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

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:

    {
      "projects": {
        "my-app": {
          "architect": {
            "build": {
              "builder": "@angular/build:application",
              "options": {
                "optimization": {
                  "styles": {
                    "inlineCritical": false
                  }
                }
              }
            }
          }
        }
      }
    }