Vulnerability · Published Aug 22, 2026 · Assigned by VulnCheck

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

High 7.5

In plain English

A high-severity security flaw in nltk from nltk. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.5%top 55.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-66393
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-66393
Debian:14	nltk	3.10.0-1	DEBIAN-CVE-2026-66393
PyPI	nltk	3.9.4	GHSA-rf74-v2fm-23pw
PyPI	nltk	3.9.4	PYSEC-2026-3724

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.9.4

Weakness: CWE-674 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

GitHub Security Advisory (GHSA-rf74-v2fm-23pw) vendor-advisory
VulnCheck Advisory: NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder third-party-advisory