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 — 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: 96%CVSS: 7.8Exploit: cisa kev · ssvc active · kev ransomware · epss top5pct · epss high · public exploitExposed 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
81
EG Risk 81/100CISA 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%
Automatability0% × 15%
CISA SSVC: Track at low, Attend at medium and Act at high mission impact.
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 within your standard update timelines at low, sooner than that at medium and as soon as possible at high mission impact; until then, restrict network exposure of the affected system or apply the vendor's mitigation.
EPSS PROB
96%
EPSS %ILE
99th
KEV
⚠ Exploited

CISA SSVCTrack at low, Attend at medium and Act at high mission impact.

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 within your standard update timelines at low, sooner than that at medium and as soon as possible at high mission impact; until then, restrict network exposure of the affected system or apply the vendor's mitigation.

Exploitation active (CISA Vulnrichment) · Automatable no (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

August 19, 2026

Advisory Details (2)

Auto-updated Jun 3, 2026
⚠️ Active exploitation confirmed. Fix referenced — no release confirmed yet. Sources: cisa, microsoft.
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-2021-26857
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-2021-26857

Vendor Advisories for CVE-2021-26857(2)

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.

Data Freshness Timeline

(refreshed 80× in last 7d / 353× 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 1,541 total refreshes for this CVE.

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

Publicly available exploits

(1 reference)

Public exploit code is referenced for this CVE (1 GitHub PoC). Defenders should treat patch urgency accordingly.

  • GitHub PoCsirpedrotavares/Proxylogon-exploit
    Published Mar 11, 2021

    proxylogon exploit - CVE-2021-26857

    Open source ↗

Related CVEs(same CWE)

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 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 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-2021-26857?
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 (CRITICAL).
Which products are affected by CVE-2021-26857?
CVE-2021-26857 affects Microsoft Exchange Server.
How do I remediate CVE-2021-26857?
A fix for CVE-2021-26857 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-2021-26857 are linked in the Vendor Advisories panel on this page.

Dependency Blast Radius

Explore the affected products and dependency analysis for CVE-2021-26857

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