CVE-2025-4138 is a high-severity path traversal vulnerability (CWE-22) affecting the Python Software Foundation's CPython implementation, specifically within the tarfile module used for extracting tar archives. The vulnerability arises when using the TarFile.extractall() or TarFile.extract() methods with the filter parameter set to "data" or "tar". This flaw allows an attacker to bypass the extraction filter, enabling symbolic link (symlink) targets within the tar archive to point outside the intended destination directory. Consequently, this can lead to unauthorized file extraction or modification outside the designated extraction path, potentially overwriting critical files or planting malicious files in arbitrary locations. Additionally, some file metadata can be altered during extraction, which may facilitate further exploitation or evasion. Notably, starting with Python 3.14, the default filter value changed from "no filtering" to "data", meaning that even users relying on default behavior in newer Python versions are affected. The vulnerability does not significantly impact the installation of source distributions packaged as tar archives, as these inherently allow arbitrary code execution during build processes. However, it remains critical to avoid installing source distributions containing suspicious symlinks. The CVSS v3.1 score is 7.5, reflecting a high severity with network attack vector, low attack complexity, no privileges or user interaction required, and a significant confidentiality impact but no integrity or availability impact. No known exploits are currently reported in the wild, and no patches are linked yet, indicating the need for vigilance and proactive mitigation.

For European organizations, this vulnerability poses a significant risk, especially those relying on Python applications that process untrusted tar archives using the affected tarfile module methods. Exploitation could lead to unauthorized disclosure of sensitive data if files outside the extraction directory are exposed, or potential disruption by overwriting or planting malicious files in critical system locations. This is particularly concerning for sectors handling sensitive or regulated data such as finance, healthcare, and government agencies within Europe. The fact that no authentication or user interaction is required increases the risk of automated exploitation in supply chain attacks or malicious archive deliveries. Organizations using Python 3.14 or later are also at risk due to the changed default filter behavior, potentially increasing the attack surface. While the vulnerability does not directly allow code execution, the ability to manipulate filesystem contents and metadata can facilitate further attacks or persistence mechanisms. Given Europe's stringent data protection regulations like GDPR, any data leakage or unauthorized file modifications could lead to compliance violations and reputational damage.

European organizations should immediately audit their Python environments to identify usage of the tarfile module's extractall() or extract() methods with the filter parameter set to "data" or "tar". Where feasible, avoid extracting untrusted tar archives or implement strict validation of archive contents before extraction. Employ sandboxing or containerization to isolate extraction processes, limiting filesystem access to designated directories. Monitor and restrict symbolic link creation and resolution during extraction. Upgrade to patched Python versions once available; until then, consider backporting fixes or applying manual checks to sanitize archive contents. Additionally, implement file integrity monitoring on critical directories to detect unauthorized changes. Educate developers and DevOps teams about the changed default filter behavior in Python 3.14 and review CI/CD pipelines that automatically process tar archives to prevent inadvertent exploitation. Finally, maintain vigilant monitoring for suspicious activity related to archive extraction and symlink manipulation.