CVE-2026-12003

Technical Description

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users (in the now superseded EXE installer), due to how Windows

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References