CVE-2021-26855, also known as Proxylogon, is a server-side request forgery (SSRF) vulnerability in Exchange that allows an attacker to send arbitrary HTTP requests and authenticate as the Exchange server. According to Orange Tsai, the researcher who discovered the vulnerabilities, CVE-2021-26855 allows code execution when chained with CVE-2021-27065 (see below). A successful exploit chain would allow an unauthenticated attacker to "execute arbitrary commands on Microsoft Exchange Server through only an open 443 port." More information and a disclosure timeline are available at https://proxylogon.com.