CVE-2021-26855: listed in the CISA KEV catalog

CVE-2021-26855 · CVSS 9.1 CRITICAL · EPSS 100.0% · KEV 2021-11-03

Microsoft Exchange Server Remote Code Execution Vulnerability

Is CVE-2021-26855 exploited?

• Listed in the CISA KEV catalog on 2021-11-03.
• Federal remediation due 2022-05-03.
• Past that date by 1565 days.
• Used in ransomware campaigns.
• EPSS puts exploitation in the next 30 days at 100.0%.
• Public exploit code: packaged in a public tool.
• Public detection rules exist.

Which products and versions are affected?

No affected package list recorded here yet.

Is there a patch?

No patch identifier recorded here yet.

What PlainSec published about CVE-2021-26855

• Public Exploits Now Feed a Custom Cobalt Strike Loader (2026-06-27)
• ShadowPad Espionage Campaign Targets Exchange and IIS Servers (2026-05-05)

Primary sources

• NVD record
• CISA KEV catalog
• Metasploit: exploit/windows/http/exchange_proxylogon_rce
• Metasploit: auxiliary/scanner/http/exchange_proxylogon
• Exploit-DB 49895
• Exploit-DB 49879
• Exploit-DB 49663
• Exploit-DB 49637
• Nuclei template

What this record does not say

• No affected package data.
• No patch identifier.

KEV and EPSS are re-checked daily. Record last updated 2026-08-11.