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CVE-2026-66393

Nltk denial of service

7.5 HIGHPublic exploitEPSS 0.52%
  1. Published22 Aug 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityNone
IntegrityNone
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

Nltk< 3.9.4

Remediation

Fixed in
Nltk≥ 3.9.4

References

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Frequently asked

Is CVE-2026-66393 being actively exploited?

CVE-2026-66393 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.52%.

How severe is CVE-2026-66393?

CVE-2026-66393 has a CVSS 3.1 base score of 7.5, rated high. EPSS estimates a 0.52% probability of exploitation.

Which products are affected by CVE-2026-66393?

Affected products include Nltk.

How do I fix CVE-2026-66393?

Upgrade affected software to a fixed version, for example Nltk 3.9.4 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.