A critical Remote Code Execution vulnerability has been uncovered in PLY (Python Lex-Yacc), described as CVE-2025-56005 and carrying a maximum CVSS score of 9.8, surfacing as the project’s maintainer announced its abandonment. Researchers identified an undocumented parameter named picklefile in the yacc() function, which is active in the PyPI release but not in the official documentation or GitHub repository, enabling unsafe deserialization via Python’s pickle[.]load() during parser initialization.
The mechanism allows an attacker who can supply a crafted pickle file to execute arbitrary code, potentially creating files or taking over a host server, according to the PoC demonstrated in the report. The vulnerability stems from this ghost parameter and the lack of validation around deserialising untrusted data, and comes as Beazley revealed that after 25 years he has decided to abandon the PLY project with no official patch forthcoming.
The mitigation is straightforward yet critical: do not use the picklefile parameter, and security teams should audit codebases to ensure user input cannot influence arguments passed to yacc(). The discovery was reported on 26 January 2026.