EUVD-2026-42338Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Lifecycle Timeline
2DescriptionCVE.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
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. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | The prerequisite is concrete: the target application must feed attacker-controlled Markdown into a Mistune parser instance older than 3.3.0, and the malicious document must contain a large number of reference-link definitions (the '[label]: url' construct) - repeated or distinct - to trigger the quadratic path in block_parser.py and ref_links handling. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, base 7.5) is internally consistent and credible: network-reachable, no privileges, no user interaction, availability-only impact - a classic algorithmic-complexity DoS. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker submits a single Markdown payload - for example a comment, wiki edit, chat message, or API document - that contains thousands of reference-link definitions to a service that renders it with a pre-3.3.0 Mistune. Because parsing scales quadratically, the request consumes disproportionate CPU and stalls the worker; repeating it across connections exhausts the server pool and denies service to legitimate users. … |
| Remediation | 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. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours: Inventory all applications and services using Mistune and document current versions. …
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External POC / Exploit Code
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