---
id: CVE-2026-43971
title: >-
  Improper Encoding or Escaping of Output vulnerability in ninenines cowlib
  allows Link header directive smuggling via unescaped special characters in
  cow_link:link/1.


  cow_link:do_link/1 in cowlib interpolates the target URI, rel value, a…
summary: >-
  Improper Encoding or Escaping of Output vulnerability in ninenines cowlib
  allows Link header directive smuggling via unescaped special characters in
  cow_link:link/1.


  cow_link:do_link/1 in cowlib interpolates the target URI, rel value, a…
severity: medium
cvss: 6.3
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N'
cwe:
  - CWE-116
vendor: ninenines
product: cowlib
affected:
  - cowlib >= 2.9.0 < 2.20.0
  - >-
    ninenines/cowlib >= 485d58dfa91b91d98135dc95e5615f421715dae5 <
    89da27ee4c241f5d649ba7d9b7f2188918af6cea
published: '2026-08-18'
updated: '2026-09-16'
sourceUpdated: '2026-09-16T21:17:12.773'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-43971'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-43971.html'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/ninenines/cowlib/commit/485d58dfa91b91d98135dc95e5615f421715dae5
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-43971'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
tags:
  - nvd
  - cve.org
epss: 0.00384
epssPercentile: 0.32347
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-08-18T14:11:55.645955Z'
cvssSource: cna
ingestedAt: '2026-09-16T21:05:36.902Z'
---

## Overview

Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.

cow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A > byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A " or \ in rel escapes the quoted string and opens new parameters. Any byte — including whitespace, =, and " — in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel="preconnect", rel="preload", and rel="prerender", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins.

This issue affects cowlib: from 2.9.0 before 2.20.0.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.