CVE-2021-26855
KEVRansomware UseMicrosoft Exchange Server Remote Code Execution Vulnerability
Description
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
Microsoft Exchange Server Remote Code Execution Vulnerability
CVSS 3.1 vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NPublishedMar 3, 2021
Last modifiedAug 19, 2026
Added to KEVNov 3, 2021
Remediation dueMay 3, 2022
References
- msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855PatchVendor Advisory
- packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.htmlExploitThird Party Advisory
- packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.htmlExploitThird Party Advisory
- packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.htmlExploitThird Party Advisory
- packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.htmlExploitThird Party Advisory
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855PatchVendor Advisory
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855US Government Resource
0 recent articles mention this CVE
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Risk assessment · Critical
Actively exploited and used by ransomware crews. Patch immediately.
- Exploitation
- Confirmed in the wild (CISA KEV)
- Federal remediation deadline
- Overdue by 1637 days
- Ransomware
- Used in known ransomware campaigns
- CVSS
- 9.1 CRITICAL
- EPSS (30-day exploit probability)
- 100.0% · top 1% of all CVEs
- Weakness
- CWE-918
- Attack vector
- Network, remotely reachable
- Preconditions
- No privileges and no user interaction
Recommended actions
- 1.Apply updates per vendor instructions.
- 2.Treat as emergency change: patch or isolate internet-facing instances first.
- 3.Review exposure: confirm which instances are reachable from the internet and restrict access until patched.
- 4.Hunt for compromise: review logs for the disclosure window, not just from the patch date.
- 5.Verify offline/immutable backups and segment the affected systems. This flaw is used for ransomware initial access.
Assessment is derived automatically from the live signals above; verify against your own asset inventory.