[LFS Trac] #5961: Python security fixes: CVE-2026-12003,11940,0864

#5961: Python security fixes: CVE-2026-12003,11940,0864
-------------------------+----------------------
 Reporter:  Joe Locash   |      Owner:  lfs-book
     Type:  enhancement  |     Status:  new
 Priority:  high         |  Milestone:  13.1
Component:  Book         |    Version:  git
 Severity:  normal       |   Keywords:
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 3 CVE's have been fixed in Python recently:

 {{{
 There is a MODERATE (CVSSv4 5.3) severity vulnerability affecting
 CPython up to (and including) 3.11.15, 3.12.13, 3.13.14, 3.14.6 and
 3.15.0b2.

 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/<arch>', 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 users (in the now superseded EXE installer), due to how Windows allows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 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.

 Reported by Jake Yamaki, Senior Consultant, Bishop Fox.

 Please see the linked CVE ID for the latest information on affected
 versions:

     https://www.cve.org/CVERecord?id=CVE-2026-12003
     https://github.com/python/cpython/pull/151545
 }}}

 {{{
 There is a HIGH severity vulnerability affecting CPython.

 tarfile.extractall() with the 'data' or 'tar' filter could be bypassed by
 a
 crafted archive where a hardlink references a symlink stored at a deeper
 name than the hardlink itself.  The extraction fallback validated the
 symlink at it's archived location but recreated it at the hardlink's
 shallower
 path, letting a relative target the filter judged contained escape the
 destination directory.  This allowed a malicious tar archive to create a
 symlink pointing outside the destination, enabling out-of-destination file
 reads or writes. This was an incomplete fix of CVE-2025-4330.

 Please see the linked CVE ID for the latest information on affected
 versions:

     https://www.cve.org/CVERecord?id=CVE-2026-11940
     https://github.com/python/cpython/pull/151559
 }}}

 {{{
 There is a MEDIUM severity vulnerability affecting CPython.

 When using the "configparser" module to write configuration files
 containing multi-line text values with carriage return characters (\r) the
 resulting file could be injected with unexpected keys and values if the
 attacker controls the written value.

 Please see the linked CVE ID for the latest information on affected
 versions:

     https://www.cve.org/CVERecord?id=CVE-2026-0864
     https://github.com/python/cpython/pull/143929
 }}}
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