CVE-2026-66393

Published Aug 22, 2026

Last updated 2 days ago

CVSS high 8.7

Overview

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.
Source
disclosure@vulncheck.com
NVD status
Received

Risk scores

CVSS 4.0

Type
Secondary
Base score
8.7
Impact score
-
Exploitability score
-
Vector string
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Severity
HIGH

CVSS 3.1

Type
Primary
Base score
7.5
Impact score
3.6
Exploitability score
3.9
Vector string
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Severity
HIGH

Weaknesses

disclosure@vulncheck.com
CWE-674

Social media

Hype score
Not currently trending

  1. CVE-2026-66393 - DoS vulnerability in NLTK. Deeply nested JSON triggers unhandled recursion crash in Python. CVSS 7.5. Update to 3.9.4 immediately. https://t.co/WAYskXri3s #Python #infosec #CVE #CVEAlert #infosec #SysAdmin #cybersecurity #Linux #devsecops #devops #developer

    @HugoValters

    23 Aug 2026

    45 Impressions

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References

Sources include official advisories and independent security research.