CVE-2026-66393

HIGHPre-NVD 7.57.5—
EchelonGraph scoreHIGH confidence

Score 7.5 from GitHub Security Advisory (severity: HIGH) published 2026-08-22. a secondary CVSS source baseline 7.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
Trending — 4 sources updated this weekElevated
7.5EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 7.5Exploit: Elevated riskExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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 v3
7.5
EG Score
7.5(high)
EG Risk
65(Attend)
EG Risk 65/100SSVC: Attend

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity75% × 45%
Exploitation40% × 40%
Automatability100% × 15%
Action: Remediate soon — notable exploitation risk.
EPSS PROB
0%
EPSS %ILE
27%
KEV
Not listed

Published

August 22, 2026

Last Modified

August 26, 2026

Advisory Details (2)

Auto-updated Aug 22, 2026
🔬 Proof of concept available. Patch available. Sources: github.
generic

NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder | Advisories | VulnCheck

https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder
github Patch Available🟡 PoC Available

Unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS · Advisory · nltk/nltk · GitHub

https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw

Vendor Advisories for CVE-2026-66393(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(1 across 1 ecosystem)
PyPI(1)
PackageVulnerable rangeFixed inDependents
nltk0.8 ... 3.9b1 (64 versions)——

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 21× in last 7d / 21× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-08-26 18:24 UTCEG score recompute
  2. 2026-08-26 18:24 UTCGHSA enrichment
  3. 2026-08-26 18:04 UTCEG score recompute
  4. 2026-08-26 18:04 UTCGHSA enrichment
  5. 2026-08-26 17:25 UTCEG score recompute
  6. 2026-08-26 17:25 UTCGHSA enrichment
  7. 2026-08-26 14:47 UTCEPSS rescore
  8. 2026-08-26 06:26 UTCGHSA enrichment
  9. 2026-08-25 17:58 UTCEG score recompute
  10. 2026-08-25 17:57 UTCGHSA enrichment
  11. 2026-08-25 13:49 UTCEPSS rescore
  12. 2026-08-25 05:32 UTCGHSA enrichment
  13. 2026-08-24 17:07 UTCEG score recompute
  14. 2026-08-24 17:06 UTCGHSA enrichment
  15. 2026-08-24 04:41 UTCGHSA enrichment
  16. 2026-08-23 16:15 UTCGHSA enrichment
  17. 2026-08-23 03:49 UTCEG score recompute
  18. 2026-08-23 03:49 UTCGHSA enrichment
  19. 2026-08-22 15:24 UTCEG score recompute
  20. 2026-08-22 14:26 UTCEG score recompute
  21. 2026-08-22 14:25 UTCMITRE cvelistV5first tracked

Frequently asked(5)

What is CVE-2026-66393?
CVE-2026-66393 is a high vulnerability published on August 22, 2026. 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…
When was CVE-2026-66393 disclosed?
CVE-2026-66393 was first published in the National Vulnerability Database on August 22, 2026, with the most recent update on August 26, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-66393 actively exploited?
CVE-2026-66393 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 73.0% of all scored CVEs.
What is the CVSS score of CVE-2026-66393?
CVE-2026-66393 has a CVSS v3 base score of 7.5 (NVD).
How do I remediate CVE-2026-66393?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-66393, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-66393

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