{"id":"CVE-2026-43971","summary":"Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1","details":"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, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A \u003e 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 onward.","aliases":["EEF-CVE-2026-43971"],"modified":"2026-08-22T03:31:16.550067354Z","published":"2026-08-18T09:01:53.199Z","database_specific":{"cna_assigner":"EEF","cwe_ids":["CWE-116"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/43xxx/CVE-2026-43971.json","unresolved_ranges":[{"extracted_events":[{"introduced":"2.9.0"},{"fixed":"*"},{"introduced":"485d58dfa91b91d98135dc95e5615f421715dae5"},{"fixed":"89da27ee4c241f5d649ba7d9b7f2188918af6cea"}],"source":"AFFECTED_FIELD"}]},"references":[{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-43971"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://cna.erlef.org/cves/CVE-2026-43971.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/43xxx/CVE-2026-43971.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-43971"},{"type":"FIX","url":"https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea"},{"type":"PACKAGE","url":"https://github.com/ninenines/cowlib"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/ninenines/cowlib","events":[{"introduced":"0"},{"fixed":"89da27ee4c241f5d649ba7d9b7f2188918af6cea"}],"database_specific":{"source":"REFERENCES"}}],"versions":["2.19.0","2.18.0","2.17.1","2.17.0","2.16.1","2.16.0","2.15.0","2.14.0","2.13.0","2.12.1","2.12.0","2.11.0","2.10.1","2.10.0","2.9.1","2.9.0","2.8.0","2.7.3","2.7.2","2.7.1","2.7.0","2.6.0","2.5.1","2.5.0","2.4.0","2.3.0","2.2.1","2.2.0","2.1.0","2.0.1","2.0.0","2.0.0-rc.1","2.0.0-pre.1","1.0.1","1.3.0","1.2.0","1.1.0","1.0.0","0.6.2","0.6.1","0.6.0","0.5.1","0.5.0","0.4.0","0.3.0","0.2.0","0.1.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-43971.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}