PyMdown Extensions: Path traversal in the b64 extension lets read files outside base_path

Exploitation

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

CVSS

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

EPSS

• CVE-2026-61632: 0.40% estimated exploitation activity in the next 30 days; percentile 34.61 (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 The b64 extension inlines images referenced by <img src="..." as base64 data URIs. When resolving the src path it joins it onto the configured basepath with os.path.normpath and opens the result directly, with no check that the resolved path stays inside basepath. A src containing ../ sequences, or an absolute path, therefore reads a file outside basepath as long as that file has an allowed image extension (.png, .jpg, .jpeg, .gif, .svg). The base64 of that file is then embedded in the rendered output, disclosing its contents. This is a separate code path from the snippets traversal issues (GHSA-jh85-wwv9-24hv, GHSA-62q4-447f-wv8h). It lives in pymdownx/b64.py and has no path restriction of any kind. Confirmed on 10.21.3 installed from PyPI. Details In pymdownx/b64.py, function replpath (around lines 68 to 90 on main): There is no startswith(basepath), no os.path.realpath comparison, and no rejection of ... Both branches are reachable from an attacker-controlled src. PoC…

Published by the source

24 Jul 2026, 16:18 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:49 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-9xwg-3r6f-jcx2

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-9xwg-3r6f-jcx2

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# Patch Next advisory brief: CVE-2026-61632

Advisory: PyMdown Extensions: Path traversal in the b64 extension lets read files outside base_path
IDs: CVE-2026-61632, GHSA-9xwg-3r6f-jcx2
Record: https://patchnext.com/advisories/cve-2026-61632 (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: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pymdown-extensions 11.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pymdown-extensions: affected <= 10.21.3; fixed in 11.0.0

## 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://osv.dev/vulnerability/GHSA-9xwg-3r6f-jcx2 (OSV record GHSA-9xwg-3r6f-jcx2 lists affected ranges: PyPI pymdown-extensions >=0 <11.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-9xwg-3r6f-jcx2 (GitHub-reviewed advisory GHSA-9xwg-3r6f-jcx2 lists affected ranges: PyPI pymdown-extensions >=0 <=10.21.3)

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