CVE-2021-26857

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 — 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: 96%CVSS: 7.8Exploit: cisa kev · ssvc active · kev ransomware · 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 Remote Code Execution Vulnerability

CVSS v3
7.8
EG Score
9.0(high)
EG Risk
81(Attend)
EG Risk 81/100SSVC: 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%
Automatability0% × 15%
Action: Remediate soon — notable exploitation risk.
EPSS PROB
96%
EPSS %ILE
100%
KEV
⚠ Exploited

Published

March 3, 2021

Last Modified

August 19, 2026

Advisory Details (2)

Auto-updated Jun 3, 2026
⚠️ Active exploitation confirmed. Patch available. Sources: cisa, microsoft.
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-2021-26857
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-2021-26857

Vendor Advisories for CVE-2021-26857(1)

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

Weakness Classification(2)

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 85× in last 7d / 357× 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 1,285 total refreshes for this CVE.

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

Publicly available exploits

(1 reference)

Working exploit code is in the public domain (1 GitHub PoC). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • GitHub PoCsirpedrotavares/Proxylogon-exploit
    First seen Mar 11, 2021

    proxylogon exploit - CVE-2021-26857

    Open source ↗

Frequently asked(6)

What is CVE-2021-26857?
CVE-2021-26857 is a critical vulnerability published on March 3, 2021. Microsoft Exchange Server Remote Code Execution Vulnerability
When was CVE-2021-26857 disclosed?
CVE-2021-26857 was first published in the National Vulnerability Database on March 3, 2021, with the most recent update on August 19, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-26857 actively exploited?
Yes. CISA added CVE-2021-26857 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 patching attention.
What is the CVSS score of CVE-2021-26857?
CVE-2021-26857 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.
Which products are affected by CVE-2021-26857?
CVE-2021-26857 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-2021-26857?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-26857, 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-2021-26857

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