GHSA-p3m8-78j2-g5p3: Path Traversal

NLTK's pathsec.py security module defaults to ENFORCE=False (line 24), which means all 8 security validation functions only emit RuntimeWarning instead of raising exceptions when violations are detected.

The pathsec module was introduced as the fix for CVE-2024-39705 (arbitrary code execution via pickle) and CVE-2026-0846 (path traversal). However, with ENFORCE=False as the default:

1. pathsec.open('/etc/passwd') succeeds (reads the file, emits warning) 2. pathsec.validatenetworkurl('http://169.254.169.254/...') succeeds (warning only) 3. pickle.loads() via nltk.data.load() proceeds despite unsafe source (warning only)

Every security gate follows the same pattern: python ENFORCE = os.environ.get('NLTKPATHSECENFORCE', '').lower() in ('1', 'true', 'yes')

def validatesomething(path): if isviolation(path): if ENFORCE: raise SecurityError('...') # Only raised when env var is set else: warnings.warn('...', RuntimeWarning) # Default: warning only # Execution continues regardless

This means the security remediations for CVE-2024-39705 and CVE-2026-0846 are effectively disabled by default. Any user who installed NLTK 3.9.x expecting the security fixes to be active is still vulnerable unless they manually set NLTKPATHSECENFORCE=1.

PoC: python import nltk.pathsec import warnings

Show that ENFORCE is False by default print(f'ENFORCE = {nltk.pathsec.ENFORCE}') # False

Attempt to read /etc/passwd through pathsec -- should be blocked with warnings.catchwarnings(record=True) as w: warnings.simplefilter('always') result = nltk.pathsec.open('/etc/passwd', 'r') print(f'File opened: {result.name}') # /etc/passwd print(f'Warning emitted: {w[0].message}') # RuntimeWarning (not an exception) # Attack succeeds -- file is readable

The correct default is fail-secure: ENFORCE should be True unless explicitly disabled. The current default makes the security module opt-in rather than opt-out, defeating its purpose.

Suggested fix: Change default to ENFORCE=True. Users who need backwards compatibility can set NLTKPATHSECENFORCE=0 to explicitly disable.