{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-86431","assignerOrgId":"83251b91-4cc7-4094-a5c7-464a1b83ea10","state":"PUBLISHED","assignerShortName":"VulnCheck","dateReserved":"2026-09-07T12:34:31.457Z","datePublished":"2026-09-07T12:53:51.246Z","dateUpdated":"2026-10-08T15:25:52.798Z"},"containers":{"cna":{"providerMetadata":{"orgId":"83251b91-4cc7-4094-a5c7-464a1b83ea10","shortName":"VulnCheck","dateUpdated":"2026-10-08T15:25:52.798Z"},"datePublic":"2026-08-09T00:00:00.000Z","title":"commonmark before 2.9.1 XSS via AttributesExtension form feed bypass","descriptions":[{"lang":"en","value":"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(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."}],"problemTypes":[{"descriptions":[{"lang":"en","description":"Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')","cweId":"CWE-79","type":"CWE"}]}],"affected":[{"vendor":"thephpleague","product":"commonmark","defaultStatus":"unaffected","versions":[{"version":"2.7.0","status":"affected","versionType":"semver","lessThan":"2.9.1"},{"version":"2.9.1","status":"unaffected","versionType":"semver"}],"packageURL":"pkg:composer/league/commonmark"}],"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:a:thephpleague:commonmark:*:*:*:*:*:*:*:*","versionStartIncluding":"2.7.0","versionEndExcluding":"2.9.1"}]}]}],"metrics":[{"format":"CVSS","cvssV4_0":{"version":"4.0","vectorString":"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","attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"NONE","privilegesRequired":"NONE","userInteraction":"NONE","vulnConfidentialityImpact":"LOW","vulnIntegrityImpact":"LOW","vulnAvailabilityImpact":"NONE","subConfidentialityImpact":"LOW","subIntegrityImpact":"LOW","subAvailabilityImpact":"NONE","baseScore":6.9,"baseSeverity":"MEDIUM"}},{"format":"CVSS","cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"NONE","scope":"CHANGED","confidentialityImpact":"LOW","integrityImpact":"LOW","availabilityImpact":"NONE","baseScore":7.2,"baseSeverity":"HIGH"}}],"references":[{"url":"https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8","tags":["vendor-advisory"],"name":"GitHub Security Advisory (GHSA-f8fg-pg57-v4j8)"},{"url":"https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea","tags":["patch"],"name":"Patch Commit"},{"name":"VulnCheck Advisory: commonmark before 2.9.1 XSS via AttributesExtension form feed bypass","tags":["third-party-advisory"],"url":"https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass"}],"credits":[{"lang":"en","value":"StarPlatinu","type":"reporter"}],"x_generator":{"engine":"vulncheck-endgame"}},"adp":[{"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-09-08T13:30:04.102356Z","id":"CVE-2026-86431","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"title":"CISA ADP Vulnrichment","providerMetadata":{"orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP","dateUpdated":"2026-09-08T13:33:06.656Z"}}]}}