CVE-2021-26858

CRITICALNVD 7.89.0▲
EchelonGraph scoreHIGH confidence

Score elevated to 9.0 because this CVE is listed on the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating real-world exploitation has been confirmed by US federal agencies. NVD baseline CVSS 7.8 retained for reference. Confidence: HIGH.

Triggered by: CISA KEV (actively exploited)
Sources: cisa_kev, epss, ghsa, nvd
Trending — 4 sources updated this weekExploited in the wild
9.0EG
EchelonGraph verdictMitigate nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS PROB: 94%CVSS: 7.8Exploit: cisa kev · ssvc active · kev ransomware · epss top5pct · epss highExposed services: Not assessed

A fix is referenced (a reference tagged "Patch", often a source commit), but no release containing it is confirmed — find the release that contains it; until it is applied, mitigate (WAF / firewall / segmentation).

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS v3
7.8
EG Score
9.0CRITICALhigh confidence
EG Risk
96
EG Risk 96/100CISA SSVC

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%
CISA SSVC: Attend at low mission impact; Act at medium or high.
Action: A fix is referenced, but no release containing it is confirmed. Check the referenced fix and the vendor's advisory for a release that contains it and apply that sooner than your standard update timelines at low mission impact and as soon as possible at medium or high; until then, restrict network exposure of the affected system or apply the vendor's mitigation.
EPSS PROB
94%
EPSS %ILE
99th
KEV
⚠ Exploited

CISA SSVCAttend at low mission impact; Act at medium or high.

A fix is referenced, but no release containing it is confirmed. Check the referenced fix and the vendor's advisory for a release that contains it and apply that sooner than your standard update timelines at low mission impact and as soon as possible at medium or high; until then, restrict network exposure of the affected system or apply the vendor's mitigation.

Exploitation active (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact total (CISA Vulnrichment). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

March 3, 2021

Last Modified

October 1, 2026

Advisory Details (2)

Auto-updated Jun 4, 2026
⚠️ Active exploitation confirmed. Fix referenced — no release confirmed yet. Sources: microsoft, cisa.
cisa🔴 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-2021-26858
microsoftMentions a patch (unverified)

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-2021-26858

Vendor Advisories for CVE-2021-26858(2)

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

Data Freshness Timeline

(refreshed 85× in last 7d / 367× 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 1,559 total refreshes for this CVE.

