NLTK: Default ENFORCE=False Disables All pathsec Security Controls
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:
- pathsec.open('/etc/passwd') succeeds (reads the file, emits warning)
- pathsec.validate_network_url('http://169.254.169.254/...') succeeds (warning only)
- pickle.loads() via nltk.data.load() proceeds despite unsafe source (warning only)
Every security gate follows the same pattern:
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 regardlessThis 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:
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 readableThe 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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | nltk | < 3.10.0 | 3.10.0 |
References
- https://www.vulncheck.com/advisories/nltk-before-insecure-default-configuration-pathsec
- 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
- https://github.com/nltk/nltk/releases/tag/v3.10.0
- https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3722.yaml
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N- Weakness (CWE)
CWE-1188- Platform
- Python (PyPI)
- Package
nltk- Disclosed
- 22.08.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-3722 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-p3m8-78j2-g5p3 - CC BY 4.0
Vulnerability history: nltk
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 52
- First → latest
- 08.2019 09.2026
- Without a fix
- 6
- Exploited in attacks
- 0
Severity breakdown
- Critical 5
- High 32
- Medium 13
- Low 2
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 2
- 2022: 1
- 2023: 0
- 2024: 1
- 2025: 0
- 2026: 47
-
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