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.

CVSS 4.0

8.7CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS 3.1

7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

0.5% chance of exploitation in the next 30 days; higher than 42% of CVEs (scores of 2026-10-25)

CWE

CWE-407

Published

2026-09-07

Modified

2026-10-08

Record from

the NVD, synced 2026-10-19

Affected

• thephpleague:commonmark

References

• https://github.com/thephpleague/commonmark/security/advisories/GHSA-j8pm-gj4c-rq4x
• https://www.vulncheck.com/advisories/league-commonmark-before-2.9.1-denial-of-service-via-parsing

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