Vulnerability · Published Jul 8, 2026 · Assigned by GitHub_M

CVE-2026-59928Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

High 7.5

In plain English

A high-severity regular-expression denial of service in mistune from lepture. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.7% (higher than 51% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.7%top 48.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:13	mistune	no fix listed	DEBIAN-CVE-2026-59928
Debian:14	mistune	no fix listed	DEBIAN-CVE-2026-59928
PyPI	mistune	3.3.0	GHSA-ffq3-xpv3-j92q
PyPI	mistune	3.3.0	PYSEC-2026-2216
Ubuntu:22.04:LTS	mistune	no fix listed	UBUNTU-CVE-2026-59928
Ubuntu:24.04:LTS	mistune	no fix listed	UBUNTU-CVE-2026-59928
Ubuntu:25.10	mistune	no fix listed	UBUNTU-CVE-2026-59928
Ubuntu:26.04:LTS	mistune	no fix listed	UBUNTU-CVE-2026-59928
Ubuntu:26.10	mistune	no fix listed	UBUNTU-CVE-2026-59928

Affected products (as reported)

Vendor	Product	Versions
lepture	mistune	< 3.3.0

Weakness: CWE-407, CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

https://github.com/lepture/mistune/security/advisories/GHSA-ffq3-xpv3-j92q x_refsource_CONFIRM
https://github.com/lepture/mistune/commit/2b04d7ba341c16ac78fe82d3076bdd5c3de87c69 x_refsource_MISC
https://github.com/lepture/mistune/releases/tag/v3.3.0 x_refsource_MISC