CVE-2026-55566: Yamcs has DOM XSS in Extension Routing

Attack type: Unauthenticated remote Impact: Execution of arbitrary JavaScript in a user’s browser. Affected components: extension.matcher.ts:12, extension.component.ts:40, app.component.ts:134.

Yamcs is vulnerable to cross-site scripting in the /ext URL endpoint. By inputting specially crafted code into the URL, an attacker can execute arbitrary JavaScript code in a user’s browser. This URL may be sent to a user via a phishing email.

Steps to Reproduce:

1. Start a Yamcs instance.
2. Insert the following URL into the browser and press enter (change ‘myproject’ to the name of your instance):

http://localhost:8090/ext/img%20src%3Dx%20onerror%3Dalert%281%29?c=myproject

3. You will receive an alert with the number ‘1’ in it.

Recommendations:

1. Use document.createElement instead of innerHTML in extension.component.ts.
2. Validate the extension against registered plugin IDs.
3. Enforce a strict custom-element-name regex before mounting.