What is the core vulnerability in CVE-2021-34473 and how does it function?

CVE-2021-34473 is a Server-Side Request Forgery (SSRF) vulnerability in Microsoft Exchange that serves as the foundation of the ProxyShell attack chain. It allows an unauthenticated attacker to make the Exchange server send requests to internal resources, such as the Exchange Web Service (EWS) or the /mapi/nspi endpoint, by crafting a specially formatted URL to the /autodiscover/autodiscover.json endpoint with a parameter like `Email=autodiscover/autodiscover.json%[email protected]`. The SSRF runs with SYSTEM privileges, enabling further exploitation.