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 Such a setup occurs with the legacy default install location for all users (in the now superseded EXE installer), due to how Windows allows all users to create folders in the root directory of their OS drive.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.

Metrics

 

CVSS Version 4.0 CVSS Version 3.x CVSS Version 2.0

NVD enrichment efforts reference publicly available information to associate vector strings. CVSS information contributed by other sources is also displayed.

CVSS 4.0 Severity and Vector Strings:

NIST: NVD

N/A

NVD assessment not yet provided.

CNA:  Python Software Foundation

CVSS-B 5.3 MEDIUM

Vector:  CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS 3.x Severity and Vector Strings:

NIST: NVD

Base Score:  N/A

NVD assessment not yet provided.

CVSS 2.0 Severity and Vector Strings:

NIST: NVD

Base Score: N/A

NVD assessment not yet provided.

http://www.openwall.com/lists/oss-security/2026/06/16/8

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 i

AV:L/AC:L/AT:P/PR:L/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CWE-427

https://github.com/python/cpython/issues/151544

https://github.com/python/cpython/pull/151545

https://https://mail.python.org/archives/list/[email protected]/thread/JIFOBO7UX3LY4VJKJUOKYJV62CFR2IRH/