{"id":"CVE-2026-86431","title":"league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 2.9.1 contain a cross-site scripting vulnerability in the AttributesExtension","summary":"league/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 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(…","severity":"high","cvss":7.2,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","cwe":["CWE-79"],"vendor":"thephpleague","product":"commonmark","affected":["commonmark >= 2.7.0, < 2.9.1"],"patched":["commonmark 2.9.1"],"published":"2026-09-07","updated":"2026-09-09","sourceUpdated":"2026-09-09T16:20:39.770","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-86431","references":[{"url":"https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea","label":"disclosure@vulncheck.com"},{"url":"https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8","label":"disclosure@vulncheck.com"},{"url":"https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass","label":"disclosure@vulncheck.com"}],"tags":["nvd","cve.org"],"epss":0.00388,"epssPercentile":0.30169,"ssvc":{"exploitation":"none","automatable":"yes","technicalImpact":"partial","timestamp":"2026-09-08T13:30:04.102356Z"},"ingestedAt":"2026-09-08T15:33:26.977Z","slug":"CVE-2026-86431","body":"## Overview\n\nleague/commonmark (thephpleague/commonmark) versions >= 2.7.0 and < 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.\n\n## Affected\n\n- `commonmark >= 2.7.0, < 2.9.1`\n\n## Remediation\n\nUpgrade past the affected range:\n\n- `commonmark 2.9.1`","depth":"twilight","depthScore":40,"depthScoreParts":{"impact":39.6,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}