A significant security flaw has been identified in the widely-used Python JSON Logger library, potentially exposing millions of systems to remote code execution (RCE) attacks. This vulnerability, tracked as CVE-2025-27607 with a CVSS score of 8.8, affects versions 3.2.0 and 3.2.1 of the package. The issue arises from a missing dependency, `msgspec-python313-pre`, which was removed by its original owner, leaving the package name unclaimed and vulnerable to exploitation.
Discovery and Exploitation Potential
Security researcher Omnigodz uncovered this vulnerability during an investigation into supply chain attacks. The `python-json-logger` package declared an optional development dependency, `msgspec-python313-pre`, in its `pyproject.toml` file. However, this dependency was not present on PyPI and was unregistered, creating an opportunity for malicious actors to claim the package name and introduce harmful code.
In a proof-of-concept demonstration, Omnigodz registered the `msgspec-python313-pre` package name and published a benign version to prevent malicious exploitation. This action highlighted the ease with which an attacker could have exploited the vulnerability. Users installing `python-json-logger` with development dependencies in Python 3.13 environments would have inadvertently downloaded and executed the malicious package, granting attackers the ability to execute arbitrary code on the affected systems.
Impact and Scope
The `python-json-logger` library is extensively used, with over 46 million monthly downloads according to PyPI’s BigQuery metrics. This widespread adoption amplifies the potential impact of the vulnerability. Successful exploitation could grant attackers full system control, compromising confidentiality, integrity, and availability. The attack requires minimal sophistication—merely publishing a malicious package to PyPI—but depends on victims using Python 3.13 with development dependencies enabled, a common configuration in CI/CD pipelines and developer workstations.
Mitigation Measures
The maintainers of `python-json-logger` have addressed the vulnerability by releasing version 3.3.0, which eliminates the `msgspec-python313-pre` dependency entirely. Users are strongly advised to upgrade to this patched version immediately to mitigate the risk. Additionally, PyPI administrators have taken steps to block the name of the missing package, preventing it from being reclaimed by malicious actors. Due to these protective measures, the severity rating has been downgraded from High to Low.
Recommendations for Developers
Developers are urged to treat dependencies as potential attack surfaces and to exercise caution when declaring optional dependencies. Regular audits of project dependencies and prompt updates to patched versions are essential practices to maintain security. Organizations should also monitor their Python environments for any instances of the `msgspec-python313-pre` dependency and remove it immediately if present.
Conclusion
This vulnerability underscores the critical importance of vigilant dependency management in software development. By promptly addressing such issues and adhering to best practices, developers can safeguard their systems against potential supply chain attacks and maintain the integrity of their software ecosystems.
