CVE-2026-86432: commonmark 2.0.0 before 2.8.4 Denial of Service via XML

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% 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-405

Description

commonmark versions from 2.0.0 before 2.8.4 contain a denial of service vulnerability in XmlRenderer that emits depth-proportional indentation for every XML tag. Attackers can provide deeply nested Markdown or AST structures to cause quadratic memory consumption and output amplification, exhausting server resources.

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-mj63-m3rc-8ppr
https://www.vulncheck.com/advisories/commonmark-2.0.0-before-2.8.4-denial-of-service-via-xml

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