CPython >3.11 Insecure Input Validation resulting in privilege escalation
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 Such a setup occurs with the legacy default install location for all users (in the now superseded EXE installer), due to how Windows allows all users to create folders in the root directory of their OS drive.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 escala
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-12003
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 4 Nov 2026, 21:29 UTC · Revision 1
Source evidence
CPython >3.11 Insecure Input Validation resulting in privilege escalation
core-osv · Original sourcePublished 16 Jun 2026, 15:18 UTC
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 escala
osv:CVE-2026-12003
Complete source record
{
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"url": "https://osv.dev/vulnerability/CVE-2026-12003",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CPython >3.11 Insecure Input Validation resulting in privilege escalation",
"summary": "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 escala",
"evidence": "osv:CVE-2026-12003",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2026-12003",
"aliases": [
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"BIT-python-2026-12003",
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"PSF-2026-28"
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"modified": "2026-10-28T02:30:39.698079379Z",
"provider": "osv",
"references": [
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