Vulnerability in Nltk
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.005 (42.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 42nd percentile — 42.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw (Exploit, vendor-advisory, Mitigation, Vendor Advisory)
- www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-66393?
- CVE-2026-66393 is a high-severity vulnerability in Nltk, classified under Uncontrolled Recursion. CVSS score: 7.5/10. Published 2026-08-22.
- How severe is CVE-2026-66393?
- High severity. CVSS v3 base score is 7.5 out of 10.