CVE-2026-12003

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users (in the now superseded EXE installer), due to how Windows allows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