CVE-2021-33766

CRITICALNVD 7.59.0▲
EchelonGraph scoreHIGH confidence

Score elevated to 9.0 because this CVE is listed on the CISA Known Exploited Vulnerabilities catalog (added 2022-01-18), indicating real-world exploitation has been confirmed by US federal agencies. NVD baseline CVSS 7.5 retained for reference. Confidence: HIGH.

Triggered by: CISA KEV (actively exploited)
Sources: cisa_kev, epss, ghsa, nvd
Trending — 5 sources updated this weekExploited in the wild
9.0EG
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS PROB: 98%CVSS: 7.5Exploit: cisa kev · ssvc active · epss top5pct · epss high · public exploitExposed: 0

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

Microsoft Exchange Server Information Disclosure Vulnerability

CVSS v3
7.5
EG Score
9.0(high)
EG Risk
96(Attend)
EG Risk 96/100SSVC: 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
Severity90% × 45%
Exploitation100% × 40%
Automatability100% × 15%
Action: Remediate soon — notable exploitation risk.
EPSS PROB
98%
EPSS %ILE
100%
KEV
⚠ Exploited

Published

July 14, 2021

Last Modified

August 10, 2026

Advisory Details (3)

Auto-updated Jun 11, 2026
⚠️ Active exploitation confirmed. Patch available. Sources: microsoft, cisa.
cisa Patch Available🔴 Active Exploitation

Known Exploited Vulnerabilities Catalog | CISA

Known Exploited Vulnerabilities Catalog | CISA. Listed in CISA Known Exploited Vulnerabilities catalog.

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766
generic

ZDI-21-798 | Zero Day Initiative

https://www.zerodayinitiative.com/advisories/ZDI-21-798/
microsoft Patch Available

Security Update Guide - Microsoft Security Response Center

Security Update Guide - Microsoft Security Response Center. Patch available via Microsoft Security Update

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766

Vendor Advisories for CVE-2021-33766(1)

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

Weakness Classification(1)

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

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 91× in last 7d / 374× 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.

Showing the most recent 100 of 1,434 total refreshes for this CVE.

