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
Elevated
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
30%
KEV
Not listed

Published

August 22, 2026

Last Modified

August 27, 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

(4 across 4 ecosystems)
Debian:12(1)
PackageVulnerable rangeFixed inDependents
nltk3.10.0-1 ... 3.9.3-1 (9 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
nltk3.10.0-1 ... 3.9.3-1 (6 versions)——
Debian:14(1)
PackageVulnerable rangeFixed inDependents
nltk3.9.1-2, 3.9.2-1, 3.9.3-13.10.0-1—
PyPI(1)
PackageVulnerable rangeFixed inDependents
nltk0.8 ... 3.9b1 (64 versions)3.9.4—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 10× in last 7d / 72× 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. 2026-09-19 00:57 UTCEG score recompute
  2. 2026-09-18 19:28 UTCEPSS rescore
  3. 2026-09-18 00:06 UTCEG score recompute
  4. 2026-09-16 21:59 UTCEG score recompute
  5. 2026-09-16 14:08 UTCEPSS rescore
  6. 2026-09-16 09:34 UTCEG score recompute
  7. 2026-09-16 05:15 UTCEPSS rescore
  8. 2026-09-15 07:31 UTCEG score recompute
  9. 2026-09-13 17:38 UTCEG score recompute
  10. 2026-09-12 16:48 UTCEG score recompute
  11. 2026-09-11 15:54 UTCEG score recompute
  12. 2026-09-11 14:53 UTCEPSS rescore
  13. 2026-09-11 09:37 UTCEPSS rescore
  14. 2026-09-10 15:04 UTCEG score recompute
  15. 2026-09-09 01:42 UTCEG score recompute
  16. 2026-09-08 22:01 UTCEPSS rescore
  17. 2026-09-08 03:52 UTCEG score recompute
  18. 2026-09-07 16:01 UTCEPSS rescore
  19. 2026-09-07 01:37 UTCEG score recompute
  20. 2026-09-06 13:47 UTCEPSS rescore
  21. 2026-09-06 00:42 UTCEG score recompute
  22. 2026-09-05 15:31 UTCEPSS rescore
  23. 2026-09-04 23:47 UTCEG score recompute
  24. 2026-09-04 10:51 UTCEG score recompute
  25. 2026-09-04 05:07 UTCEPSS rescore
Show 47 more
  1. 2026-09-02 21:29 UTCEG score recompute
  2. 2026-09-02 14:12 UTCEPSS rescore
  3. 2026-09-02 09:02 UTCGHSA enrichment
  4. 2026-09-01 20:31 UTCEG score recompute
  5. 2026-09-01 20:31 UTCGHSA enrichment
  6. 2026-09-01 13:54 UTCEPSS rescore
  7. 2026-09-01 08:06 UTCEG score recompute
  8. 2026-09-01 08:05 UTCGHSA enrichment
  9. 2026-09-01 04:40 UTCEPSS rescore
  10. 2026-08-31 19:36 UTCGHSA enrichment
  11. 2026-08-31 06:52 UTCEG score recompute
  12. 2026-08-31 06:52 UTCGHSA enrichment
  13. 2026-08-30 19:17 UTCEPSS rescore
  14. 2026-08-30 18:27 UTCGHSA enrichment
  15. 2026-08-30 06:02 UTCEG score recompute
  16. 2026-08-30 06:02 UTCGHSA enrichment
  17. 2026-08-30 01:22 UTCEPSS rescore
  18. 2026-08-29 17:36 UTCGHSA enrichment
  19. 2026-08-29 04:59 UTCEG score recompute
  20. 2026-08-29 04:59 UTCGHSA enrichment
  21. 2026-08-28 21:42 UTCEPSS rescore
  22. 2026-08-28 16:30 UTCGHSA enrichment
  23. 2026-08-28 03:23 UTCGHSA enrichment
  24. 2026-08-27 14:58 UTCEG score recompute
  25. 2026-08-27 14:58 UTCGHSA enrichment
  26. 2026-08-27 06:50 UTCGHSA enrichment
  27. 2026-08-26 18:24 UTCEG score recompute
  28. 2026-08-26 18:24 UTCGHSA enrichment
  29. 2026-08-26 18:04 UTCEG score recompute
  30. 2026-08-26 18:04 UTCGHSA enrichment
  31. 2026-08-26 17:25 UTCEG score recompute
  32. 2026-08-26 17:25 UTCGHSA enrichment
  33. 2026-08-26 14:47 UTCEPSS rescore
  34. 2026-08-26 06:26 UTCGHSA enrichment
  35. 2026-08-25 17:58 UTCEG score recompute
  36. 2026-08-25 17:57 UTCGHSA enrichment
  37. 2026-08-25 13:49 UTCEPSS rescore
  38. 2026-08-25 05:32 UTCGHSA enrichment
  39. 2026-08-24 17:07 UTCEG score recompute
  40. 2026-08-24 17:06 UTCGHSA enrichment
  41. 2026-08-24 04:41 UTCGHSA enrichment
  42. 2026-08-23 16:15 UTCGHSA enrichment
  43. 2026-08-23 03:49 UTCEG score recompute
  44. 2026-08-23 03:49 UTCGHSA enrichment
  45. 2026-08-22 15:24 UTCEG score recompute
  46. 2026-08-22 14:26 UTCEG score recompute
  47. 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 27, 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 69.9% 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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