Yamcs is a mission control framework. Prior to 5.12.8 and 5.13.2, Yamcs processes attacker-controlled data from the /ext URL route in yamcs-web/src/main/webapp/projects/webapp/src/app/core/routes/extension.matcher.ts, extension.component.ts, and app.component.ts without checking registered plugin IDs before DOM rendering through innerHTML. A crafted URL can execute JavaScript when opened by a user. The script can read data available to the Yamcs web application and perform actions in the user context. This issue is fixed in versions 5.12.8 and 5.13.2.

Affected: Maven: org.yamcs:yamcs-core

**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. <img width="940" height="294" alt="image" src="https://github.com/user-attachments/assets/d886d65c-9bb5-4599-ac11-55db83aafc0e" />

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.