CVE-2021-26855: 
vulnerability analysis and mitigation

CVE-2021-26855, also known as ProxyLogon, is a Microsoft Exchange Server Remote Code Execution Vulnerability that was disclosed on March 2, 2021. This critical vulnerability affects multiple versions of Microsoft Exchange Server, including versions 2013, 2016, and 2019. The vulnerability is part of a chain of exploits targeting Exchange Servers and has been assigned a CVSS v3.1 base score of 9.8 (Critical) by NIST (NVD).

The vulnerability is classified as a Server-Side Request Forgery (SSRF) vulnerability (CWE-918). It allows remote attackers to execute arbitrary code on affected Exchange Servers without authentication. The CVSS vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) indicates that the vulnerability can be exploited over the network with no privileges or user interaction required, and can result in high impacts to confidentiality, integrity, and availability (NVD).

The vulnerability enables attackers to gain complete access to Exchange Servers, potentially allowing them to read emails, download attachments, and gain persistent system access. When successfully exploited, it can lead to unauthorized access to email accounts and potential remote code execution on the affected server (NVD).

Microsoft has released security updates to address this vulnerability. Organizations are strongly advised to apply the security update KB5000871 to affected Exchange Server versions. The patch is available for Exchange Server 2013, 2016, and 2019 through various cumulative updates. Microsoft recommends prioritizing the updates for internet-facing Exchange Servers (Microsoft Support).