Vulnerability in Thephpleague Commonmark
CVE-2026-86429
The league/commonmark (thephpleague/commonmark) library in versions >= 1.5.0 and < 2.9.1 contains quadratic parsing complexity in its SmartPunctExtension and AttributesExtension. When either extension is explicitly registered on the Environment (they are not enabled by default and are excluded from the standard CommonMark and GitHub-Flavored Markdown converters), an unauthenticated attacker can submit small, specially crafted Markdown documents — such as text alternating with unpaired quotes, contiguous runs of block-level attribute blocks, or repeated class attributes — to trigger disproportionate CPU consumption and cause a denial of service. Fixed in 2.9.1.
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 (43.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 43.0% 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-jjv6-8j6v-6j52 (vendor-advisory, Mitigation, Vendor Advisory)
- www.vulncheck.com/advisories/commonmark-before-2.9.1-denial-of-service-via-smar… (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-86429?
- CVE-2026-86429 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-2026-86429?
- High severity. CVSS v3 base score is 7.5 out of 10.