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](/news/proxyshell-exploitation-analysis-1-cve-2021-34473). 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.