GHSA-mh25-x5hq-wrqp league/commonmark: Denial of service via colliding heading slugs

• Packagist league/commonmark: >= 2.0.0, < 2.9.0 → fixed in 2.9.0

Impact `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. The 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. Workarounds Integrators who cannot upgrade immediately can: • 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). • Disable `HeadingPermalinkExtension` (and `TableOfContentsExtension`, which depends on it), and `FootnoteExtension` where anonymous footnotes reach the same normalizer, for untrusted Markdown. • Cap the accepted document size / heading count upstream so K cannot reach the quadratic danger zone. Each 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.