{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "extracted_events": [
              {
                "range": [
                  {
                    "introduced": "0"
                  },
                  {
                    "fixed": "2.9.0"
                  }
                ],
                "source": "DESCRIPTION"
              }
            ]
          },
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5703d83ba3da3b2e356a5fedc848ed6d8ffb6529"
            }
          ],
          "repo": "https://github.com/thephpleague/commonmark",
          "type": "GIT"
        }
      ]
    }
  ],
  "aliases": [
    "GHSA-mh25-x5hq-wrqp"
  ],
  "database_specific": {
    "cna_assigner": "VulnCheck",
    "cwe_ids": [
      "CWE-407"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86434.json",
    "unresolved_ranges": [
      {
        "extracted_events": [
          {
            "range": [
              {
                "introduced": "2.0.0"
              },
              {
                "fixed": "2.8.4"
              }
            ],
            "source": "AFFECTED_FIELD",
            "strategy": "StandardRange"
          }
        ]
      },
      {
        "extracted_events": [
          {
            "range": [
              {
                "introduced": "2.0.0"
              },
              {
                "fixed": "2.8.4"
              }
            ],
            "source": "CPE_FIELD",
            "strategy": "CPEApplicability"
          }
        ]
      }
    ]
  },
  "details": "league/commonmark versions \u003e= 2.0.0 and \u003c 2.8.4 (patched in 2.9.0) contain a denial of service vulnerability in UniqueSlugNormalizer::normalize(), which restarts its numeric-suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that collapse to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can force many headings onto a single base slug (e.g., via empty ATX headings, identical heading text, or punctuation-only headings) in a small Markdown document, consuming excessive CPU and denying service.",
  "id": "CVE-2026-86434",
  "modified": "2026-10-21T02:30:57.775173437Z",
  "published": "2026-09-07T12:53:53.194Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86434.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/thephpleague/commonmark/security/advisories/GHSA-mh25-x5hq-wrqp"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-86434"
    },
    {
      "type": "ADVISORY",
      "url": "https://www.vulncheck.com/advisories/commonmark-2.0.0-through-2.8.3-denial-of-service-via-slug-collision"
    }
  ],
  "schema_version": "1.9.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision"
}