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Mistune CVE-2026-59925

| EUVDEUVD-2026-42335 HIGH
Inefficient Algorithmic Complexity (CWE-407)
2026-07-08 GitHub_M GHSA-4j32-57v6-6g45 PYSEC-2026-2213
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

Unauthenticated remote input to a default parser (AV:N/AC:L/PR:N/UI:N) with a trivially crafted payload; impact is CPU-exhaustion DoS only, so C:N/I:N/A:H.

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 - 16:51 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, long sequences of well-formed double-asterisk or triple-asterisk emphasis pairs around a character cause quadratic work in src/mistune/inline_parser.py because the parser scans forward for matching close markers from every potential opening run, allowing denial of service in default Mistune parsing. 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 widely used pure-Python Markdown parser, affects all versions prior to 3.3.0 where the default inline parser exhibits quadratic (O(n²)) behavior on long runs of well-formed double- or triple-asterisk emphasis pairs. Because the parser rescans forward for matching close markers from every candidate opening run, a small crafted Markdown payload forces disproportionate CPU consumption, exhausting resources on any service that renders untrusted Markdown. There is no public exploit identified at time of analysis and it is not listed in CISA KEV, but the low attack complexity and unauthenticated network vector make availability impact realistic against default configurations.

Technical ContextAI

Mistune (project id lepture/mistune, per CPE cpe:2.3:a:lepture:mistune) is a fast, extensible Markdown-to-HTML library commonly embedded in Python web apps, documentation tooling, static site generators, and comment/wiki systems. The root cause is CWE-407 (Inefficient Algorithmic Complexity / algorithmic denial of service): the emphasis-handling logic in src/mistune/inline_parser.py treats every potential opening emphasis run as a starting point and scans forward looking for a matching closing marker. When input contains long sequences of nested or repeated /* emphasis pairs around a character, the number of scan operations grows quadratically with input length, so linearly larger input yields super-linear parsing time. This is the classic algorithmic-complexity DoS pattern (analogous to catastrophic backtracking) rather than a memory-corruption or injection flaw - hence the availability-only impact.

RemediationAI

Vendor-released patch: upgrade to Mistune 3.3.0 (https://github.com/lepture/mistune/releases/tag/v3.3.0), which rewrites the emphasis-scanning logic per commit 5de41fb8e527004dbc363e047a3c380c9288c74f; review the advisory at https://github.com/lepture/mistune/security/advisories/GHSA-4j32-57v6-6g45 for details. Update pinned dependencies (requirements.txt/poetry/pipenv) and rebuild any vendored copies. If immediate upgrade is not possible, apply compensating controls at the ingestion boundary: cap the maximum size/length of Markdown input accepted from untrusted sources (long asterisk runs require sizable input to be effective), and enforce a wall-clock timeout or run rendering in a separate worker/subprocess with a CPU limit so a single request cannot pin a thread - the trade-off is that legitimate very large documents may be rejected or truncated. Rate-limiting Markdown-rendering endpoints and placing them behind authentication reduces the blast radius but does not remove the flaw. These mitigations are stopgaps; upgrading to 3.3.0 is 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-2026-59925 vulnerability details – vuln.today
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