A misconfiguration discovered in the Google Cloud Platform could allow threat actors to gain complete control over virtual devices by exploiting legitimate features in the system, researchers at Mitiga, a Cloud Incident Response firm, stated.
Mitiga uncovered a misconfiguration several months ago while examining Google Cloud Platform’s Compute Engine (GCP), specifically virtual machine (VM) services. The Cloud incident response vendor identified a misconfiguration that allowed attackers to send and receive data from the VM and possibly secure complete control over the system.
However, Mitiga emphasizes that this is not a security loophole, or system error – it’s described as a “dangerous functionality”.
Mitiga notes that malicious actors could use a compromised metadata API, named “getSerialPortOutput”, which is used for the purpose of tracking and reading serial port keys. The researchers described the API call as a “legacy method of debugging systems”, as serial ports are not ports in the TCP/UP sense, but rather files of the form /dev/ttySX, given that this is Linux.
"We at Mitiga believe that this misconfiguration is likely common enough to warrant concern; however, with proper access control to the GCP environment there is no exploitable flaw," Andrew Johnston, principal consultant at Mitiga, stated.
After reporting the findings to Google, the company agreed that misconfiguration could be exploited to bypass firewall settings. Mitiga proposed two changes to the getSerialPortOutput function by Google, including restricting its use to only higher-tiered permission roles and allowing organizations to disable any additions or alterations of VM metadata at runtime.
Additionally, the company advised Google to revise its GCP documentation, to further clarify that firewalls and other network access controls don’t fully restrict access to VMs.
However, Google disagreed with a majority of the recommendations.
"After a long exchange, Google did ultimately concur that certain portions of their documentation could be made clearer and agreed to make changes to documentation that indicated the control plane can access VMs regardless of firewall settings. Google did not acknowledge the other recommendations nor speak to specifics regarding whether a GCP user could evade charges by using the getSerialPortOutput method," Johnston wrote in the report.
