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Yamcs has DOM XSS in Extension Routing

Moderate severity GitHub Reviewed Published Jul 14, 2026 in yamcs/yamcs • Updated Aug 28, 2026

Package

maven org.yamcs:yamcs-core (Maven)

Affected versions

>= 5.13.0, <= 5.13.1
<= 5.12.7

Patched versions

5.13.2
5.12.8

Description

Attack type: Unauthenticated remote
Impact: Execution of arbitrary JavaScript in a user’s browser.
Affected components: extension.matcher.ts:12, extension.component.ts:40, app.component.ts:134.

Yamcs is vulnerable to cross-site scripting in the /ext URL endpoint. By inputting specially crafted code into the URL, an attacker can execute arbitrary JavaScript code in a user’s browser. This URL may be sent to a user via a phishing email.

image

Steps to Reproduce:

  1. Start a Yamcs instance.
  2. Insert the following URL into the browser and press enter (change ‘myproject’ to the name of your instance):
http://localhost:8090/ext/img%20src%3Dx%20onerror%3Dalert%281%29?c=myproject
  1. You will receive an alert with the number ‘1’ in it.

Recommendations:

  1. Use document.createElement instead of innerHTML in extension.component.ts.
  2. Validate the extension against registered plugin IDs.
  3. Enforce a strict custom-element-name regex before mounting.

References

@fqqb fqqb published to yamcs/yamcs Jul 14, 2026
Published to the GitHub Advisory Database Aug 28, 2026
Reviewed Aug 28, 2026
Last updated Aug 28, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(19th percentile)

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2026-55566

GHSA ID

GHSA-9272-wg2r-7xmx

Source code

Credits

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