CVE-2026-44307

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.get_template(), allowing reads of files outside the configured template directory.

Details

The root cause is a mismatch between posixpath (used for URI normalization in get_template()) 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. get_template() 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("..") check passes.
3. os.path.isfile() on Windows interprets \ as a path separator, resolving the .. traversal and finding files outside the template directory.

Affected code

• mako/lookup.py: TemplateLookup.get_template() uses posixpath.normpath/posixpath.join for path construction but os.path.isfile() for existence check
• mako/template.py: Template.__init__() URI validation uses os.path.normpath() which on Windows resolves backslash traversal to a form that passes the startswith("..") guard

If an application passes user-controlled template names or include paths to TemplateLookup.get_template(), an attacker on Windows may be able to load and disclose readable files outside the configured template directory. The primary impact is local file disclosure. If the targeted file contains Mako/Python template syntax, it may also be parsed and executed as a template.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

Kodem intelligence

The fix should normalize backslashes to forward slashes early in the URI processing pipeline, before any path operations, to ensure consistent behavior across platforms.

1. What is CVE-2026-44307? CVE-2026-44307 is a high-severity path traversal vulnerability in Mako (pip), affecting versions <= 1.3.11. It is fixed in 1.3.12. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. Which versions of Mako are affected by CVE-2026-44307? Mako (pip) versions <= 1.3.11 is affected.
3. Is there a fix for CVE-2026-44307? Yes. CVE-2026-44307 is fixed in 1.3.12. Upgrade to this version or later.
4. Is CVE-2026-44307 exploitable, and should I be worried? Whether CVE-2026-44307 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-44307 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-44307? Upgrade Mako to 1.3.12 or later.