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
….Modulessetup.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Such a setup occurs with the legacy default install location for all users (in the now superseded EXE installer), due to how Windows allows all users to create folders in the root directory of their OS drive.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 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 dont 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.
Weakness
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.
Extended Description
Although this weakness can occur with any type of resource, it is frequently introduced when a product uses a directory search path to find executables or code libraries, but the path contains a directory that can be modified by an attacker, such as “/tmp” or the current working directory. In Windows-based systems, when the LoadLibrary or LoadLibraryEx function is called with a DLL name that does not contain a fully qualified path, the function follows a search order that includes two path elements that might be uncontrolled:
In some cases, the attack can be conducted remotely, such as when SMB or WebDAV network shares are used. One or more locations in that path could include the Windows drive root or its subdirectories. This often exists in Linux-based code assuming the controlled nature of the root directory (/) or its subdirectories (/etc, etc), or a code that recursively accesses the parent directory. In Windows, the drive root and some of its subdirectories have weak permissions by default, which makes them uncontrolled. In some Unix-based systems, a PATH might be created that contains an empty element, e.g. by splicing an empty variable into the PATH. This empty element can be interpreted as equivalent to the current working directory, which might be an untrusted search element. In software package management frameworks (e.g., npm, RubyGems, or PyPi), the framework may identify dependencies on third-party libraries or other packages, then consult a repository that contains the desired package. The framework may search a public repository before a private repository. This could be exploited by attackers by placing a malicious package in the public repository that has the same name as a package from the private repository. The search path might not be directly under control of the developer relying on the framework, but this search order effectively contains an untrusted element.
Potential Mitigations
Related Attack Patterns
References
- https://github.com/python/cpython/issues/151544
- https://github.com/python/cpython/pull/151545
- https://https://mail.python.org/archives/list/security-announce@python.org/thread/JIFOBO7UX3LY4VJKJUOKYJV62CFR2IRH/
- http://www.openwall.com/lists/oss-security/2026/06/16/8