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.
References
GrahamCampbell Finder
Impact
UniqueSlugNormalizer::normalize()makes each slug document-unique by searching for an unused numeric suffix, but restarts that search from1on 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
HeadingPermalinkExtensionis registered (itsHeadingPermalinkProcessornormalizes every heading), independently throughFootnoteExtension(itsAnonymousFootnoteRefParsernormalizes every^[label]reference), and on anyTableOfContentsExtensionsite (which requiresHeadingPermalinkExtensionto be co-registered). The defaultslug_normalizer/uniquesetting (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.UniqueSlugNormalizerwas 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:
slug_normalizer/uniquetofalse/UniqueSlugNormalizerInterface::DISABLED, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor).HeadingPermalinkExtension(andTableOfContentsExtension, which depends on it), andFootnoteExtensionwhere anonymous footnotes reach the same normalizer, for untrusted Markdown.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.
References