Angular has XSS Vulnerability via Unsanitized SVG Script Attributes

A Cross-Site Scripting (XSS) vulnerability has been identified in the Angular Template Compiler. The vulnerability exists because Angular’s internal sanitization schema fails to recognize the href and xlink:href attributes of SVG <script> elements as a Resource URL context.

In a standard security model, attributes that can load and execute code (like a script's source) should be strictly validated. However, because the compiler does not classify these specific SVG attributes correctly, it allows attackers to bypass Angular's built-in security protections.

When template binding is used to assign user-controlled data to these attributes for example, <script [attr.href]="userInput"> the compiler treats the value as a standard string or a non-sensitive URL rather than a resource link. This enables an attacker to provide a malicious payload, such as a data:text/javascript URI or a link to an external malicious script.

Impact

When successfully exploited, this vulnerability allows for arbitrary JavaScript execution within the context of the victim's browser session. This can lead to:

• Session Hijacking: Stealing session cookies, localStorage data, or authentication tokens.
• Data Exfiltration: Accessing and transmitting sensitive information displayed within the application.
• Unauthorized Actions: Performing state-changing actions (like clicking buttons or submitting forms) on behalf of the authenticated user.

Attack Preconditions

1. The victim application must explicitly use SVG <script> elements within its templates.
2. The application must use property or attribute binding (interpolation) for the href or xlink:href attributes of those SVG scripts.
3. The data bound to these attributes must be derived from an untrusted source (e.g., URL parameters, user-submitted database entries, or unsanitized API responses).

Patches

• 19.2.18
• 20.3.16
• 21.0.7
• 21.1.0-rc.0

Workarounds

Until the patch is applied, developers should:

• Avoid Dynamic Bindings: Do not use Angular template binding (e.g., [attr.href]) for SVG <script> elements.
• Input Validation: If dynamic values must be used, strictly validate the input against a strict allowlist of trusted URLs on the server side or before it reaches the template.

Resources

• https://github.com/angular/angular/pull/66318