PYSEC-2026-597

NLTK version 3.9.4 is vulnerable to a path traversal attack due to an incomplete fix for GitHub Issue #3504. The _UNSAFE_NO_PROTOCOL_RE regex in nltk/data.py checks for literal ../ sequences but fails to account for percent-encoded traversal sequences such as ..%2f. The url2pathname() function decodes these sequences after the validation step, allowing an attacker to bypass the protection. This vulnerability enables an attacker to read arbitrary files accessible to the Python process by controlling the resource name parameter passed to nltk.data.load() or nltk.data.find(). The issue affects applications that rely on NLTK for resource loading, including NLP web applications, Jupyter notebooks, and CLI tools. The default pathsec.ENFORCE=False setting exacerbates the impact by not blocking the file read at the open() stage.