Summary Arbitrary classes can be loaded and instantiated using a HTTP PUT request to the /api/system/cluster_config/ 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 https://localhost:9000/api/system/cluster_config/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…Read More