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: 94%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
94%
EPSS %ILE
100%
KEV
⚠ Exploited

Published

March 3, 2021

Last Modified

December 18, 2025

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

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 52× in last 7d / 313× 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 655 total refreshes for this CVE.

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

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 December 18, 2025. 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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