  1. 2026-09-25 16:11 UTCVendor advisory
  2. 2026-09-25 16:11 UTCGHSA enrichment
  3. 2026-09-25 14:55 UTCCISA KEV update
  4. 2026-09-25 12:13 UTCVendor advisory
  5. 2026-09-25 12:13 UTCGHSA enrichment
  6. 2026-09-25 08:14 UTCVendor advisory
  7. 2026-09-25 08:14 UTCGHSA enrichment
  8. 2026-09-25 04:16 UTCVendor advisory
  9. 2026-09-25 04:16 UTCGHSA enrichment
  10. 2026-09-25 00:18 UTCVendor advisory
  11. 2026-09-25 00:18 UTCGHSA enrichment
  12. 2026-09-24 20:19 UTCVendor advisory
  13. 2026-09-24 20:19 UTCGHSA enrichment
  14. 2026-09-24 19:31 UTCCISA KEV update
  15. 2026-09-24 16:20 UTCVendor advisory
  16. 2026-09-24 16:20 UTCGHSA enrichment
  17. 2026-09-24 12:22 UTCVendor advisory
  18. 2026-09-24 12:22 UTCGHSA enrichment
  19. 2026-09-24 08:23 UTCVendor advisory
  20. 2026-09-24 08:23 UTCGHSA enrichment
  21. 2026-09-24 04:25 UTCVendor advisory
  22. 2026-09-24 04:25 UTCGHSA enrichment
  23. 2026-09-24 00:26 UTCVendor advisory
  24. 2026-09-24 00:26 UTCGHSA enrichment
  25. 2026-09-23 20:28 UTCVendor advisory
Show 75 more
  1. 2026-09-23 20:28 UTCGHSA enrichment
  2. 2026-09-23 16:30 UTCEG score recompute
  3. 2026-09-23 16:30 UTCVendor advisory
  4. 2026-09-23 16:29 UTCGHSA enrichment
  5. 2026-09-23 13:44 UTCEPSS rescore
  6. 2026-09-23 12:31 UTCVendor advisory
  7. 2026-09-23 12:31 UTCGHSA enrichment
  8. 2026-09-23 08:33 UTCVendor advisory
  9. 2026-09-23 08:33 UTCGHSA enrichment
  10. 2026-09-23 04:34 UTCVendor advisory
  11. 2026-09-23 04:34 UTCGHSA enrichment
  12. 2026-09-23 00:35 UTCVendor advisory
  13. 2026-09-23 00:35 UTCGHSA enrichment
  14. 2026-09-22 20:37 UTCVendor advisory
  15. 2026-09-22 20:36 UTCGHSA enrichment
  16. 2026-09-22 19:36 UTCCISA KEV update
  17. 2026-09-22 16:38 UTCVendor advisory
  18. 2026-09-22 16:38 UTCGHSA enrichment
  19. 2026-09-22 12:39 UTCVendor advisory
  20. 2026-09-22 12:39 UTCGHSA enrichment
  21. 2026-09-22 08:40 UTCVendor advisory
  22. 2026-09-22 08:40 UTCGHSA enrichment
  23. 2026-09-22 04:41 UTCVendor advisory
  24. 2026-09-22 04:41 UTCGHSA enrichment
  25. 2026-09-22 00:42 UTCVendor advisory
  26. 2026-09-22 00:42 UTCGHSA enrichment
  27. 2026-09-21 20:44 UTCVendor advisory
  28. 2026-09-21 20:44 UTCGHSA enrichment
  29. 2026-09-21 19:09 UTCCISA KEV update
  30. 2026-09-21 16:45 UTCVendor advisory
  31. 2026-09-21 16:45 UTCGHSA enrichment
  32. 2026-09-21 12:47 UTCVendor advisory
  33. 2026-09-21 12:47 UTCGHSA enrichment
  34. 2026-09-21 08:49 UTCVendor advisory
  35. 2026-09-21 08:49 UTCGHSA enrichment
  36. 2026-09-21 04:50 UTCVendor advisory
  37. 2026-09-21 04:50 UTCGHSA enrichment
  38. 2026-09-21 00:48 UTCVendor advisory
  39. 2026-09-21 00:48 UTCGHSA enrichment
  40. 2026-09-20 20:49 UTCVendor advisory
  41. 2026-09-20 20:49 UTCGHSA enrichment
  42. 2026-09-20 16:51 UTCVendor advisory
  43. 2026-09-20 16:51 UTCGHSA enrichment
  44. 2026-09-20 12:53 UTCVendor advisory
  45. 2026-09-20 12:52 UTCGHSA enrichment
  46. 2026-09-20 08:53 UTCVendor advisory
  47. 2026-09-20 08:53 UTCGHSA enrichment
  48. 2026-09-20 04:54 UTCVendor advisory
  49. 2026-09-20 04:53 UTCGHSA enrichment
  50. 2026-09-20 00:54 UTCVendor advisory
  51. 2026-09-20 00:54 UTCGHSA enrichment
  52. 2026-09-19 20:56 UTCVendor advisory
  53. 2026-09-19 20:56 UTCGHSA enrichment
  54. 2026-09-19 16:58 UTCVendor advisory
  55. 2026-09-19 16:58 UTCGHSA enrichment
  56. 2026-09-19 12:57 UTCVendor advisory
  57. 2026-09-19 12:57 UTCGHSA enrichment
  58. 2026-09-19 08:59 UTCVendor advisory
  59. 2026-09-19 08:59 UTCGHSA enrichment
  60. 2026-09-19 05:00 UTCVendor advisory
  61. 2026-09-19 05:00 UTCGHSA enrichment
  62. 2026-09-19 01:01 UTCVendor advisory
  63. 2026-09-19 01:01 UTCGHSA enrichment
  64. 2026-09-18 21:03 UTCVendor advisory
  65. 2026-09-18 21:03 UTCGHSA enrichment
  66. 2026-09-18 19:20 UTCCISA KEV update
  67. 2026-09-18 17:05 UTCVendor advisory
  68. 2026-09-18 17:04 UTCGHSA enrichment
  69. 2026-09-18 14:15 UTCCISA KEV update
  70. 2026-09-18 13:05 UTCVendor advisory
  71. 2026-09-18 13:05 UTCGHSA enrichment
  72. 2026-09-18 09:07 UTCVendor advisory
  73. 2026-09-18 09:07 UTCGHSA enrichment
  74. 2026-09-18 05:08 UTCVendor advisory
  75. 2026-09-18 05:08 UTCGHSA enrichment

Publicly available exploits

(3 references)

Working exploit code is in the public domain (2 GitHub PoCs). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • GitHub PoCdemossl/CVE-2021-33766-ProxyToken
    First seen Sep 15, 2021

    CVE-2021-33766-poc

    Open source ↗
  • GitHub PoCbhdresh/CVE-2021-33766
    First seen Aug 31, 2021

    ProxyToken (CVE-2021-33766) : An Authentication Bypass in Microsoft Exchange Server POC exploit

    Open source ↗
  • Nucleihttp/cves/2021/CVE-2021-33766.yaml
    First seen Jan 1, 2021

    Microsoft Exchange - Authentication Bypass

    Open source ↗

Related CVEs(same CWE)

Frequently asked(6)

What is CVE-2021-33766?
CVE-2021-33766 is a critical vulnerability published on July 14, 2021. Microsoft Exchange Server Information Disclosure Vulnerability
When was CVE-2021-33766 disclosed?
CVE-2021-33766 was first published in the National Vulnerability Database on July 14, 2021, with the most recent update on August 10, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-33766 actively exploited?
Yes. CISA added CVE-2021-33766 to the Known Exploited Vulnerabilities catalog on January 18, 2022, affecting Microsoft Exchange Server. KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate patching attention.
What is the CVSS score of CVE-2021-33766?
CVE-2021-33766 has a CVSS v3 base score of 7.5 (NVD). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 9.0.
Which products are affected by CVE-2021-33766?
CVE-2021-33766 affects Microsoft Exchange Server. The full affected-products list, including version ranges and fixed versions, is shown in the Affected Packages section of this page.
How do I remediate CVE-2021-33766?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-33766, 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

Explore the affected products and dependency analysis for CVE-2021-33766

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