{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "cpe": "cpe:2.3:a:thephpleague:commonmark:*:*:*:*:*:*:*:*",
            "extracted_events": [
              {
                "introduced": "2.7.0"
              },
              {
                "fixed": "2.9.1"
              }
            ],
            "source": [
              "CPE_RANGE",
              "REFERENCES"
            ]
          },
          "events": [
            {
              "introduced": "6fbb36d44824ed4091adbcf4c7d4a3923cdb3405"
            },
            {
              "fixed": "73cb188c785abfa7a7bec73487148202968274c6"
            },
            {
              "fixed": "43207253ea5f14867c77c697cd3838c446cadcea"
            }
          ],
          "repo": "https://github.com/thephpleague/commonmark",
          "type": "GIT"
        }
      ]
    }
  ],
  "aliases": [
    "GHSA-f8fg-pg57-v4j8"
  ],
  "database_specific": {
    "cna_assigner": "VulnCheck",
    "cwe_ids": [
      "CWE-79"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86431.json"
  },
  "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.",
  "id": "CVE-2026-86431",
  "modified": "2026-09-11T03:30:40.174002368Z",
  "published": "2026-09-07T12:53:51.246Z",
  "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"
    }
  ],
  "schema_version": "1.9.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "commonmark before 2.9.1 XSS via AttributesExtension form feed bypass"
}