CVE-2021-26855
Published: November 3, 2021
Official Description
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
CISA KEV Advisory
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
Apply updates per vendor instructions.
Risk Analysis
Microsoft Exchange Server has an unspecified vulnerability that allows for remote code execution. This vulnerability is a component of the ProxyLogon exploit chain, and its high EPSS score of 0.94308 signifies a very high likelihood of exploitation.
This vulnerability is actively exploited in the wild as part of the ProxyLogon exploit chain and is listed in CISA's KEV catalog. This indicates a severe and immediate threat, as it is remotely exploitable.
Immediate application of all available security updates for Microsoft Exchange Server is critical. Organizations should also implement robust network segmentation and monitoring for Exchange servers.
Technical Analysis
CVE-2021-26855 requires local access, meaning attackers must already have a foothold on the target system.
Exploitation requires some privileges, which limits the exposure to scenarios where an attacker has already gained initial access.
CISA has added CVE-2021-26855 to the Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. U.S. federal agencies are required to patch this within the mandated timeframe, and all organizations should treat remediation as urgent.
Affected Vendors & Products
Exploit & PoC Resources
Quick Facts
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2021-26855 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts
- !CISA KEV: Federal agencies must patch per BOD 22-01 timeline
- !Active exploitation confirmed — treat as P1