Angular: SSR XSS via Unescaped <template> Content Across DocumentFragment Boundaries in Fallback Raw-Content Elements

Angular is a development platform for building mobile and desktop web applications using TypeScript/JavaScript and other languages. Prior to 20.3.30, 21.2.22, and 22.1.4, Angular server-side rendering (SSR) in @angular/platform-server serializes untrusted input inside template content nested in fallback raw-content elements such as noscript, iframe, noembed, and noframes. The Domino serializer's fallbackRawContentTags traversal stopped at the DocumentFragment used by template.content, so matching closing tags in xmp, style, script, comments, or text nodes were not escaped. Standard interpolation with comments or text nodes is reachable without relaxed schemas; literal xmp or style requires CUSTOM_ELEMENTS_SCHEMA or NO_ERRORS_SCHEMA, while Renderer2 imperative DOM construction is unconditionally affected. When HTML5 RAWTEXT browser parsing encounters the unescaped closing tag, it exits the fallback container and interprets trailing markup as active DOM elements, enabling arbitrary JavaScript execution. This issue is fixed in versions 20.3.30, 21.2.22, and 22.1.4.