Process crash via unbounded JSON recursion
Published Aug 22, 2026 · Updated Aug 22, 2026
Uncontrolled recursion in JSONTaggedDecoder in NLTK before 3.9.4 allows attackers to crash a Python process with deeply nested JSON. The decoder recursively traverses every dictionary value and list element without tracking depth, so input beyond Python's recursion limit raises an unhandled RecursionError. Exploitation requires calling code to pass attacker-supplied JSON to the decoder and interrupts only that Python process.
Summary
What happened
Uncontrolled recursion in JSONTaggedDecoder in NLTK before 3.9.4 allows attackers to crash a Python process with deeply nested JSON. The decoder recursively traverses every dictionary value and list element without tracking depth, so input beyond Python's recursion limit raises an unhandled RecursionError. Exploitation requires calling code to pass attacker-supplied JSON to the decoder and interrupts only that Python process.
The record
- CVE
- CVE-2026-66393
- Published
- Aug 22, 2026
- Updated
- Aug 22, 2026
- Vendor
- NLTK Project
- Product
- NLTK
- Classifications
- CWE-674, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 22, 2026CVE publishedPublication date reported by the CVE source.
- Aug 22, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <3.9.4
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- JSONTaggedDecoder deeply nested JSON proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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