Summary

Arbitrary classes can be loaded and instantiated using a HTTP PUT request to the /api/system/clusterconfig/ endpoint.

Details

Graylog's cluster config system uses fully qualified class names as config keys. To validate the existence of the requested class before using them, Graylog loads the class using the class loader.

https://github.com/Graylog2/graylog2-server/blob/e458db8bf4f789d4d19f1b37f0263f910c8d036c/graylog2-server/src/main/java/org/graylog2/rest/resources/system/ClusterConfigResource.java#L208-L214

PoC A request of the following form will output the content of the /etc/passwd file:

curl -u admin:<admin-password> -X PUT http://localhost:9000/api/system/clusterconfig/java.io.File \ -H "Content-Type: application/json" \ -H "X-Requested-By: poc" \ -d '"/etc/passwd"'

To perform the request, authorization is required. Only users posessing the clusterconfigentry:create and clusterconfigentry:edit permissions are allowed to do so. These permissions are usually only granted to Admin users.

Impact

If a user with the appropriate permissions performs the request, arbitrary classes with 1-arg String constructors can be instantiated.

This will execute arbitrary code that is run during class instantiation.

In the specific use case of java.io.File, the behaviour of the internal web-server stack will lead to information exposure by including the entire file content in the response to the REST request.

Credits

Analysis provided by Fabian Yamaguchi - Whirly Labs (Pty) Ltd

Impact Reauthenticating with an existing session cookie would re-use that session id, even if for different user credentials. In this case, the pre-existing session could be used to gain elevated access to an existing Graylog login session, provided the malicious user could successfully inject their session cookie into someone else's browser.

The complexity of such an attack is high, because it requires presenting a spoofed login screen and injection of a session cookie into an existing browser, potentially through an XSS attack. No such attack has been discovered.

Patches Graylog 5.1.11 and 5.2.4, and any versions of the 6.0 development branch contain patches to not re-use sessions under any circumstances, making this type of attack impossible.

Workarounds Using short session expiration and explicit log outs of unused sessions can help limiting the attack vector. Unpatched this vulnerability exists, but is relatively hard to exploit. A proxy could be leveraged to clear the authentication cookie for the Graylog server URL for the /api/system/sessions endpoint, as that is the only one vulnerable.

Analysis provided by Fabian Yamaguchi - Whirly Labs (Pty) Ltd