Mistune plugins/formatting: quadratic-time parsing on long runs of ~~x~~, ==x==, and ^^x^^ markers (strikethrough / mark / insert)

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-2026-59922: 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 denial of service. A run of N closed pairs ~~x~~~~x~~... (or the analogous ==x== for mark, ^^x^^ for insert) causes O(N²) work in the formatting parser. With the strikethrough, mark, or insert plugin enabled, an 8 KB input pegs the CPU for ~4 seconds; 16 KB → ~17 seconds. File: src/mistune/plugins/formatting.py, lines 13-15 (the STRIKEEND / MARKEND / INSERTEND patterns and their per-position scan). Root cause: for each opening ~~/==/^^ the parser scans forward for the matching close pattern. The scan itself uses a bounded regex, but the parser tries the close-scan at every potential start position. For input shaped like ~~x~~ repeated N times, every ~~ is examined as a possible start, each scan covers up to the end of input. Total work is O(N²). Default config without these plugins handles the same input in linear time (4 ms for 4000 reps), confirming the cost is in the formatting plugin's per-marker scan, not in core parsing. Affected Code File:…

Published by the source

20 Jul 2026, 21:34 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-c8j7-8cv4-2xmq

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-c8j7-8cv4-2xmq

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# Patch Next advisory brief: CVE-2026-59922

Advisory: Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)
IDs: CVE-2026-59922, GHSA-c8j7-8cv4-2xmq
Record: https://patchnext.com/advisories/cve-2026-59922 (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-c8j7-8cv4-2xmq (OSV record GHSA-c8j7-8cv4-2xmq lists affected ranges: PyPI mistune >=0 <3.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-c8j7-8cv4-2xmq (GitHub-reviewed advisory GHSA-c8j7-8cv4-2xmq lists affected ranges: PyPI mistune >=0 <3.3.0)

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