CVE-2021-27056 is a vulnerability that affects the Microsoft Exchange Server. This flaw allows an attacker to execute arbitrary code on the server with SYSTEM privileges, which means they can take full control of the system. The vulnerability was caused by improper validation of user input, specifically in the Exchange Control Panel (ECP) component.
The vulnerability was discovered and reported to Microsoft by researchers from the National Security Agency (NSA), who classified it as a critical vulnerability. Microsoft released a security update to address the vulnerability on April 13, 2021, as part of its monthly Patch Tuesday updates. The update fixed four vulnerabilities in total, including CVE-2021-27056.
However, even after the patch was released, many organizations were still vulnerable to attacks exploiting this flaw. This was due to several factors, including delayed patching, insufficient awareness, and the complexity of updating large-scale Exchange Server environments. As a result, attackers were able to exploit the vulnerability and gain access to sensitive data and systems for several months after the patch was released. In response, Microsoft released additional guidance and tools to help organizations protect themselves against these attacks.