NLTK: Default ENFORCE=False Disables All pathsec Security Controls

Story map

Why it is Amber

• CVSS 8.7 high
• Remotely exploitable without logging in
• On your watchlist: PyPI

Affected packages

References

• https://github.com/advisories/GHSA-p3m8-78j2-g5p3
• https://github.com/nltk/nltk/security/advisories/GHSA-p3m8-78j2-g5p3
• https://nvd.nist.gov/vuln/detail/CVE-2026-62388
• https://github.com/nltk/nltk/pull/3593
• https://github.com/nltk/nltk/commit/155e40343cff0bf50d233e274a12e04d1428b1d9
• https://github.com/nltk/nltk/releases/tag/v3.10.0
• https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3722.yaml
• https://www.vulncheck.com/advisories/nltk-before-insecure-default-configuration-pathsec
• https://github.com/advisories/GHSA-p3m8-78j2-g5p3

Original description

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.validate_network_url('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('NLTK_PATHSEC_ENFORCE', '').lower() in ('1', 'true', 'yes') def validate_something(path): if is_violation(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 NLTK_PATHSEC_ENFORCE=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.catch_warnings(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 NLTK_PATHSEC_ENFORCE=0 to explicitly disable.