Mistune inline_parser: quadratic-time parsing on long runs of **x** and ***x*** emphasis pairs

Exploitation

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

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-59925: 0.64% estimated exploitation activity in the next 30 days; percentile 50.92 (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 Type: Algorithmic-complexity DoS in core emphasis parsing. A long sequence of well-formed x (strong) or x (strong-emphasis combined) pairs causes O(N²) parser work. Distinct from the bracket-bomb DoS ([ repetition) and from the formatting-plugin DoS (~~/==/^^); this one fires on default-config mistune with no plugins required. File: src/mistune/inlineparser.py lines 41-48 (the EMPHASISENDRE family) and the surrounding emphasis dispatch. Root cause: for every opening run of s the parser scans forward using one of EMPHASISENDRE[''] / [''] / [''] to find the matching close. Each scan is bounded per call, but the parser invokes the scan from every potential start position. For input shaped x repeated N times, every is treated as a potential start, each scan can cover up to the end of input. Total work is O(N²). The triple-emphasis variant x is slightly worse due to the extra alternation between , , and close patterns. Reproducible against default mistune with no plugins. Affected…

Published by the source

20 Jul 2026, 21:32 UTC

Last modified by the source

20 Jul 2026, 21:46 UTC

Source last checked

26 Oct 2026, 13:49 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4j32-57v6-6g45

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4j32-57v6-6g45

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# Patch Next advisory brief: CVE-2026-59925

Advisory: Mistune inline_parser: quadratic-time parsing on long runs of `**x**` and `***x***` emphasis pairs
IDs: CVE-2026-59925, GHSA-4j32-57v6-6g45
Record: https://patchnext.com/advisories/cve-2026-59925 (revision 2, source checked 26 Oct 2026, 13:49 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
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-4j32-57v6-6g45 (OSV record GHSA-4j32-57v6-6g45 lists affected ranges: PyPI mistune >=0 <3.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-4j32-57v6-6g45 (GitHub-reviewed advisory GHSA-4j32-57v6-6g45 lists affected ranges: PyPI mistune >=0 <3.3.0)

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