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Mistune CVE-2026-59927

| EUVDEUVD-2026-42339 MEDIUM
Uncontrolled Recursion (CWE-674)
2026-07-08 GitHub_M GHSA-8mpj-m6qm-5qr8 PYSEC-2026-2215
Denial Of Service Python Red Hat Suse Mistune Lepture
5.3
CVSS 3.1 · Vendor: GitHub_M
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Severity by source

Vendor (GitHub_M) PRIMARY
5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
vuln.today AI
3.1 LOW

Triggering the cycle requires writing two mutually-referencing files to the server, implying PR:L and AC:H rather than vendor's PR:N/AC:L.

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

Lifecycle Timeline

2
Patch available
Jul 08, 2026 - 18:02 EUVD
Analysis Generated
Jul 08, 2026 - 17:34 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, the Include directive in src/mistune/directives/include.py detects only direct self-includes and not indirect cycles, allowing two markdown files that include each other to trigger unbounded recursion, raise RecursionError, and crash the rendering request. This issue is fixed in version 3.3.0.

AnalysisAI

Unbounded recursion in Mistune's Include directive (all versions prior to 3.3.0) can be triggered by a pair of mutually-referencing markdown files, bypassing the cycle detector that only checks for direct self-inclusion. An attacker who can supply markdown content processed by an application using the Include directive may crash the rendering request with a Python RecursionError, resulting in a denial of service. No public exploit has been identified at time of analysis and the vulnerability is not listed in CISA KEV; a confirmed fix is available in version 3.3.0.

Technical ContextAI

Mistune (CPE: cpe:2.3:a:lepture:mistune:*:*:*:*:*:*:*:*) is a Python Markdown parser that supports an optional Include directive implemented in src/mistune/directives/include.py, which reads and inlines external markdown files at render time. The root cause is CWE-674 (Uncontrolled Recursion): the guard logic only detects a file directly naming itself as an include target and does not maintain a call-stack or visited-file set across the full inclusion chain. When file A includes file B and file B includes file A, the directive handler recurses without bound until Python's default recursion limit is hit, raising an uncaught RecursionError that aborts the rendering request. The flaw is entirely within the parser library and is independent of the underlying markdown content format.

RemediationAI

Upgrade Mistune to version 3.3.0, which introduces indirect cycle detection for the Include directive. The fix is confirmed in the official release at https://github.com/lepture/mistune/releases/tag/v3.3.0 and the underlying code change is visible at https://github.com/lepture/mistune/commit/1bef343ade163fc3bb95572b15be720084cdb993. If an immediate upgrade is not possible, disable the Include directive feature in the application's Mistune configuration to eliminate the vulnerable code path entirely - this is the most effective workaround with no security trade-off, though it removes include functionality. Alternatively, restrict the markdown rendering endpoint to authenticated or trusted users to reduce the attacker-controlled input surface; this reduces risk but does not eliminate the flaw for insider or compromised-account scenarios. Content validation to block or sanitize include directives before they reach Mistune is also a viable compensating control but requires careful implementation to avoid bypass.

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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-2026-59927 vulnerability details – vuln.today
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