CVE-2026-62388: NLTK Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-p3m8-78j2-g5p3",
  "modified" : "2026-09-02T15:40:33Z",
  "published" : "2026-09-02T15:40:33Z",
  "aliases" : [ "CVE-2026-62388" ],
  "summary" : "NLTK: Default ENFORCE=False Disables All pathsec Security Controls",
  "details" : "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.\n\nThe 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:\n\n1. pathsec.open('/etc/passwd') succeeds (reads the file, emits warning)\n2. pathsec.validate_network_url('http://169.254.169.254/...') succeeds (warning only)\n3. pickle.loads() via nltk.data.load() proceeds despite unsafe source (warning only)\n\nEvery security gate follows the same pattern:\n```python\nENFORCE = os.environ.get('NLTK_PATHSEC_ENFORCE', '').lower() in ('1', 'true', 'yes')\n\ndef validate_something(path):\n    if is_violation(path):\n        if ENFORCE:\n            raise SecurityError('...')  # Only raised when env var is set\n        else:\n            warnings.warn('...', RuntimeWarning)  # Default: warning only\n    # Execution continues regardless\n```\n\nThis 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.\n\nPoC:\n```python\nimport nltk.pathsec\nimport warnings\n\n# Show that ENFORCE is False by default\nprint(f'ENFORCE = {nltk.pathsec.ENFORCE}')  # False\n\n# Attempt to read /etc/passwd through pathsec -- should be blocked\nwith warnings.catch_warnings(record=True) as w:\n    warnings.simplefilter('always')\n    result = nltk.pathsec.open('/etc/passwd', 'r')\n    print(f'File opened: {result.name}')  # /etc/passwd\n    print(f'Warning emitted: {w[0].message}')  # RuntimeWarning (not an exception)\n    # Attack succeeds -- file is readable\n```\n\nThe 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.\n\nSuggested fix: Change default to ENFORCE=True. Users who need backwards compatibility can set NLTK_PATHSEC_ENFORCE=0 to explicitly disable.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.9.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-p3m8-78j2-g5p3"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-62388"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/pull/3593"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/155e40343cff0bf50d233e274a12e04d1428b1d9"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/releases/tag/v3.10.0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3722.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-before-insecure-default-configuration-pathsec"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1188" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:40:33Z",
    "nvd_published_at" : null
  }
}