  1. 2026-10-06 21:24 UTCVendor advisory
  2. 2026-10-06 21:24 UTCGHSA enrichment
  3. 2026-10-06 17:16 UTCVendor advisory
  4. 2026-10-06 17:16 UTCGHSA enrichment
  5. 2026-10-06 13:08 UTCVendor advisory
  6. 2026-10-06 13:08 UTCGHSA enrichment
  7. 2026-10-06 09:00 UTCVendor advisory
  8. 2026-10-06 09:00 UTCGHSA enrichment
  9. 2026-10-06 04:52 UTCVendor advisory
  10. 2026-10-06 04:51 UTCGHSA enrichment
  11. 2026-10-06 00:44 UTCVendor advisory
  12. 2026-10-06 00:43 UTCGHSA enrichment
  13. 2026-10-05 20:35 UTCVendor advisory
  14. 2026-10-05 20:35 UTCGHSA enrichment
  15. 2026-10-05 16:27 UTCVendor advisory
  16. 2026-10-05 16:27 UTCGHSA enrichment
  17. 2026-10-05 12:19 UTCVendor advisory
  18. 2026-10-05 12:19 UTCGHSA enrichment
  19. 2026-10-05 08:12 UTCVendor advisory
  20. 2026-10-05 08:12 UTCGHSA enrichment
  21. 2026-10-05 04:04 UTCVendor advisory
  22. 2026-10-05 04:04 UTCGHSA enrichment
  23. 2026-10-04 23:55 UTCEG score recompute
  24. 2026-10-04 23:55 UTCVendor advisory
  25. 2026-10-04 23:54 UTCGHSA enrichment
Show 75 more
  1. 2026-10-04 23:19 UTCEPSS rescore
  2. 2026-10-04 19:47 UTCVendor advisory
  3. 2026-10-04 19:47 UTCGHSA enrichment
  4. 2026-10-04 15:39 UTCVendor advisory
  5. 2026-10-04 15:39 UTCGHSA enrichment
  6. 2026-10-04 11:30 UTCVendor advisory
  7. 2026-10-04 11:30 UTCGHSA enrichment
  8. 2026-10-04 07:22 UTCVendor advisory
  9. 2026-10-04 07:22 UTCGHSA enrichment
  10. 2026-10-04 03:14 UTCVendor advisory
  11. 2026-10-04 03:14 UTCGHSA enrichment
  12. 2026-10-03 23:06 UTCVendor advisory
  13. 2026-10-03 23:06 UTCGHSA enrichment
  14. 2026-10-03 18:55 UTCVendor advisory
  15. 2026-10-03 18:55 UTCGHSA enrichment
  16. 2026-10-03 14:47 UTCVendor advisory
  17. 2026-10-03 14:47 UTCGHSA enrichment
  18. 2026-10-03 10:39 UTCVendor advisory
  19. 2026-10-03 10:39 UTCGHSA enrichment
  20. 2026-10-03 06:29 UTCVendor advisory
  21. 2026-10-03 06:29 UTCGHSA enrichment
  22. 2026-10-03 02:20 UTCVendor advisory
  23. 2026-10-03 02:20 UTCGHSA enrichment
  24. 2026-10-02 22:10 UTCVendor advisory
  25. 2026-10-02 22:10 UTCGHSA enrichment
  26. 2026-10-02 18:02 UTCVendor advisory
  27. 2026-10-02 18:02 UTCGHSA enrichment
  28. 2026-10-02 13:53 UTCVendor advisory
  29. 2026-10-02 13:53 UTCGHSA enrichment
  30. 2026-10-02 09:44 UTCVendor advisory
  31. 2026-10-02 09:44 UTCGHSA enrichment
  32. 2026-10-02 05:36 UTCVendor advisory
  33. 2026-10-02 05:36 UTCGHSA enrichment
  34. 2026-10-02 01:28 UTCVendor advisory
  35. 2026-10-02 01:28 UTCGHSA enrichment
  36. 2026-10-01 21:20 UTCVendor advisory
  37. 2026-10-01 21:20 UTCGHSA enrichment
  38. 2026-10-01 20:22 UTCVendor advisory
  39. 2026-10-01 20:22 UTCGHSA enrichment
  40. 2026-10-01 19:52 UTCVendor advisory
  41. 2026-10-01 19:52 UTCGHSA enrichment
  42. 2026-10-01 15:44 UTCVendor advisory
  43. 2026-10-01 15:44 UTCGHSA enrichment
  44. 2026-10-01 11:36 UTCVendor advisory
  45. 2026-10-01 11:36 UTCGHSA enrichment
  46. 2026-10-01 07:26 UTCVendor advisory
  47. 2026-10-01 07:26 UTCGHSA enrichment
  48. 2026-10-01 03:18 UTCVendor advisory
  49. 2026-10-01 03:18 UTCGHSA enrichment
  50. 2026-09-30 23:10 UTCVendor advisory
  51. 2026-09-30 23:10 UTCGHSA enrichment
  52. 2026-09-30 19:01 UTCGHSA enrichment
  53. 2026-09-30 14:53 UTCVendor advisory
  54. 2026-09-30 14:52 UTCGHSA enrichment
  55. 2026-09-30 10:44 UTCVendor advisory
  56. 2026-09-30 10:44 UTCGHSA enrichment
  57. 2026-09-30 06:36 UTCVendor advisory
  58. 2026-09-30 06:36 UTCGHSA enrichment
  59. 2026-09-30 02:27 UTCVendor advisory
  60. 2026-09-30 02:27 UTCGHSA enrichment
  61. 2026-09-29 22:19 UTCVendor advisory
  62. 2026-09-29 22:19 UTCGHSA enrichment
  63. 2026-09-29 18:07 UTCVendor advisory
  64. 2026-09-29 18:07 UTCGHSA enrichment
  65. 2026-09-29 13:54 UTCVendor advisory
  66. 2026-09-29 13:54 UTCGHSA enrichment
  67. 2026-09-29 09:46 UTCVendor advisory
  68. 2026-09-29 09:46 UTCGHSA enrichment
  69. 2026-09-29 05:38 UTCVendor advisory
  70. 2026-09-29 05:38 UTCGHSA enrichment
  71. 2026-09-29 01:30 UTCEG score recompute
  72. 2026-09-29 01:30 UTCVendor advisory
  73. 2026-09-29 01:30 UTCGHSA enrichment
  74. 2026-09-28 21:22 UTCVendor advisory
  75. 2026-09-28 21:22 UTCGHSA enrichment

Frequently asked(6)

What is CVE-2021-26858?
CVE-2021-26858 is a critical vulnerability published on March 3, 2021. Microsoft Exchange Server Remote Code Execution Vulnerability
When was CVE-2021-26858 disclosed?
CVE-2021-26858 was first published on March 3, 2021, with the most recent update on October 1, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-26858 actively exploited?
Yes. CISA added CVE-2021-26858 to the Known Exploited Vulnerabilities catalog on November 3, 2021, affecting Microsoft Exchange Server. KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate attention. A fix is referenced, but no release containing it is confirmed. Check the referenced fix and the vendor's advisory for a release that contains it and apply that; until then, restrict network exposure of the affected system or apply the vendor's mitigation.
What is the CVSS score of CVE-2021-26858?
CVE-2021-26858 has a CVSS v3 base score of 7.8 (NVD). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 9.0 (CRITICAL).
Which products are affected by CVE-2021-26858?
CVE-2021-26858 affects Microsoft Exchange Server.
How do I remediate CVE-2021-26858?
A fix for CVE-2021-26858 is referenced (a reference tagged "Patch", often a source commit), but EchelonGraph has no release containing it on record. Check that reference and the vendor's advisory for a release that contains the fix and update to it; until then, restrict network exposure of the affected system or apply the vendor's mitigation. The vendor advisories EchelonGraph has for CVE-2021-26858 are linked in the Vendor Advisories panel on this page.

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