Mako vulnerable to path traversal via backslash URI on Windows in TemplateLookup

Exploitation

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

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-44307: 0.74% estimated exploitation activity in the next 30 days; percentile 54.71 (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 URI using backslash traversal (e.g. \..\..\ secret.txt) bypasses the directory traversal check in Template.init and the posixpath-based normalization in TemplateLookup.gettemplate(), allowing reads of files outside the configured template directory. Details The root cause is a mismatch between posixpath (used for URI normalization in gettemplate()) and os.path (used for file access via os.path.isfile() and validation via os.path.normpath() in Template.init). On Windows, os.path is ntpath, which treats \ as a path separator, while posixpath treats it as a literal character. The vulnerability chain: 1. gettemplate() strips only leading / via re.sub(r"^\/+", "", uri) and normalizes with posixpath — backslash \ is treated as a literal character, so \..\ secret.txt passes through with .. undetected. 2. Template.init() validation uses os.path.normpath() — on Windows this resolves \..\ secret.txt to \secret.txt, which does not start with .., so the startswith("..")…

Published by the source

6 May 2026, 21:45 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:41 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-2h4p-vjrc-8xpq

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-2h4p-vjrc-8xpq

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# Patch Next advisory brief: CVE-2026-44307

Advisory: Mako vulnerable to path traversal via backslash URI on Windows in TemplateLookup
IDs: CVE-2026-44307, GHSA-2h4p-vjrc-8xpq
Record: https://patchnext.com/advisories/cve-2026-44307 (revision 3, source checked 26 Oct 2026, 13:41 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mako 1.3.12. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mako: affected <= 1.3.11; fixed in 1.3.12

## 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://osv.dev/vulnerability/GHSA-2h4p-vjrc-8xpq (OSV record GHSA-2h4p-vjrc-8xpq lists affected ranges: PyPI mako >=0 <1.3.12 (fixed))
- Verified: https://github.com/advisories/GHSA-2h4p-vjrc-8xpq (GitHub-reviewed advisory GHSA-2h4p-vjrc-8xpq lists affected ranges: PyPI mako >=0 <=1.3.11)

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