What’s Vulnerable › Vulnerabilities

CVE-2026-86434: commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-07 · updated 2026-10-08
• Weakness: CWE-407

Description

league/commonmark versions >= 2.0.0 and < 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.

Affected products

• thephpleague commonmark: from 2.0.0 before 2.8.4 — fixed in 2.8.4

How to fix

Upgrade to a fixed version: thephpleague commonmark 2.8.4. Check the vendor advisory for mitigations.

References

• https://github.com/thephpleague/commonmark/security/advisories/GHSA-mh25-x5hq-wrqp
• https://www.vulncheck.com/advisories/commonmark-2.0.0-through-2.8.3-denial-of-service-via-slug-collision

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