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 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().

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22

Also known as

CVE-2026-12243, PYSEC-2026-597

• github.com/nltk/nltk/security/advisories/GHSA-m42h-3232-vpv3
• nvd.nist.gov/vuln/detail/CVE-2026-12243
• github.com/nltk/nltk/issues/3504
• github.com/nltk/nltk/pull/3522
• github.com/nltk/nltk/commit/aec4fce1b84ad725b8975f7365b23a4f626572a9
• github.com/nltk/nltk
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-597.yaml
• huntr.com/bounties/39aa9354-54ca-4e77-96da-580eb1fe6ed1
• securityinfinity.com/research/path-traversal-in-nltks-nltk-data-load-via-percent-encoded-sequences