Reference page — cumulative record through Sunday, October 4, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2026-66393

CVE-2026-66393HIGH

NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder

  AV  AC  AT  PR  UI  VC  VI  VA   CVSS    EPSS   %ile   KEV
   N   L   N   N   N   N   N   H    8.7   .0052   42.2     —

AFFECTED
  Product  Versions     Fixed
  nltk     unspecified  3.9.4

TIMELINE
  Jul 26  Reserved by VulnCheck
  Aug 22  Published (CNA: VulnCheck)
  Aug 27  EXPLOIT PUBLISHED — CVE-2026-66393 (nltk). Public exploit reference added.

CWE-674 · CNA: VulnCheck · CVSS v4.0 · 2 references · NVD status: Analyzed

Description

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.

Lifecycle

Complete event history — 3 events, chronological
Date	Event	Detail
July 26, 2026	Reserved	Reserved by VulnCheck
August 22, 2026	Published	Published (CNA: VulnCheck)
August 27, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2026-66393 (nltk). Public exploit reference added.

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
nltk	nltk	—	—	3.9.4

Weaknesses

CWE-674

References (2)

https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw [Exploit, Mitigation, Vendor Advisory]
https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder [Third Party Advisory]

Authoritative record: CVE-2026-66393 at cve.org

Vendors: nltk

Weaknesses: CWE-674

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2026-66393 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Sunday, October 4, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.