CVE-2026-66393
NLTK <3.9.4 unbounded recursion in JSON parsing allows DoS/crash via nested JSON
Is CVE-2026-66393 being exploited?
Not confirmed. CVE-2026-66393 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.36% probability of exploitation in the next 30 days.
How severe is CVE-2026-66393?
CVE-2026-66393 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-66393?
Yes. A fix has been recorded for CVE-2026-66393. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-66393 affect?
CVE-2026-66393 affects NLTK < 3.9.4, Python apps using JSONTaggedDecoder, NLP services using NLTK. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-66393?
Upgrade NLTK to >=3.9.4 and validate/truncate nested JSON inputs.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Attacker-supplied deeply nested JSON triggers unhandled RecursionError, crashing Python processes and causing denial of service.
Patch when possible
- affected< 3.9.4
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.