{"id":"CVE-2026-86431","summary":"commonmark before 2.9.1 XSS via AttributesExtension form feed bypass","details":"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.","aliases":["GHSA-f8fg-pg57-v4j8"],"modified":"2026-09-10T03:30:44.365471349Z","published":"2026-09-07T12:53:51.246Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86431.json","cna_assigner":"VulnCheck","cwe_ids":["CWE-79"]},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86431.json"},{"type":"ADVISORY","url":"https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-86431"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass"},{"type":"FIX","url":"https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/thephpleague/commonmark","events":[{"introduced":"6fbb36d44824ed4091adbcf4c7d4a3923cdb3405"},{"fixed":"43207253ea5f14867c77c697cd3838c446cadcea"}],"database_specific":{"extracted_events":[{"introduced":"2.7.0"},{"fixed":"2.9.1"}],"source":["AFFECTED_FIELD","REFERENCES"]}}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-86431.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}