CVE-2026-86434: high-severity vulnerability in thephpleague commonmark
commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision
Published · Updated
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21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 8.7epss 0.5%
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.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Affected products
thephpleague · commonmarkRelated CVEs — thephpleague commonmark
In the same product, most dangerous first.
CVE-2026-71488HIGHleague/commonmark: Quadratic-time denial of service when parsing crafted MarkdownEPSS 0.6%CVE-2026-86429HIGHcommonmark before 2.9.1 Denial of Service via SmartPunct and AttributesEPSS 0.5%CVE-2026-86433HIGHcommonmark 1.5.0 before 2.8.4 Denial of Service via AttributesEPSS 0.5%CVE-2026-86430HIGHleague/commonmark before 2.9.1 Denial of Service via parsingEPSS 0.5%CVE-2026-86428HIGHcommonmark 1.5.0 before 2.10.0 Denial of Service via AttributesEPSS 0.5%CVE-2024-58382HIGHleague/commonmark before 2.6.0 Denial of Service via Quadratic ComplexityEPSS 0.5%