{"id":"CVE-2026-86431","aliases":["DEBIAN-CVE-2026-86431","EUVD-2026-72389","GHSA-m24x-mf52-x8gc","UBUNTU-CVE-2026-86431"],"status":"published","title":"commonmark before 2.9.1 XSS via AttributesExtension form feed bypass","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","severity":{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-12","score":0.00388,"percentile":0.33391,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-79"],"published":"2026-09-07T12:53:51Z","modified":"2026-10-28T02:31:13Z","fixes":[{"ecosystem":"Debian:14","package":"php-league-commonmark","fixed_in":["2.10.0-1"],"sources":["osv"]}],"affected":[{"source":"osv","ecosystem":"Debian:12","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.10.0-1","2.10.1-1","2.10.3-1","2.3.9-1","2.3.9-1+deb12u1","2.4.0-1","2.4.1-1","2.4.2-1","2.4.2-2","2.5.0-1","2.5.1-1","2.5.3-1","2.6.0-1","2.6.1-1","2.6.1-2","2.6.2-1","2.7.0-1","2.7.1-1","2.7.1-2","2.8.0-1","2.8.1-1","2.8.2-1","2.8.2-2","2.8.3-1","2.9.0-1"]},{"source":"osv","ecosystem":"Debian:13","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.10.0-1","2.10.1-1","2.10.3-1","2.7.0-1","2.7.0-1+deb13u1","2.7.1-1","2.7.1-2","2.8.0-1","2.8.1-1","2.8.2-1","2.8.2-2","2.8.3-1","2.9.0-1"]},{"source":"osv","ecosystem":"Debian:14","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.10.0-1"}]}],"versions":["2.7.0-1","2.7.1-1","2.7.1-2","2.8.0-1","2.8.1-1","2.8.2-1","2.8.2-2","2.8.3-1","2.9.0-1"]},{"source":"osv","ecosystem":"Ubuntu:Pro:20.04:LTS","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.1.2-1","1.2.0-1","1.3.0-1","1.3.1-1","1.3.1-1ubuntu2","1.3.1-1ubuntu2+esm1"]},{"source":"osv","ecosystem":"Ubuntu:Pro:22.04:LTS","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.5.7-2build1","1.6.6-1build1","1.6.7-1","1.6.7-1ubuntu0.1~esm1"]},{"source":"osv","ecosystem":"Ubuntu:Pro:24.04:LTS","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.3.9-1","2.4.1-1","2.4.2-1","2.4.2-2","2.4.2-2ubuntu0.1~esm1"]},{"source":"osv","ecosystem":"Ubuntu:26.04:LTS","package":"php-league-commonmark","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.7.0-1","2.7.1-2","2.8.0-1"]}],"products":[{"vendor":"thephpleague","product":"commonmark"}],"references":[{"url":"https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8","tags":["vendor-advisory","Exploit","Mitigation","Vendor Advisory","advisory","report"]},{"url":"https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea","tags":["patch","Patch","fix"]},{"url":"https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass","tags":["third-party-advisory","Patch","Third Party Advisory","advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-86431","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-m24x-mf52-x8gc"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86431.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-86431","tags":["advisory"]},{"url":"https://ubuntu.com/security/CVE-2026-86431","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-86431","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-86431","title":"commonmark before 2.9.1 XSS via AttributesExtension form feed bypass","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T12:53:51.246Z","modified":"2026-10-08T15:25:52.798Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N","level":"medium","assessor":"cna"},{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high","assessor":"cna"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T13:49:31.546614Z"}],"euvd":[{"id":"EUVD-2026-72389","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T12:53:51Z","modified":"2026-10-08T15:25:52Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N","level":"medium"}],"updated_at":"2026-11-02T17:27:28.564399Z"}],"ghsa":[{"id":"GHSA-m24x-mf52-x8gc","title":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site...","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T15:33:53Z","modified":"2026-09-07T15:34:01Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high"},{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/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-79"],"updated_at":"2026-11-02T15:11:27.182088Z"}],"nvd":[{"id":"CVE-2026-86431","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T13:20:42.46Z","modified":"2026-10-08T16:17:57.507Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/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"},{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high","assessor":"secondary"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T14:12:44.436435Z"}],"osv":[{"id":"CVE-2026-86431","title":"commonmark before 2.9.1 XSS via AttributesExtension form feed bypass","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T12:53:51.246Z","modified":"2026-10-28T02:31:13.48327733Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N","level":"medium"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T15:45:18.825071Z"},{"id":"DEBIAN-CVE-2026-86431","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T13:20:42.46Z","modified":"2026-10-09T09:47:29.874703022Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/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":"2026-11-02T15:46:05.022702Z"},{"id":"UBUNTU-CVE-2026-86431","description":"league/commonmark (thephpleague/commonmark) versions \u003e= 2.7.0 and \u003c 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension. Prefixing an attribute name with a single U+000C form feed byte (e.g. {\\x0Conclick=\"alert(1)\"}) bypasses the AttributesHelper::filterAttributes() 'on*' event-handler filter because PHP's trim() does not strip U+000C, causing the attribute to be written verbatim into the output where browsers parse it as a genuine event handler. The same prefix also defeats the allow_unsafe_links check, allowing javascript: URIs through href/src attributes even when allow_unsafe_links is false. Exploitation requires processing untrusted Markdown with the AttributesExtension enabled; the injected script executes when the rendered HTML is viewed. Fixed in 2.9.1.","status":"published","published":"2026-09-07T13:20:00Z","modified":"2026-09-16T14:03:21.177135181Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high"},{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N","level":"medium"}],"updated_at":"2026-11-02T16:57:24.828975Z"}]}}