The vulnerability in CPython's tarfile module (CVE-2025-4138) involves improper limitation of pathnames (CWE-22) during extraction of tar archives. Specifically, when using the extraction filters "data" or "tar" with TarFile.extractall() or TarFile.extract(), the filter can be ignored, allowing symbolic link targets to escape the intended extraction directory. This can lead to modification of file metadata outside the target directory. The issue affects Python versions 3.10.0 through 3.14.0a1, with the default filter changing to "data" in Python 3.14, increasing the risk for code relying on default behavior. The vulnerability does not significantly affect source distribution installations due to their inherent arbitrary code execution risk during build.

An attacker providing a crafted tar archive can cause extraction operations to write files or modify metadata outside the intended directory via symlink traversal, potentially leading to unauthorized file modifications. The CVSS score of 7.5 (high) reflects the network attack vector, low complexity, no privileges required, no user interaction, and high confidentiality impact. There is no indication of integrity or availability impact. No known exploits in the wild have been reported.

No official patch or fix information is provided in the available data. Patch status is not yet confirmed — check the Python Software Foundation advisory for current remediation guidance. Users should avoid extracting untrusted tar archives with the affected filter parameters until a fix is available. For Python 3.14 or later, be aware that the default filter value changed to "data", which is affected. Exercise caution when installing source distributions with suspicious symbolic links, although this vulnerability does not significantly increase risk in that context.