Mistune: Potential DoS via quadratic-time parsing in parse_link_text

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-49851: 0.52% estimated exploitation activity in the next 30 days; percentile 44.25 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parselinktext. A relatively small input consisting of repeated [ characters causes significant parsing slowdown. Affected component mistune/inlineparser.py → parselinktext Description When parsing Markdown containing many consecutive [ characters, parselinktext 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. Root cause The vulnerability stems from a two-loop interaction: - The outer loop in InlineParser.parse() (inlineparser.py) advances only 1 character at a time when parselink() returns None - Each failed attempt calls parselinktext() which performs an O(n) scan to the end of the string looking for a closing ] - With n consecutive characters, this…

Published by the source

9 Jul 2026, 23:52 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:46 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-qcq2-496w-v96p

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qcq2-496w-v96p

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# Patch Next advisory brief: CVE-2026-49851

Advisory: Mistune: Potential DoS via quadratic-time parsing in parse_link_text
IDs: CVE-2026-49851, GHSA-qcq2-496w-v96p
Record: https://patchnext.com/advisories/cve-2026-49851 (revision 3, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mistune 3.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mistune: affected < 3.3.0; fixed in 3.3.0

## Suggested inspection (read-only)
- `python -m pip show mistune`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mistune.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-qcq2-496w-v96p (OSV record GHSA-qcq2-496w-v96p lists affected ranges: PyPI mistune >=0 <3.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-qcq2-496w-v96p (GitHub-reviewed advisory GHSA-qcq2-496w-v96p lists affected ranges: PyPI mistune >=0 <3.3.0)

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