Angular is a development platform for building mobile and desktop web applications using TypeScript/JavaScript and other languages. Prior to 21.0.2, 20.3.15, and 19.2.17, A Stored Cross-Site Scripting (XSS) vulnerability has been identified in the Angular Template Compiler. It occurs because the compiler's internal security schema is incomplete, allowing attackers to bypass Angular's built-in security sanitization.

Specifically, the schema fails to classify certain URL-holding attributes (e.g., those that could contain javascript: URLs) as requiring strict URL security, enabling the injection of malicious scripts. This vulnerability is fixed in 21.0.2, 20.3.15, and 19.2.17.