CVE-2026-66393: NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-08-22 · updated 2026-10-14
Weakness: CWE-674

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.

Affected products

nltk: before 3.9.4 — fixed in 3.9.4

How to fix

Upgrade to a fixed version: nltk 3.9.4. Check the vendor advisory for mitigations.

References

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

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