{"id":"CVE-2026-43971","aliases":["AZL-96873","DEBIAN-CVE-2026-43971","EEF-CVE-2026-43971","EUVD-2026-60573","GHSA-gg23-fwhr-prjh","RHSA-2026:66210","UBUNTU-CVE-2026-43971"],"status":"published","title":"Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1","description":"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 before 2.20.0.","severity":{"version":"4.0","score":6.3,"vector":"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","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-09","score":0.00524,"percentile":0.44703,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-116"],"published":"2026-08-18T09:01:53Z","modified":"2026-11-05T05:40:51Z","fixes":[{"ecosystem":"Azure Linux:3","package":"rabbitmq-server","fixed_in":["3.13.7-9"],"sources":["osv"]},{"ecosystem":"Debian:14","package":"erlang-cowlib","fixed_in":["2.20.0-1"],"sources":["osv"]},{"ecosystem":"Hex","package":"cowlib","fixed_in":["2.20.0"],"sources":["osv"]},{"ecosystem":"Red Hat:hummingbird:1","package":"rabbitmq-server4.3","fixed_in":["0:4.3.5-1.1.hum1"],"sources":["osv"]}],"affected":[{"source":"osv","ecosystem":"Azure Linux:3","package":"rabbitmq-server","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.13.7-9"}]}]},{"source":"osv","ecosystem":"Debian:14","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.20.0-1"}]}],"versions":["1.3.0-3","2.17.1-1","2.17.1-2","2.18.0-1","2.19.0-1"]},{"source":"osv","ecosystem":"Hex","package":"cowlib","ranges":[{"type":"SEMVER","events":[{"introduced":"2.9.0"},{"fixed":"2.20.0"}]}],"versions":["2.10.0","2.10.1","2.11.0","2.12.0","2.12.1","2.13.0","2.14.0","2.15.0","2.16.0","2.16.1","2.17.0","2.17.1","2.18.0","2.19.0","2.9.0","2.9.1"]},{"source":"osv","ecosystem":"Red Hat:hummingbird:1","package":"rabbitmq-server4.3","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0:4.3.5-1.1.hum1"}]}]},{"source":"osv","ecosystem":"Ubuntu:16.04:LTS","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-1"]},{"source":"osv","ecosystem":"Ubuntu:18.04:LTS","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-2"]},{"source":"osv","ecosystem":"Ubuntu:20.04:LTS","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3"]},{"source":"osv","ecosystem":"Ubuntu:22.04:LTS","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3"]},{"source":"osv","ecosystem":"Ubuntu:24.04:LTS","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3"]},{"source":"osv","ecosystem":"Ubuntu:26.10","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3build1","2.19.0-1"]},{"source":"osv","ecosystem":"Ubuntu:26.04:LTS","package":"erlang-cowlib","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-3","1.3.0-3build1"]}],"products":[{"vendor":"ninenines","product":"cowlib"}],"references":[{"url":"https://cna.erlef.org/cves/CVE-2026-43971.html","tags":["related","third-party-advisory","advisory","web","article","report"]},{"url":"https://osv.dev/vulnerability/EEF-CVE-2026-43971","tags":["related","web","article","report"]},{"url":"https://github.com/ninenines/cowlib/commit/485d58dfa91b91d98135dc95e5615f421715dae5","tags":["related","fix","web"]},{"url":"https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea","tags":["patch","fix"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-43971","tags":["web","advisory"]},{"url":"https://github.com/advisories/GHSA-gg23-fwhr-prjh"},{"url":"https://github.com","tags":["web"]},{"url":"https://repo.hex.pm","tags":["web"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/43xxx/CVE-2026-43971.json","tags":["advisory"]},{"url":"https://github.com/ninenines/cowlib","tags":["package"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-43971","tags":["advisory"]},{"url":"https://hex.pm/packages/cowlib","tags":["package"]},{"url":"https://access.redhat.com/errata/RHSA-2026:66210","tags":["advisory"]},{"url":"https://access.redhat.com/security/cve/CVE-2026-43971","tags":["report"]},{"url":"https://access.redhat.com/security/updates/classification/","tags":["article"]},{"url":"https://images.redhat.com/","tags":["article"]},{"url":"https://security.access.redhat.com/data/csaf/v2/advisories/2026/rhsa-2026_66210.json","tags":["advisory"]},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2517738","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-43971","tags":["advisory","report"]},{"url":"https://ubuntu.com/security/CVE-2026-43971","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-43971","title":"Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:01:53.199Z","modified":"2026-09-16T20:24:09.279Z","cvss":[{"version":"4.0","score":6.3,"vector":"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","level":"medium","assessor":"cna"}],"cwes":["CWE-116"],"updated_at":"2026-11-02T13:37:36.275898Z"}],"euvd":[{"id":"EUVD-2026-60573","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:01:53Z","modified":"2026-09-16T20:24:09Z","cvss":[{"version":"4.0","score":6.3,"vector":"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","level":"medium"}],"updated_at":"2026-11-02T17:28:14.33177Z"}],"ghsa":[{"id":"GHSA-gg23-fwhr-prjh","title":"Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header...","description":"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.","status":"published","published":"2026-08-18T09:30:27Z","modified":"2026-09-16T21:31:37Z","cvss":[{"version":"4.0","score":6.3,"vector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium"}],"cwes":["CWE-116"],"updated_at":"2026-11-02T15:12:02.095044Z"}],"nvd":[{"id":"CVE-2026-43971","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:17:14.34Z","modified":"2026-10-22T22:10:00.25Z","cvss":[{"version":"4.0","score":6.3,"vector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium","assessor":"secondary"}],"cwes":["CWE-116"],"updated_at":"2026-11-02T14:12:38.647684Z"}],"osv":[{"id":"CVE-2026-43971","title":"Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:01:53.199Z","modified":"2026-10-28T02:31:04.361933121Z","cvss":[{"version":"4.0","score":6.3,"vector":"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","level":"medium"}],"cwes":["CWE-116"],"updated_at":"2026-11-02T15:45:04.820169Z"},{"id":"DEBIAN-CVE-2026-43971","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:17:14.34Z","modified":"2026-10-23T17:47:50.235386616Z","cvss":[{"version":"4.0","score":6.3,"vector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium"}],"updated_at":"2026-11-02T15:45:55.669088Z"},{"id":"EEF-CVE-2026-43971","title":"Link Header Directive Smuggling via Unescaped target/rel/Attribute Keys in cow_link:link/1","description":"## Summary\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\n`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.\n\nThis issue affects cowlib: from 2.9.0 before 2.20.0.\n\n## Workarounds\n\nSanitize values before passing them to `cow_link:link/1`: reject or strip any target value containing `\u003e`, any rel value containing `\"` or `\\`, and any attribute key containing characters outside the HTTP token grammar. Alternatively, ensure all Link entry fields are derived exclusively from trusted, application-controlled values rather than untrusted input.\n\n## Configurations\n\nThe application must pass values that originate from, or can be influenced by, untrusted input into `cow_link:link/1` — specifically the target URI, rel value, or attribute keys. Applications that construct Link entries exclusively from trusted, application-controlled values are not affected.","status":"published","published":"2026-08-18T09:01:53.199Z","modified":"2026-09-16T20:30:35.688617126Z","cvss":[{"version":"4.0","score":6.3,"vector":"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","level":"medium"}],"cwes":["CWE-116"],"updated_at":"2026-11-02T15:46:33.047032Z"},{"id":"RHSA-2026:66210","title":"Red Hat Security Advisory: Red Hat Hardened Images RPMs Security Update","status":"published","published":"2026-09-10T10:17:13Z","modified":"2026-09-10T10:31:21.652611693Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T15:53:22.406388Z"},{"id":"UBUNTU-CVE-2026-43971","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:17:00Z","modified":"2026-10-17T12:58:18.681655384Z","cvss":[{"version":"4.0","score":6.3,"vector":"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","level":"medium"}],"updated_at":"2026-11-02T16:56:22.075798Z"},{"id":"AZL-96873","title":"CVE-2026-43971 affecting package rabbitmq-server for versions less than 3.13.7-9","description":"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 before 2.20.0.","status":"published","published":"2026-08-18T09:17:14Z","modified":"2026-11-05T05:40:51Z","updated_at":"2026-11-05T16:41:26.280152Z"}]}}