November 3, 2021 · Applied Cybernetics Group
CVE-2021-26857 — Microsoft Exchange Server
known ransomware use
Microsoft Exchange Server Remote Code Execution Vulnerability
- Added to KEV
2021-11-03- Federal due date
2022-05-03- Vendor
- Microsoft
- Product
- Exchange Server
- EPSS
- 99.9th percentile (score 0.958, as of
2026-09-16) - NVD CVSS v3.1
- 7.8 (HIGH)
- Ransomware use
- Known
- ATT&CK
- T1133 , T1505.003 · signal rollup
- Upstream
- https://nvd.nist.gov/vuln/detail/CVE-2021-26857
CISA short description
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
Required action
Apply updates per vendor instructions.
NVD description
Microsoft Exchange Server Remote Code Execution Vulnerability