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Mistune CVE-2026-59922

| EUVDEUVD-2026-42329 HIGH
Inefficient Algorithmic Complexity (CWE-407)
2026-07-08 GitHub_M GHSA-c8j7-8cv4-2xmq PYSEC-2026-2210
Python Denial Of Service Mistune Suse Red Hat
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 typically reachable remotely without auth (AV:N/PR:N/UI:N) and the payload is trivial to craft (AC:L); availability-only impact yields A:H, C:N, I:N.

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 - 16:52 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 run of closed tilde, equals-sign, or caret marker pairs around a character causes quadratic work in src/mistune/plugins/formatting.py when the strikethrough, mark, or insert plugin scans for matching markers from each possible start position, 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, a Python Markdown parser, affects all versions prior to 3.3.0 when the strikethrough, mark, or insert plugins are enabled. Remote attackers can submit crafted Markdown containing runs of closed tilde (~), equals (=), or caret (^) marker pairs that trigger quadratic-time scanning in the formatting plugin, exhausting CPU and rendering the parsing service unresponsive. No public exploit identified at time of analysis, and the flaw is not in CISA KEV; impact is limited to availability with no data exposure.

Technical ContextAI

Mistune (PyPI package 'mistune', maintained by lepture) is a pure-Python Markdown-to-HTML/AST parser widely embedded in web apps, documentation tooling, and notebook renderers. The root cause is CWE-407 (Inefficient Algorithmic Complexity): in src/mistune/plugins/formatting.py, the strikethrough (~~), mark (==), and insert (^^) plugins scan for a matching closing marker from each possible start position. When the input contains a dense run of closed marker pairs around a character, this per-start-position scan produces O(n^2) work relative to input length. The affected component is the inline formatting plugin layer, so exposure depends on which plugins the embedding application registers rather than on Mistune's core Markdown grammar. CPE cpe:2.3:a:lepture:mistune:*:*:*:* confirms the vendor/product with all versions below the fix in scope.

RemediationAI

Vendor-released patch: upgrade Mistune to version 3.3.0 or later, which fixes the quadratic scanning in src/mistune/plugins/formatting.py (fix commit 96d0f57f8fe9eeb06bb4cff521962a27d7c402e7, release https://github.com/lepture/mistune/releases/tag/v3.3.0). Review the advisory at https://github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmq. If immediate upgrade is not possible, the most direct compensating control is to disable the strikethrough, mark, and insert plugins in the Mistune configuration for any pipeline that processes untrusted input - the trade-off is loss of ~~, ==, and ^^ rendering for legitimate users. Additional mitigations: enforce a strict maximum input length on Markdown submitted to the parser (caps the n^2 blow-up but may truncate legitimate long documents), and run parsing under a per-request CPU/time budget or worker timeout so a single malicious payload cannot monopolize a core (adds latency-handling complexity but preserves service availability).

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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-2026-59922 vulnerability details – vuln.today
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