Mako, a Python template library, prior to version 1.3.12, contains a path traversal vulnerability (CWE-22) on Windows platforms. The issue arises because the directory traversal check in Template.__init__ and the normalization in TemplateLookup.get_template() rely on posixpath-based logic, which does not correctly handle Windows backslash (\) traversal sequences. This allows an attacker to craft a URI with backslash traversal (e.g., \..\..\secret.txt) to read files outside the configured template directory. The vulnerability is addressed in Mako version 1.3.12.

An attacker can read arbitrary files outside the intended template directory on Windows systems using Mako versions prior to 1.3.12. This can lead to unauthorized disclosure of sensitive information. The CVSS 4.0 base score is 8.7 (high severity), indicating network exploitable without privileges or user interaction, with high impact on confidentiality.

Upgrade Mako to version 1.3.12 or later, where this path traversal vulnerability is fixed. Since the vendor advisory states the issue is resolved in 1.3.12, applying this official fix fully mitigates the vulnerability. No additional mitigations are specified or required.