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Mistune CVE-2026-59928

| EUVDEUVD-2026-42338 HIGH
Inefficient Algorithmic Complexity (CWE-407)
2026-07-08 GitHub_M GHSA-ffq3-xpv3-j92q PYSEC-2026-2216
Python Denial Of Service Mistune Red Hat Suse
7.5
CVSS 3.1 · Vendor: GitHub_M
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Severity by source

Vendor (GitHub_M) PRIMARY
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
vuln.today AI
7.5 HIGH

Untrusted Markdown is parsed remotely with no auth or interaction (AV:N/AC:L/PR:N/UI:N); algorithmic DoS impacts only availability (A:H), with no confidentiality or integrity effect and no scope change.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Red Hat
6.5 MEDIUM
qualitative

Primary rating from Vendor (GitHub_M).

CVSS VectorVendor: GitHub_M

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

2
Patch available
Jul 08, 2026 - 18:02 EUVD
Analysis Generated
Jul 08, 2026 - 17:23 vuln.today

Blast Radius

ecosystem impact
† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth
  • 1 pypi packages depend on mistune (1 direct, 0 indirect)

Ecosystem-wide dependent count for version 3.3.0.

DescriptionCVE.org

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.

Articles & Coverage 1

News 10 New Python Vulnerabilities - 2 Critical, 8 High, 1 With POC Jul 09, 2026

AnalysisAI

Denial of service in Mistune (the lepture Python Markdown parser) before 3.3.0 lets remote unauthenticated attackers exhaust CPU by submitting a Markdown document packed with many repeated or distinct reference-link definitions, which triggers quadratic-time processing in the block parser and its ref_links dictionary handling. Any service that renders attacker-supplied Markdown through Mistune (wikis, comment systems, docs pipelines, notebook converters) can be stalled with a single crafted input. No public exploit is identified at time of analysis and it is not in CISA KEV, but the low-complexity, no-auth vector makes weaponization trivial.

Technical ContextAI

Mistune is a pure-Python Markdown parser exposing renderers and plugins; it is embedded in many web apps and documentation toolchains to convert user Markdown into HTML. The flaw sits in src/mistune/block_parser.py and the ref_links environment dictionary that stores link reference definitions (the '[label]: url' syntax). CWE-407 (Algorithmic Complexity / Inefficient Algorithmic Complexity) is the root-cause class: the parser's handling of reference-link definitions scales quadratically (O(n^2)) with the number of definitions, so input size grows work super-linearly. An attacker does not need a memory bomb or code execution - they simply exploit the parsing algorithm's poor worst-case complexity to burn CPU cycles. The affected package is identified by CPE cpe:2.3:a:lepture:mistune across all versions prior to the fix.

RemediationAI

Vendor-released patch: upgrade Mistune to 3.3.0 or later (pip install --upgrade 'mistune>=3.3.0' and repin the dependency in requirements/lockfiles), per advisory GHSA-ffq3-xpv3-j92q and release https://github.com/lepture/mistune/releases/tag/v3.3.0; the corrective change is commit 2b04d7ba341c16ac78fe82d3076bdd5c3de87c69. Where an immediate upgrade is not possible, apply compensating controls at the point untrusted Markdown enters the parser: cap the maximum size/length of Markdown input accepted from users (trades off legitimate long documents), run parsing under a strict per-request CPU/wall-clock timeout or in a bounded worker/subprocess so a single request cannot pin a core, and rate-limit or queue Markdown-rendering endpoints to blunt repeated abuse. These controls reduce but do not eliminate the quadratic blowup, so upgrading remains the durable fix.

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SUSE Linux Enterprise Server 16.0 SUSE Linux Enterprise Server for SAP applications 16.0 Fixed
openSUSE Leap 16.0 Fixed
SUSE Linux Enterprise Desktop 15 SP7 Not-Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Not-Affected
SUSE Linux Enterprise Module for Package Hub 15 SP7 Not-Affected

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CVE-2026-59928 vulnerability details – vuln.today
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