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CVE-2026-62388: CVE-2026-62388: Insecure Default Security Enforcement in Natural Language Toolkit (NLTK) Path Security Module

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CVE-2026-62388: Insecure Default Security Enforcement in Natural Language Toolkit (NLTK) Path Security Module

Vulnerability ID: CVE-2026-62388
CVSS Score: 7.5
Published: 2026-09-02

CVE-2026-62388 represents a critical design flaw in the Natural Language Toolkit (NLTK) before version 3.10.0. The central security module (nltk/pathsec.py) initialized its validation enforcement flag to false by default. This fail-open configuration rendered security controls—such as path traversal checks, zip archive audits, and SSRF validations—non-blocking, only emitting warnings while permitting arbitrary file operations and code execution.

TL;DR

NLTK versions prior to 3.10.0 initialized path security in an inactive 'fail-open' state by default, allowing attackers to bypass path traversal, SSRF, and zip-extraction protections.

⚠️ Exploit Status: POC

Technical Details

  • CWE ID: CWE-1188
  • Attack Vector: Network
  • CVSS v3.1 Score: 7.5 (High)
  • CVSS v4.0 Score: 8.7 (High)
  • EPSS Score: 0.00457
  • Exploit Status: PoC / Structural Bypass documented
  • KEV Status: Not Listed

Affected Systems

  • Natural Language Toolkit (NLTK)
  • NLTK: < 3.10.0 (Fixed in: 3.10.0)

Code Analysis

Commit: 155e403

Fix insecure default configuration in pathsec.py, harden Windows file:// schema extraction, and implement dedicated downloader concurrency lock

Exploit Details

Mitigation Strategies

  • Upgrade NLTK to version 3.10.0 or later.
  • Programmatically override the enforcement state via nltk.pathsec.ENFORCE = True in application entrypoints.
  • Implement network-level egress filtering to prevent SSRF vulnerabilities resulting from resource retrieval.
  • Configure Python's warning filters to raise errors on RuntimeWarnings triggered by NLTK's security validations.

Remediation Steps:

  1. Identify all active deployments using NLTK versions prior to 3.10.0.
  2. Execute 'pip install --upgrade nltk>=3.10.0' to obtain the patched release.
  3. Verify the fix by checking that 'nltk.pathsec.ENFORCE' evaluates to True under default initialization.
  4. For systems where upgrading is deferred, inject a configuration script at bootstrap: 'import nltk.pathsec; nltk.pathsec.ENFORCE = True'.

References

Read the full report for CVE-2026-62388 on our website for more details including interactive diagrams and full exploit analysis.

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