Missing Fallback Raw-Content Serialization Escaping leads to Cross-Site Scripting (XSS) in Angular SSR

A Cross-Site Scripting (XSS) vulnerability exists in `@angular/platform-server`'s DOM emulation dependency (`domino`) when serializing the content of fallback raw-content elements (`<iframe>`, `<noembed>`, `<noframes>`, and `<noscript>`). When rendering dynamic text content inside fallback raw-content elements via template bindings, the template engine expects the browser to render the content safely. Under Server-Side Rendering (SSR), `domino` is configured with scripting enabled, meaning these elements are treated as raw-text elements. However, `domino`'s serializer previously did not escape text nodes within fallback raw-content elements (`<iframe>`, `<noembed>`, `<noframes>`, `<noscript>`) during DOM serialization. As a result, any occurrence of closing tags in the bound dynamic text was not escaped. The unescaped closing tag could be serialized directly into the output HTML. When parsed by a browser or re-parsed during SSR post-processing without preserving raw-content parser state, an injected closing tag closes the element early, allowing an injected script block to execute in the user's browser context, causing same-origin Cross-Site Scripting (XSS). ### Impact This vulnerability allows an attacker to perform same-origin Cross-Site Scripting (XSS) attacks against any user visiting an SSR-rendered page that binds user-controlled data inside fallback raw-content elements (`<iframe>`, `<noembed>`, `<noframes>`, `<noscript>`). This can lead to session hijacking, credentials theft, unauthorized actions on behalf of users, and defacement. ### Patched Versions - 22.0.7 - 21.2.19 - 20.3.27 ### Workarounds If you cannot immediately update your dependencies, you can mitigate this issue using any of the following approaches: - **Disable critical CSS inlining**: Critical CSS inlining in Angular SSR post-processes the rendered HTML using `domino`. Disabling this step prevents `domino` from re-parsing and re-serializing the HTML during server-side rendering. - In `angular.json`, set `inlineCritical` to `false` under style optimization options: ```json { "projects": { "my-app": { "architect": { "build": { "builder": "@angular/build:application", "options": { "optimization": { "styles": { "inlineCritical": false } } } } } } } } ``` - When rendering programmatically with `CommonEngine`, set `inlineCriticalCss: false` in your render options. - **Avoid binding user-controlled values** inside fallback raw-content elements (`<iframe>`, `<noembed>`, `<noframes>`, `<noscript>`). - **Sanitize user input** placed inside these elements to explicitly strip or escape closing tags before passing it to the template. ### References - https://github.com/angular/domino/pull/32 - https://github.com/angular/angular/pull/69675 - https://github.com/angular/angular/pull/69714 - https://github.com/angular/angular/pull/69929 - https://github.com/angular/angular/pull/69930

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