CVE-2021-26855

CRITICALNVD 9.89.8—
EchelonGraph scoreHIGH confidence

Score elevated to 9.8 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 9.8 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
9.8
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS: 100%CVSS: 9.8Exploit: NoneExposed: 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
9.8
EG Score
9.8(high)
EG Risk
99(Act)
EG Risk 99/100SSVC: Act

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
Severity98% × 45%
Exploitation100% × 40%
Automatability100% × 15%
Action: Fix now — active exploitation, automatable, high impact.
EPSS
100.0%
KEV
⚠ Exploited

Published

March 3, 2021

Last Modified

June 17, 2026

Advisory Details (5)

Auto-updated Jun 2, 2026
Patch available. Sources: microsoft.
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-2021-26855
generic

Packet Storm

http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
generic

Packet Storm

http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
generic

Packet Storm

http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
generic

Packet Storm

http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html

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 80× in last 7d / 371× 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 669 total refreshes for this CVE.

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

Publicly available exploits

(10 references)

Working exploit code is in the public domain (6 GitHub PoCs) (4 Exploit-DB entries). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • Exploit-DBEDB-49895
    First seen May 21, 2021

    Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit)

    Open source ↗
  • Exploit-DBEDB-49879
    First seen May 18, 2021

    Microsoft Exchange 2019 - Unauthenticated Email Download

    Open source ↗
  • GitHub PoChosch3n/ProxyVulns
    First seen Apr 14, 2021

    [ProxyLogon] CVE-2021-26855 & CVE-2021-27065 Fixed RawIdentity Bug Exploit. [ProxyOracle] CVE-2021-31195 & CVE-2021-31196 Exploit Chains. [ProxyShell] CVE-2021-34473 & CVE-2021-34523 & CVE-2021-31207 Exploit Chains.

    Open source ↗
  • GitHub PoCFlangvik/SharpProxyLogon
    First seen Mar 29, 2021

    C# POC for CVE-2021-26855 aka ProxyLogon, supports the classically semi-interactive web shell as well as shellcode injection

    Open source ↗
  • GitHub PoCpraetorian-inc/proxylogon-exploit
    First seen Mar 24, 2021

    Proof-of-concept exploit for CVE-2021-26855 and CVE-2021-27065. Unauthenticated RCE in Exchange.

    Open source ↗
  • GitHub PoCp0wershe11/ProxyLogon
    First seen Mar 17, 2021

    ProxyLogon(CVE-2021-26855+CVE-2021-27065) Exchange Server RCE(SSRF->GetWebShell)

    Open source ↗
  • Exploit-DBEDB-49663
    First seen Mar 14, 2021

    Microsoft Exchange 2019 - Server-Side Request Forgery

    Open source ↗
  • Exploit-DBEDB-49637
    First seen Mar 11, 2021

    Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC)

    Open source ↗
  • GitHub PoChackerschoice/CVE-2021-26855
    First seen Mar 11, 2021

    PoC of proxylogon chain SSRF(CVE-2021-26855) to write file by testanull, censored by github

    Open source ↗
  • GitHub PoCalt3kx/CVE-2021-26855_PoC
    First seen Mar 10, 2021
    Open source ↗

Frequently asked(6)

What is CVE-2021-26855?
CVE-2021-26855 is a critical vulnerability published on March 3, 2021. Microsoft Exchange Server Remote Code Execution Vulnerability
When was CVE-2021-26855 disclosed?
CVE-2021-26855 was first published in the National Vulnerability Database on March 3, 2021, with the most recent update on June 17, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-26855 actively exploited?
Yes. CISA added CVE-2021-26855 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-2021-26855?
CVE-2021-26855 has a CVSS v3 base score of 9.8 (NVD).
Which products are affected by CVE-2021-26855?
CVE-2021-26855 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-2021-26855?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-26855, 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-2021-26855

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