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 — 5 sources updated this weekExploited in the wild
9.8EG
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS PROB: 100%CVSS: 9.8Exploit: cisa kev · ssvc active · kev ransomware · Metasploit · 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
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 PROB
100%
EPSS %ILE
100%
KEV
⚠ Exploited

Published

March 3, 2021

Last Modified

August 19, 2026

Advisory Details (7)

Auto-updated Sep 16, 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-2021-26855
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 Security

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

Packet Storm Security

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

Packet Storm Security

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

Packet Storm Security

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

Security Update Guide - Microsoft Security Response Center

Security Update Guide - Microsoft Security Response Center. Patch available via Microsoft Security Update

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855

Vendor Advisories for CVE-2021-26855(1)

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

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 94× in last 7d / 382× 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 1,373 total refreshes for this CVE.

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

Publicly available exploits

(10 references)

Working exploit code is in the public domain (8 GitHub PoCs) (2 Exploit-DB entries). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • GitHub PoCssrsec/Microsoft-Exchange-RCE
    First seen Feb 2, 2023

    Microsoft Exchange CVE-2021-26855&CVE-2021-27065

    Open source ↗
  • GitHub PoCkh4sh3i/ProxyLogon
    First seen Jun 27, 2022

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

    Open source ↗
  • GitHub PoCthau0x01/poc_proxylogon
    First seen Dec 4, 2021

    Microsoft Exchange ProxyLogon PoC (CVE-2021-26855)

    Open source ↗
  • 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 ↗
  • GitHub PoCZephrFish/Exch-CVE-2021-26855_Priv
    First seen Mar 15, 2021

    patched to work

    Open source ↗

Related CVEs(same vendor + same CWE)

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 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-2021-26855 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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