Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

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-2026-59928: 0.65% estimated exploitation activity in the next 30 days; percentile 51.25 (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 Type: Algorithmic-complexity DoS in reference-link definition handling. A markdown document with N reference-link definitions of the same key (or many distinct keys) takes O(N²) parser time. 5000 repeated [a]: u\n definitions take ~1.1 second; 10000 → ~4.5 seconds. File: src/mistune/blockparser.py (reference-link def parsing) and the surrounding reflinks env-dictionary handling. Root cause: every reference definition is parsed by scanning forward from each candidate position. The unikey normalisation runs per-def, the dictionary insert is per-def, and the lookup-by-label-then-iterate-defs path is linear in the number of stored defs. For input with N defs, the total work is O(N²). Affected Code src/mistune/blockparser.py — reference-definition rule fires on every line that matches [label]: url. For each one: - unikey(label) is called (linear scan of the label). - The def is appended to state.env['reflinks']. - Later inline-link resolution looks up by unikey(label) in the dict…

Published by the source

20 Jul 2026, 21:24 UTC

Last modified by the source

20 Jul 2026, 21:30 UTC

Source last checked

26 Oct 2026, 13:49 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-ffq3-xpv3-j92q

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-ffq3-xpv3-j92q

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# Patch Next advisory brief: CVE-2026-59928

Advisory: Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
IDs: CVE-2026-59928, GHSA-ffq3-xpv3-j92q
Record: https://patchnext.com/advisories/cve-2026-59928 (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-ffq3-xpv3-j92q (OSV record GHSA-ffq3-xpv3-j92q lists affected ranges: PyPI mistune >=0 <3.3.0 (fixed))
- Verified: https://github.com/advisories/GHSA-ffq3-xpv3-j92q (GitHub-reviewed advisory GHSA-ffq3-xpv3-j92q lists affected ranges: PyPI mistune >=0 <3.3.0)

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