CVE-2025-56005

CVE-2025-56005 is an undocumented feature in the PLY (Python Lex-Yacc) library version 3.11 that enables remote code execution through the `picklefile` parameter in the `yacc()` function. This parameter accepts a `.pkl` file, which is deserialized using `pickle.load()` without any validation. Since Python's `pickle` module permits code execution via the `__reduce__()` method, a malicious `.pkl` file can trigger arbitrary code execution. The parameter is not referenced in official documentation or the project's GitHub repository but remains active in the PyPI-distributed version, posing a stealthy backdoor and persistence risk. The vulnerability, published on 2026-01-20, carries a CVSS v3.1 score of 9.8 and is classified under CWE-502 (Deserialization of Untrusted Data).

A remote attacker can exploit this vulnerability by supplying a crafted `.pkl` file to the `picklefile` parameter when the `yacc()` function is invoked, requiring no privileges, user interaction, or special access (AV:N/AC:L/PR:N/UI:N). Successful exploitation leads to high-impact confidentiality, integrity, and availability violations (C:H/I:H/A:H), allowing full remote code execution on the target system running affected PLY code.

References indicate significant controversy regarding the vulnerability's validity, with no official patches or mitigations detailed. A proof-of-concept is provided in repositories like https://github.com/bohmiiidd/Undocumented-RCE-in-PLY and https://github.com/bohmiiidd/Undocumument_RCE_PLY-yacc-CVE-2025-56005, but a third-party analysis at https://github.com/tom025/ply_exploit_rejection and https://github.com/tom025/ply_exploit_rejection/issues/1 argues for rejection, stating the PoC fails to demonstrate arbitrary code execution. An oss-security mailing list discussion at http://www.openwall.com/lists/oss-security/2026/01/23/4 further highlights this debate. Practitioners should avoid the `picklefile` parameter and monitor for updates from PLY maintainers.