CVE-2021-26858
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
This unspecified vulnerability in Microsoft Exchange Server allows for remote code execution and is part of the critical ProxyLogon exploit chain. Its inclusion in the KEV catalog and a high EPSS score indicate a significant and confirmed risk of exploitation.
This vulnerability is actively being exploited in the wild and is listed in CISA's KEV catalog, confirming its real-world exploitation.
Apply all available security updates and patches for Microsoft Exchange Server immediately. Implement strong network segmentation and monitor Exchange servers for any signs of compromise.
Technical Analysis
CVE-2021-26858 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-2021-26858 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-2021-26858 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