CVE-2026-12243

# 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(resource_name)) if os.path.exists(p): return FileSystemPathPointer(p) # Proof of Concept import nltk.data nltk.data.path = ["/home/user/nltk_data"] %2e%2e decodes to .. via url2pathname(), escaping the data dir data = nltk.data.load("%2e%2e/SECRET_credentials.txt", format="raw") print(data) b'AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\nDATABASE_PASS=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 resource_name = unquote(resource_name) # 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().

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.