{"cve":"CVE-2026-86431","nvd":{"published":"2026-09-07T13:20:42.460","modified":"2026-10-08T16:17:57.507","status":"Analyzed","description":"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.","cvss_v4":{"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","score":6.9},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","score":7.2},"cwes":["CWE-79"],"references":["https://github.com/thephpleague/commonmark/commit/43207253ea5f14867c77c697cd3838c446cadcea","https://github.com/thephpleague/commonmark/security/advisories/GHSA-f8fg-pg57-v4j8","https://www.vulncheck.com/advisories/commonmark-before-2.9.1-xss-via-attributesextension-form-feed-bypass"],"products":["thephpleague:commonmark"],"source":"nvd","synced_at":"2026-10-19T22:52:03.409947763+00:00"},"epss":{"cve":"CVE-2026-86431","score":0.00388,"percentile":0.32627,"date":"2026-10-25"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}