Hi Shane,
All Telerik UI for ASP.NET AJAX versions prior to R1 2020 (2020.1.114) are vulnerable to some pretty severe vulnerabilities. You can find detailed information about them in the following articles:
The only way to prevent the vulnerabilities is to upgrade to R1 2020 (2020.1.114) or better the latest R3 2023 (2023.3.1010) version (which also offers support for the latest browsers, many new components, features, improvements, WCAG 2.1 accessibility, and security improvements).
Best
Regards,
Rumen
Progress Telerik