CVE-2026-12003 CPython >3.11 Insecure Input Validation resulting in privilege escalation
Low-privilege users can escalate privileges by creating specific files in the parent directory of a legacy Windows Python installation, altering the module search path.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-12003?
CVE-2026-12003 is a medium-severity vulnerability in CPython, published on 16 Jun 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Official 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Such a setup occurs with the legacy default install location for all users (in the now superseded EXE installer), due to how Windows allows all users to create folders in the root directory of their OS drive.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.
Weakness: CWE-427
Which versions are affected by CVE-2026-12003?
3 open source packages ship an affected version, including libpython, python and python-min.
| Package | Affected | Fixed in |
|---|---|---|
libpython Bitnami | All versions | 3.11.16, 3.12.14, 3.13.15, 3.14.7 |
python Bitnami | All versions | 3.11.16, 3.12.14, 3.13.15, 3.14.7 |
python-min Bitnami | All versions | 3.11.16, 3.12.14, 3.13.15, 3.14.7 |
How do I fix CVE-2026-12003?
Upgrade libpython to a fixed version for your release line: 3.11.16, 3.12.14, 3.13.15 or 3.14.7.
Is CVE-2026-12003 being exploited?
There is no known exploitation of CVE-2026-12003 in the wild and no public exploit code.
Sources
Also known as AZL-90182, BIT-libpython-2026-12003, BIT-python-2026-12003, BIT-python-min-2026-12003, DEBIAN-CVE-2026-12003, ECHO-d378-1b5a-f13e and 8 more.