Microsoft has released a security update for the Exchange Server that addresses four vulnerabilities with severity scores ranging from high to critical. All the security flaws lead to remote code execution on a vulnerable machine and were discovered and reported to Microsoft by the U.S. National Security Agency (NSA). Microsoft also found some of the flaws internally. The security flaws affect on-premises Exchange Server versions 2013 through 2019 and while there is no evidence of being exploited in the wild, Microsoft theorizes that threat actors are likely to leverage them as soon as they create an exploit. The NSA says that the discovery of critical vulnerabilities in the Microsoft Exchange server is recent and that they reported them immediately through the “disclosure process to secure the nation and our allies.” The four vulnerabilities received tracking numbers (CVE-2021-28480, CVE-2021-28481, CVE-2021-28482, CVE-2021-28483). The most severe of them have a Critical severity score of 9.8 out of 10 and could be exploited before authentication, according to Microsoft senior threat intelligence analyst Kevin Beaumont. Another critical one is 9/10, and the least severe one is 8.8/10. All of them lead to remote code execution and received patches through this month’s updates from Microsoft, which fix a total of 108 vulnerabilities, five of them being zero-days.