Vulnerability in Thephpleague Commonmark
CVE-2026-86430
league/commonmark versions before 2.9.1 contain multiple denial of service vulnerabilities in fenced code block detection, reference link label lookup, and emphasis delimiter processing that perform super-linear work on crafted input. Attackers can submit specially crafted Markdown with long backtick runs, nested brackets, or delimiter sequences to consume disproportionate CPU time and prevent legitimate requests from completing.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.005 (41.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 41st percentile — 41.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.005 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/thephpleague/commonmark/security/advisories/GHSA-j8pm-gj4c-rq4x (vendor-advisory, Patch, Mitigation, Vendor Advisory)
- www.vulncheck.com/advisories/league-commonmark-before-2.9.1-denial-of-service-v… (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-86430?
- CVE-2026-86430 is a high-severity vulnerability in Thephpleague Commonmark, classified under Inefficient Algorithmic Complexity. CVSS score: 7.5/10. Published 2026-09-07.
- How severe is CVE-2026-86430?
- High severity. CVSS v3 base score is 7.5 out of 10.