nltk: Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences

Summary nltk.data.load and nltk.data.find resolve user-supplied resource names to filesystem paths using url2pathname, which decodes percent-encoded sequences e.g. %2e%2e to ... Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e...

CVE-2026-12243

NLTK version 3.9.4 is vulnerable to a path traversal attack due to an incomplete fix for GitHub Issue 3504. The UNSAFENOPROTOCOLRE 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...