Mako: Path traversal via drive-letter URI on Windows in TemplateLookup

Exploitation

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

CVSS

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

EPSS

• CVE-2026-102991: 0.36% estimated exploitation activity in the next 30 days; percentile 29.39 (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 On Windows, a TemplateLookup URI beginning with a drive designator (e.g. C:/../../secret.txt or C:\..\..\secret.txt) bypasses the directory traversal check in Template.init, allowing reads of files outside the configured template directory. This is a third, independent instance of the root cause behind CVE-2026-41205 (the // prefix) and CVE-2026-44307 (the backslash form). Both of those fixes normalized a separator spelling. Neither addressed the other way posixpath and ntpath disagree: the drive designator. Both prior fixes remain effective on their own terms; this variant survives them for an independent reason. Details The root cause is the same posixpath / os.path mismatch: resolution is done with posixpath, confinement is checked with os.path, which is ntpath on Windows. posixpath has no concept of a drive, so it treats C: as an ordinary path component; the .. segments pop C: and then escape the template root. ntpath, by contrast, splits the drive off and treats the…

Published by the source

5 Oct 2026, 23:42 UTC

Last modified by the source

15 Oct 2026, 14:55 UTC

Source last checked

26 Oct 2026, 13:55 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-5639-2j2p-m4mx

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-5639-2j2p-m4mx

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-5639-2j2p-m4mx

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# Patch Next advisory brief: CVE-2026-102991

Advisory: Mako: Path traversal via drive-letter URI on Windows in TemplateLookup
IDs: CVE-2026-102991, GHSA-5639-2j2p-m4mx
Record: https://patchnext.com/advisories/cve-2026-102991 (revision 3, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mako 1.4.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mako: affected <= 1.4.1; fixed in 1.4.2

## Suggested inspection (read-only)
- `python -m pip show mako`: 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 mako.
- 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-5639-2j2p-m4mx (GitHub-reviewed advisory GHSA-5639-2j2p-m4mx lists affected ranges: PyPI mako >=0 <=1.4.1)
- Verified: https://osv.dev/vulnerability/GHSA-5639-2j2p-m4mx (OSV record GHSA-5639-2j2p-m4mx lists affected ranges: PyPI mako >=0 <1.4.2 (fixed))

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