CVE-2026-49851
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedMistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. When parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior. An attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload. This vulnerability is fixed in 3.3.0.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceMistune Markdown parser prior to version 3.3.0 contains a CPU exhaustion vulnerability in parse_link_text caused by superlinear O(n²) complexity. When parsing Markdown with many consecutive [ characters, the function repeatedly scans the input using regex search inside a loop, re-scanning large portions of the remaining string on each iteration, enabling attackers to cause excessive CPU usage with small payloads.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify if Mistune is installedRun 'pip show mistune' or check your project's requirements.txt/poetry.lock for the mistune packageAffected if Mistune package is present in the environment
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Determine installed Mistune versionRun 'pip show mistune' and look for the 'Version' field, or import mistune and print(mistune.__version__)Affected if The installed version is lower than 3.3.0 (e.g., 3.2.x, 3.1.x, 3.0.x, 2.x.x series)
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Check for markdown processing of untrusted inputSearch codebases for calls to mistune markdown functions such as mistune.markdown(), mistune.html(), or MD() with user-supplied inputAffected if The application passes untrusted or externally-sourced Markdown text to Mistune's parsing functions
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Verify parse_link_text is reachableConfirm that parsed Markdown contains or could contain link syntax with consecutive [ characters (e.g., [[text or multiple adjacent brackets)Affected if The Markdown content being parsed includes sequences of '[' characters that would trigger the vulnerable code path in parse_link_text
If Mistune version is below 3.3.0 AND the application processes untrusted Markdown with link syntax containing consecutive brackets, the environment is affected by this CPU exhaustion vulnerability.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scopedUpgrade Mistune to version 3.3.0 or later to remediate this vulnerability.
3.3.0
- Check your current mistune version by running `pip show mistune` or inspecting your requirements.txt/pipfile
- Upgrade mistune to version 3.3.0 or later using `pip install --upgrade mistune>=3.3.0` or `pip install mistune==3.3.0`
- Verify the upgrade was successful by running `pip show mistune` and confirming the version is 3.3.0 or higher
- If using a dependency lock file (e.g., requirements-lock.txt, Pipfile.lock), regenerate it to capture the new version
- Test that your markdown parsing functionality works correctly with the upgraded version
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation1.0 h
- Implementation1.0 h
- Testing2.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-49851 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-49851 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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