Python CPython up to 3.14.x on Windows Installation Directory Modules/setup.local VPATH uncontrolled search path

Summaryinfo

A vulnerability has been found in Python CPython up to 3.14.x on Windows and classified as problematic. This issue affects some unknown processing of the file Modules/setup.local of the component Installation Directory Handler. This manipulation of the argument VPATH causes uncontrolled search path. This vulnerability appears as CVE-2026-12003. The attack requires local access. There is no available exploit. The affected component should be upgraded.

Detailsinfo

A vulnerability has been found in Python CPython up to 3.14.x on Windows and classified as critical. Affected by this vulnerability is an unknown part of the file Modules/setup.local of the component Installation Directory Handler. The manipulation of the argument VPATH with an unknown input leads to a uncontrolled search path vulnerability. The CWE definition for the vulnerability is CWE-427. The product uses a fixed or controlled search path to find resources, but one or more locations in that path can be under the control of unintended actors. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

To allow builds of Python to be run from an in-tree layout (rather than an installed file layout), the VPATH variable is defined at build time and used to locate certain landmarks - specifically, Modules/setup.local. When this landmark is found relative to VPATH relative to the executable, Python assumes it is running in a source tree and generates a different default sys.path. This code remains in release builds, so that release-ready builds can be built in-tree. On Windows, since builds are written to 'PCbuild/', the value of VPATH is set to '..\..', which results in a landmark of '..\..\Modules\setup.local'. This path is outside the install directory of Python, and may have different permissions, potentially allowing a low-privilege user to create the landmark and an alternative `Lib` folder that will be discovered by an otherwise restricted install. Such a setup occurs with the legacy default install location for all users (in the now superseded EXE installer), due to how Windows allows all users to create folders in the root directory of their OS drive. Our recommended mitigation on Windows is to migrate away from the legacy installer and use the new [Python install manager](https://www.python.org/downloads/latest/pymanager/) to install for the current user. Installs where the directory two levels above the Python installation directory have equivalent permissions are unaffected (in general, a per-user install cannot be modified at all by other users, removing any escalation of privilege risk, and could be directly modified by a privileged user, making the potential tampering irrelevant). Alternative mitigations might include preemptively creating and restricting access to a `Modules` directory. Be aware that only 3.13 and 3.14 will receive updated legacy installers - earlier fixes are only provided as sources. Platforms other than Windows allow VPATH to be overridden, but as they don't usually use a separated directory in the build for binaries, are unlikely to have a landmark reference outside of the install directory. The landmark detection involving VPATH is a fallback for when a more specific landmark - .\pybuilddir.txt - is absent, and was included for compatibility. Future releases of Python will no longer include the fallback, and so builds will need to generate or preserve the pybuilddir.txt file in order to work in-tree. This landmark file has been generated on Windows since 3.11, and on other platforms for longer.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-12003 since 06/11/2026. The exploitation appears to be difficult. An attack has to be approached locally. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1574 for this issue.

Upgrading to version 3.15.0 eliminates this vulnerability. Applying a patch is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Type

• Programming Language Software

Vendor

• Python

Name

• CPython

Version

• 3.0
• 3.1
• 3.2
• 3.3
• 3.4
• 3.5
• 3.6
• 3.7
• 3.8
• 3.9
• 3.10
• 3.11
• 3.12
• 3.13
• 3.14

License

• open-source

Website

• Product: https://github.com/python/cpython/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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