Read the docs
TI-2026-47152 Aliases CVE-2026-66393GHSA-rf74-v2fm-23pw

NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder

Priority HIGH 77CWE-674Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Attack requirements
None
Impact type
Denial of service
Confidentiality · integrity · availability
None · None · High
Fix status
Partial Red Hat: 1 without a fix

Attack profile from CVSS 4.0 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-674.

Summary

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00521 44th percentileScored 2026-11-07
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Partial nltk (nltk): 3.9.4nltk (PyPI): 3.9.4Red Hat: 1 without a fix

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
nltknltkbefore 3.9.43.9.4Patch availableCNA
—nltk (PyPI)before 3.9.43.9.4Patch availableGHSA-rf74-v2fm-23pw
  • nltk Vendornltk Affected before 3.9.4 Fixed in 3.9.4 Patch available CNA
  • nltk (PyPI) Vendor— Affected before 3.9.4 Fixed in 3.9.4 Patch available GHSA-rf74-v2fm-23pw

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:nltk:nltk:*:*:*:*:*:*:*:*before 3.9.4Vulnerablenltk
  • cpe:2.3:a:nltk:nltk:*:*:*:*:*:*:*:* Versions before 3.9.4 Vulnerable ·nltk

Remediation

  • Fix · CNA nltk (nltk) Fixed in 3.9.4
  • Fix · GHSA-rf74-v2fm-23pw nltk (PyPI) Fixed in 3.9.4 Advisory →

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

  • Release ·

    nltk 3.9.4

    2 records Highest priority Priority HIGH 2 HIGH

    Other records, highest priority first

    • TI-2026-49337 NLTK before 3.9.4 Symlink Escape via CorpusReaderPriority HIGH 70
    Show all 2 records →

Vendor assessments

Red Hat

Red Hat rating: Important. Red Hat's page for this CVE →

ProductStatusPlan or fix
Red Hat Ansible Automation Platform 2AffectedNo fix planned: Will not fix
Not affected: 4 products
  • Exploit Intelligence
  • Lightspeed Core
  • OpenShift Lightspeed
  • Red Hat OpenShift AI (RHOAI)

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-66393
  2. 2026-10-03 Alias added: GHSA-rf74-v2fm-23pw

References

Exploit 1

Advisory 1

Other 3

  • TI-2026-00629 Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading CVSS 3.0CRITICAL 10.0
  • TI-2025-12800 Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution CVSS 3.0CRITICAL 10.0
  • TI-2026-55173 NLTK before 3.10.3 JVM Argument Injection via Per-Call Options CVSS 4.0CRITICAL 9.3
  • TI-2026-55155 NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization CVSS 4.0CRITICAL 9.3
  • TI-2026-56370 nltk PorterStemmer before 3.10.3 Quadratic-time DoS CVSS 4.0HIGH 8.7
  • TI-2026-55172 NLTK 3.10.2 Path Traversal via corpus-reader constructors CVSS 4.0HIGH 8.8
See all 41 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences