{"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.\n\ncow_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.\n\ncow_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":0.32215,"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"partial","timestamp":"2026-08-18T14:11:55.645955Z"},"cvssSource":"cna","ingestedAt":"2026-09-16T21:05:36.902Z","slug":"CVE-2026-43971","body":"## Overview\n\nImproper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.\n\ncow_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.\n\nThis issue affects cowlib: from 2.9.0 before 2.20.0.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":35,"depthScoreParts":{"impact":34.7,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}