GHSA-mh25-x5hq-wrqp: league/commonmark Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mh25-x5hq-wrqp",
  "modified" : "2026-08-06T20:41:45Z",
  "published" : "2026-08-06T20:41:45Z",
  "aliases" : [ ],
  "summary" : "league/commonmark: Denial of service via colliding heading slugs",
  "details" : "### Impact\n\n`UniqueSlugNormalizer::normalize()` makes each slug document-unique by searching for an unused numeric suffix, but **restarts that search from `1` on every collision**. The k-th heading that collapses to the same base slug performs k−1 array lookups, so K colliding slugs cost Σ(k−1) = **O(K²)**. An attacker can force every heading onto a single base slug trivially — many empty ATX headings, identical heading text, or punctuation-only headings that normalize to the empty string.\n\nThe path is reached whenever the shared slug normalizer runs over attacker-controlled text. That happens when `HeadingPermalinkExtension` is registered (its `HeadingPermalinkProcessor` normalizes every heading), independently through `FootnoteExtension` (its `AnonymousFootnoteRefParser` normalizes every `^[label]` reference), and on any `TableOfContentsExtension` site (which requires `HeadingPermalinkExtension` to be co-registered). The default `slug_normalizer/unique` setting (`UniqueSlugNormalizerInterface::PER_DOCUMENT`) accumulates collisions across the whole document. No authentication is required — a small document body turns into seconds of CPU and denies service. Availability impact only. **`UniqueSlugNormalizer` was introduced in 2.0.0 (first shipped in 2.0.0-beta1, May 2021); the 1.x heading-permalink slug generator performed no de-duplication and is not affected. All 2.x releases (including 2.8.x) are affected.**\n\n### Workarounds\n\nIntegrators who cannot upgrade immediately can:\n\n- **Set `slug_normalizer/unique` to `false` / `UniqueSlugNormalizerInterface::DISABLED`**, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor).\n- **Disable `HeadingPermalinkExtension`** (and `TableOfContentsExtension`, which depends on it), and `FootnoteExtension` where anonymous footnotes reach the same normalizer, for untrusted Markdown.\n- **Cap the accepted document size / heading count upstream** so K cannot reach the quadratic danger zone.\n\nEach of these trades off functionality or correctness; upgrading to the patched release (which removes the quadratic behavior while keeping unique ids and identical output) is the recommended remediation.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "league/commonmark"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "2.9.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/thephpleague/commonmark/security/advisories/GHSA-mh25-x5hq-wrqp"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/thephpleague/commonmark"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/thephpleague/commonmark/releases/tag/2.9.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-06T20:41:45Z",
    "nvd_published_at" : null
  }
}