@angular/platform-server: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Overview

A Cross-Site Scripting (XSS) vulnerability exists in @angular/platform-server's DOM emulation dependency (domino) when serializing the content of raw-text elements (such as <script>, <style>, and <iframe>).

domino supports escaping raw-text elements during serialization to prevent closing-tag breakout. However, a Unicode index alignment bug existed in this escaping logic.

In JavaScript, string lengths and character indices are calculated based on UTF-16 code units (where astral characters—such as emojis—occupy 2 code units / 4 bytes). If the bound dynamic text contained astral Unicode characters before the closing tag (e.g. </script>, </style>, or </iframe>), the index offset calculation in domino's replacement logic shifted.

This misalignment caused domino to fail to replace or escape the closing tag, leaving it raw and unescaped in the output HTML.

An attacker who controls the dynamic text can supply a payload containing both an astral Unicode character and a closing tag (e.g., 😀</iframe><script>alert(1)</script>). When serialized on the server during SSR, the browser parses the unescaped closing tag, exits the raw-text context early, and executes the subsequent <script> block, leading to 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 raw-text elements. This can lead to session hijacking, credentials theft, unauthorized actions on behalf of users, and defacement.

Patched Versions

• 22.0.0-rc.2
• 21.2.16
• 20.3.24
• 19.2.25

Workarounds

If you cannot immediately update your dependencies, you can:

• Avoid binding user-controlled values inside <iframe> or other raw-text elements.
• Sanitize any user input placed inside raw-text elements to explicitly strip closing tags before passing it to the template.

Affected packages

• @angular/platform-server >= 22.0.0-next.0, < 22.0.0-rc.2
• @angular/platform-server >= 21.0.0-next.0, < 21.2.16
• @angular/platform-server >= 20.0.0-next.0, < 20.3.24
• @angular/platform-server >= 19.0.0-next.0, < 19.2.25
• @angular/platform-server <= 18.2.14

Remediation

Upgrade to a patched release:

• @angular/platform-server 22.0.0-rc.2
• @angular/platform-server 21.2.16
• @angular/platform-server 20.3.24
• @angular/platform-server 19.2.25