CVE-2026-55549: Yamcs reflected XSS in authorize endpoint

Executive brief

Yamcs is a satellite operations control system used to manage and monitor space missions. A reflected cross-site scripting vulnerability in the authorize endpoint allows attackers to inject malicious JavaScript into a user's browser via a crafted URL. This enables session hijacking—stealing access and refresh tokens to impersonate legitimate users and take over their accounts.

Technical details

A reflected XSS vulnerability exists in Yamcs versions 5.8.6 and earlier in the OAuth2 authorize endpoint (/auth/authorize). The vulnerability resides in the authorize.html, AuthHandler.java, and HandlerContext.java components, which fail to properly sanitize user-supplied input from URL parameters (such as redirect_uri or other query parameters) before rendering them in the HTML response. An attacker can craft a malicious URL containing JavaScript payload that executes in the browser context of any logged-in user who clicks the link. This does not require user authentication by the attacker, only that a victim is logged into Yamcs. Once executed, the injected script can access HTTP-only cookies containing session tokens and exfiltrate them to an attacker-controlled server, allowing complete account takeover. The vulnerability has been patched in version 5.9.4.

Affected products

• Yamcs yamcs-core < 5.9.4

Timeline

• 2026-08-28: disclosed: Published to GitHub Advisory Database
• 2026: patched: Fixed in version 5.9.4

References

• https://api.github.com/users/suffs811
• https://github.com/suffs811
• https://api.github.com/users/suffs811/gists%7B/gist_id%7D
• https://api.github.com/users/suffs811/repos
• https://avatars.githubusercontent.com/u/84633135?v=4
• https://api.github.com/users/suffs811/events%7B/privacy%7D