Yamcs has Reflected XSS in the URL of the Authorize Endpoint

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

EPSS

• CVE-2026-55549: 1.31% estimated exploitation activity in the next 30 days; percentile 70.57 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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"…

Published by the source

28 Aug 2026, 17:17 UTC

Last modified by the source

28 Aug 2026, 17:40 UTC

Source last checked

26 Oct 2026, 16:55 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rxpg-wjf8-qv9c

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-rxpg-wjf8-qv9c

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# Patch Next advisory brief: CVE-2026-55549

Advisory: Yamcs has Reflected XSS in the URL of the Authorize Endpoint
IDs: CVE-2026-55549, GHSA-rxpg-wjf8-qv9c
Record: https://patchnext.com/advisories/cve-2026-55549 (revision 2, source checked 26 Oct 2026, 16:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.yamcs:yamcs-core 5.9.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.yamcs:yamcs-core: affected < 5.9.4; fixed in 5.9.4

## Suggested inspection (read-only)
- Maven org.yamcs:yamcs-core: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.yamcs:yamcs-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-rxpg-wjf8-qv9c (OSV record GHSA-rxpg-wjf8-qv9c lists affected ranges: Maven org.yamcs:yamcs-core >=0 <5.9.4 (fixed))
- Verified: https://github.com/advisories/GHSA-rxpg-wjf8-qv9c (GitHub-reviewed advisory GHSA-rxpg-wjf8-qv9c lists affected ranges: Maven org.yamcs:yamcs-core >=0 <5.9.4)

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