pymdown-extensions: exponential-backtracking ReDoS in caret, tilde, betterem, and magiclink inline processors

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-2026-67422: 0.61% estimated exploitation activity in the next 30 days; percentile 49.13 (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 Four inline processors in pymdown-extensions contain regular expressions with exponential backtracking. A single untrusted Markdown line under 50 bytes drives markdown.markdown() into unbounded CPU on the rendering thread (seconds at ~45 bytes, growing exponentially with each added character). All four fire in the extension's default configuration and are reachable through the documented public API. The caret/tilde/ betterem blow-up was introduced by the emphasis-pattern rewrite in PR #2547 (first released in 10.13, Dec 2024) — earlier releases used a linear (.+?) / ([^\s]+?) content group — and is present through 11.0 (latest); magiclink's host pattern is long-standing and affects effectively all releases. Likely CWE-1333 (Inefficient Regular Expression Complexity). This is a distinct issue from CVE-2025-68142 (ReDoS in pymdownx.blocks.caption, REFIGNUM, fixed in 10.16.1): different extensions, different regexes, and a different root cause (delimiter-run partition ambiguity…

Published by the source

7 Aug 2026, 18:26 UTC

Last modified by the source

7 Aug 2026, 18:26 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-gm37-52c6-37mw

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# Patch Next advisory brief: CVE-2026-67422

Advisory: pymdown-extensions: exponential-backtracking ReDoS in caret, tilde, betterem, and magiclink inline processors
IDs: CVE-2026-67422, GHSA-gm37-52c6-37mw
Record: https://patchnext.com/advisories/cve-2026-67422 (revision 1, source checked 26 Oct 2026, 13:50 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: pymdown-extensions 11.0.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pymdown-extensions: affected <= 11.0.0; fixed in 11.0.1

## Suggested inspection (read-only)
- `python -m pip show pymdown-extensions`: 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 pymdown-extensions.
- 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://github.com/advisories/GHSA-gm37-52c6-37mw (GitHub-reviewed advisory GHSA-gm37-52c6-37mw lists affected ranges: PyPI pymdown-extensions >=0 <=11.0.0)

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