CVE-2026-12003 (GCVE-0-2026-12003)

Vulnerability from cvelistv5 – Published: 2026-06-16 15:18 – Updated: 2026-06-16 19:12
VLAI
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 Such a setup occurs with the legacy default install location for all users (in the now superseded EXE installer), due to how Windows allows all users to create folders in the root directory of their OS drive.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 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.
Severity
5.3 (Medium)
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
SSVC
Exploitation: none Automatable: no Technical Impact: total
CISA Coordinator (v2.0.3)
CWE
Assigner
PSF
References
Impacted products
Vendor Product Version
Python Software Foundation CPython Affected: 0 , < 3.15.0 (python)
Create a notification for this product.
Credits
Jake Yamaki (https://github.com/b6938236) Steve Dower (https://github.com/zooba)
Show details on NVD website
To clipboard 

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This path is outside the install directory\u003cbr\u003eof Python, and may have different permissions, potentially allowing a\u003cbr\u003elow-privilege user to create the landmark and an alternative `Lib`\u003cbr\u003efolder that will be discovered by an otherwise restricted install.\u003cbr\u003e\u003cbr\u003eSuch a setup occurs with the legacy default install location for all\u003cbr\u003eusers (in the now superseded EXE installer), due to how Windows allows\u003cbr\u003eall users to create folders in the root directory of their OS drive.\u003cbr\u003e\u003cbr\u003eOur recommended mitigation on Windows is to migrate away from the\u003cbr\u003elegacy installer and use the new [Python install\u003cbr\u003emanager](https://www.python.org/downloads/latest/pymanager/) to install\u003cbr\u003efor the current user. Installs where the directory two levels above the\u003cbr\u003ePython installation directory have equivalent permissions are unaffected\u003cbr\u003e(in general, a per-user install cannot be modified at all by other\u003cbr\u003eusers, removing any escalation of privilege risk, and could be directly\u003cbr\u003emodified by a privileged user, making the potential tampering\u003cbr\u003eirrelevant). Alternative mitigations might include preemptively creating\u003cbr\u003eand restricting access to a `Modules` directory. Be aware that only 3.13\u003cbr\u003eand 3.14 will receive updated legacy installers - earlier fixes are only\u003cbr\u003eprovided as sources.\u003cbr\u003e\u003cbr\u003ePlatforms other than Windows allow VPATH to be overridden, but as they\u003cbr\u003edon\u0027t usually use a separated directory in the build for binaries, are\u003cbr\u003eunlikely to have a landmark reference outside of the install directory.\u003cbr\u003e\u003cbr\u003eThe landmark detection involving VPATH is a fallback for when a more\u003cbr\u003especific landmark - .\\\\pybuilddir.txt - is absent, and was included for\u003cbr\u003ecompatibility. Future releases of Python will no longer include the\u003cbr\u003efallback, and so builds will need to generate or preserve the\u003cbr\u003epybuilddir.txt file in order to work in-tree. This landmark file has\u003cbr\u003ebeen generated on Windows since 3.11, and on other platforms for longer.\"}]}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-427\", \"description\": \"CWE-427\"}]}], \"providerMetadata\": {\"orgId\": \"28c92f92-d60d-412d-b760-e73465c3df22\", \"shortName\": \"PSF\", \"dateUpdated\": \"2026-06-16T15:27:24.909Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-12003\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-06-16T19:12:36.706Z\", \"dateReserved\": \"2026-06-11T17:05:37.519Z\", \"assignerOrgId\": \"28c92f92-d60d-412d-b760-e73465c3df22\", \"datePublished\": \"2026-06-16T15:18:42.998Z\", \"assignerShortName\": \"PSF\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}


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Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.