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Vulnerability record · CVE-2026-66393 · published 22 August 2026

CVE-2026-66393: Nltk vulnerability

Nltk · Nltk

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.

8.7 CVSS 4.0 High EPSS 0.52% · top 58.1% CWE-674 · CWE-674
8.7CVSS 4.0 base score
0.52%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References, 2 tagged exploit
27 Aug 2026Last modified by NVD

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.

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:

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw ExploitMitigationVendor Advisory
https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder Third Party Advisory
https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw ExploitMitigationVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-66393), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.