A vulnerability in the Rackspace Windows Agent and Updater was discovered that allows for modified Agent binaries to be remotely uploaded (without authentication) to Rackspace Cloud Server guest instances. Modified Agent binaries are processed as an update for the Agent and arbitrary code can then be executed after the service is restarted. CloudPassage disclosed the vulnerability to Rackspace and CVE-2013-6795 was issued by MITRE Corporation.
The Windows Agent and Updater is used by Windows Cloud Server instances on OpenStack Nova to handle boot configurations for Windows guests running on the Xen hypervisor. The agent was created by Rackspace for their Windows instances and both the Agent and Updater services run under the LocalSystem account.
Previous versions of the Updater (before 126.96.36.199) allowed for unsigned agent updates utilizing a specially crafted .NET remote call to TCP port 1984. The Update service takes a single .NET serializable object with a URL and an MD5 checksum. Once the sequence is triggered, a ZIP file is downloaded, verified using the checksum, and extracted into the program folder of the Agent service before the service is restarted. No authentication is performed by the .NET remoting service, making it possible to deploy a modified Agent update that overwrites the running Agent service binary. A proof of concept tool was developed to trigger the sequence with an arbitrary download URL using the original .NET libraries from a target.
The copy of the Rackspace Cloud Agent Service binary was injected with a backdoored downloader using the und3ath Injector tool. The modification triggered the downloader payload in the OnStart method – which is called when the service is started.
The backdoored file was archived to emulate the distribution packaging of a valid Agent Update package. The payload triggered by the downloader was a simple bind-shell created using msfvenom from the Metasploit project.
CloudPassage responsibly disclosed the finding to Rackspace and, as of version 188.8.131.52, the Updater has been changed to use IPC with XenStore and no longer listens on port 1984. Rackspace recommends that users running the Windows agent less than version 184.108.40.206 update to the latest version, available on GitHub at https://github.com/rackerlabs/openstack-guest-agents-windows-xenserver.