High severity · published Aug 22, 2026
CVE-2026-66393
NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder
7.5
CVSS
0.5%
EPSS
No
On KEV
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 vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 44% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
NL
nltk · nltk
before 3.9.4 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-66393
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-66393
Exploit
https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw
Third Party Advisory
https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder
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