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Vulnerabilities

Security fix · CVE-2026-86434

league/commonmark: Denial of service via colliding heading slugs

Published Aug 6, 2026 · catalog updated Oct 20, 2026 · league/commonmark: Denial of service

Check if you run it

No confirmed exploitation is on record for CVE-2026-86434. EPSS is 0.51%. Act only if a listed version of league/commonmark: Denial of service is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (league/commonmark: Denial of service).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
High7.5
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.51%
Remediation
Advisory link
Affected products
Packagist league/commonmark

### Impact makes each slug document-unique by searching for an unused numeric suffix, but **restarts that search from 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 is registered (its normalizes every heading), independently through (its normalizes every reference), and on any site (which requires to be co-registered). The default setting ( ) 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. ** 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 to / **, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor). - **Disable ** (and , which depends on it), and 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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Packagist league/commonmark2.0.0 → 2.9.0

How to fix CVE-2026-86434

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-86434. EPSS is 0.51%. Act only if a listed version of league/commonmark: Denial of service is installed.

Which versions are affected?

Compare what you have installed with 2.0.0 → 2.9.0. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-407. EPSS 0.51%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Aug 6, 2026 · GitHub Advisory Database

    Detected

    ### Impact makes each slug document-unique by searching for an unused numeric suffix, but **restarts that search from 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 punctuat

  2. Oct 20, 2026 · GitHub Advisory Database

    Fix available

    league/commonmark fixed in 2.9.0

  3. Oct 20, 2026 · GitHub Advisory Database

    Fix available

    league/commonmark fixed in 2.9.0

  4. Oct 20, 2026 · GitHub Advisory Database

    Updated

    ### Impact makes each slug document-unique by searching for an unused numeric suffix, but **restarts that search from 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 punctuat

Sources

References

Freshness

Last verified in this catalog Oct 23, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.