CVE-2026-66393

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

8.7CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS 3.1

7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

0.5% chance of exploitation in the next 30 days; higher than 43% of CVEs (scores of 2026-10-25)

CWE

CWE-674

Published

2026-08-22

Modified

2026-10-14

Record from

the NVD, synced 2026-10-19

Affected

• nltk:nltk

Packages (OSV)

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

Check your stackJSON