CVE-2026-66393

HIGH

7.5

CVSS v3

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.

CWE-674

How to fix

Remediation Available

nltkDebian

Fixed in:3.10.0-1CVE-2026-66393

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityNone

AvailabilityHigh

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

Exploit Intelligence

0.36%probability of exploitation in 30 days

29thpercentile

Low risk: more likely to be exploited than 29% of all known CVEs.

References

Exploit1

github.com

Third-Party Advisory1

vulncheck.com

Related Vulnerabilities

Other CWE-674 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2021-45105	Medium	5.9	100%	-	Fix
CVE-2024-25111	High	8.6	65%	-	Fix
CVE-2023-50269	High	8.6	58%	-	Fix
CVE-2021-42697	High	7.5	36%	-	Fix
CVE-2018-0739	Medium	6.5	19%	-	Fix
CVE-2007-1285	High	7.5	18%	-	Fix

Common questions

How do I fix CVE-2026-66393?

Upgrade nltk on debian unstable to 3.10.0-1 or later.

Is CVE-2026-66393 being actively exploited?

Not that we know of. CVE-2026-66393 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.36% is the estimated probability that it will be exploited in the next 30 days. That is higher than 29% of all scored CVEs.

How severe is CVE-2026-66393?

CVE-2026-66393 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-66393 affect?

Published advisories record a fix for nltk (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-08-27.