{"id":"UBUNTU-CVE-2026-43971","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. 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 \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. This issue affects cowlib: from 2.9.0 onward.","modified":"2026-09-16T14:02:03.757465091Z","published":"2026-08-18T09:17:00Z","upstream":["CVE-2026-43971"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2026-43971"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2026-43971"},{"type":"REPORT","url":"https://cna.erlef.org/cves/CVE-2026-43971.html"},{"type":"REPORT","url":"https://osv.dev/vulnerability/EEF-CVE-2026-43971"}],"affected":[{"package":{"name":"erlang-cowlib","ecosystem":"Ubuntu:16.04:LTS","purl":"pkg:deb/ubuntu/erlang-cowlib?arch=source&distro=xenial"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-1"],"ecosystem_specific":{"binaries":[{"binary_name":"erlang-cowlib","binary_version":"1.3.0-1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-43971.json"}},{"package":{"name":"erlang-cowlib","ecosystem":"Ubuntu:18.04:LTS","purl":"pkg:deb/ubuntu/erlang-cowlib?arch=source&distro=bionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-2"],"ecosystem_specific":{"binaries":[{"binary_name":"erlang-cowlib","binary_version":"1.3.0-2"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-43971.json"}},{"package":{"name":"erlang-cowlib","ecosystem":"Ubuntu:20.04:LTS","purl":"pkg:deb/ubuntu/erlang-cowlib?arch=source&distro=focal"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3"],"ecosystem_specific":{"binaries":[{"binary_name":"erlang-cowlib","binary_version":"1.3.0-3"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-43971.json"}},{"package":{"name":"erlang-cowlib","ecosystem":"Ubuntu:22.04:LTS","purl":"pkg:deb/ubuntu/erlang-cowlib?arch=source&distro=jammy"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3"],"ecosystem_specific":{"binaries":[{"binary_name":"erlang-cowlib","binary_version":"1.3.0-3"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-43971.json"}},{"package":{"name":"erlang-cowlib","ecosystem":"Ubuntu:24.04:LTS","purl":"pkg:deb/ubuntu/erlang-cowlib?arch=source&distro=noble"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3"],"ecosystem_specific":{"binaries":[{"binary_name":"erlang-cowlib","binary_version":"1.3.0-3"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-43971.json"}},{"package":{"name":"erlang-cowlib","ecosystem":"Ubuntu:26.04:LTS","purl":"pkg:deb/ubuntu/erlang-cowlib?arch=source&distro=resolute"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3","1.3.0-3build1"],"ecosystem_specific":{"binaries":[{"binary_name":"erlang-cowlib","binary_version":"1.3.0-3build1"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-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"},{"type":"Ubuntu","score":"medium"}]}