CVE-2026-12243

Name of the Vulnerable Software and Affected Versions NLTK version 3.9.4

Description A path traversal flaw exists due to improper input validation and an incomplete fix for a previous issue. The UNSAFE NO PROTOCOL RE regex in nltk/data.py blocks literal ../ sequences but fails to detect percent-encoded traversal sequences such as ..%2f. Because the url2pathname() function decodes these sequences after the validation step, an attacker can bypass the protection. This allows an attacker to read arbitrary files accessible to the Python process by controlling the resource name parameter passed to the nltk.data.load() and nltk.data.find() functions. The impact is increased when the pathsec.ENFORCE=False setting is used, as it does not block the file read during the open() stage. This issue affects applications relying on NLTK for resource loading, such as NLP web applications, Jupyter notebooks, and CLI tools.

Recommendations At the moment, there is no information about a newer version that contains a fix for this vulnerability.