A Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF) vulnerability exists when Power BI Report Server Template file (pbix) containing HTML files is uploaded to the server and HTML files are accessed directly by the victim.
Combining these 2 vulnerabilities together, an attacker is able to upload malicious Power BI templates files to the server using the victim's session and run scripts in the security context of the user and perform privilege escalation in case the victim has admin privileges when the victim access one of the HTML files present in the malicious Power BI template uploaded.
The security update addresses the vulnerability by helping to ensure that Power BI Report Server properly sanitize file uploads.