revised on 18 August 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from OSV.

References

github.com (Fix)
cna.erlef.org (Web)
hex.pm (Package)

Other advisories for this package

cowlib has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')) in other software:

Details

Severity LOW
CVSS Score 2.0
CVSS 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
CWE CWE-93
Public Exploit ✅ No
Source OSV
Published 2026-05-11
Updated 2026-08-20
Modified 2026-08-18

Affected Packages

Software From version Fixed in
cowlib — —

Similar Threats

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