We have discovered two avenues of local privilege escalation for AWS macOS EC2 instances. On macOS, Launch Daemons that are spawned from configurations specified within "/Library/LaunchDaemons/" are run with root privileges. By default, macOS EC2 instances have the following Launch Daemon configurations:
[email protected] ~ % ls -1 /Library/LaunchDaemons
Of these, com.amazon.ec2.macos-init.plist and com.amazon.ec2.monitoring.agents.cpuutilization.plist point to the following executables:
[email protected] ~ % ls -al /usr/local/Cellar/ec2-macos-init/1.4.1/bin/ec2-macos-init
-r-xr-xr-x 1 ec2-user admin 6874432 Mar 15 20:23 /usr/local/Cellar/ec2-macos-init/1.4.1/bin/ec2-macos-init
[email protected] ~ % ls -al /usr/local/Cellar/ec2-macos-system-monitor/1.2.0/libexec/bin/send-cpu-utilization
-rwxr-xr-x 1 ec2-user admin 2815880 Mar 2 00:22 /usr/local/Cellar/ec2-macos-system-monitor/1.2.0/libexec/bin/send-cpu-utilization
As can be seen above, the file owners of the above executables are ec2-user. This allows ec2-user to modify these files without restriction, such as swapping the binaries with something malicious, and have it executed as root the next time launchd spawns the corresponding the LaunchDaemon. This effectively allows for a privilege escalation from ec2-user to root.
To note, we are fully aware that this is not a "real" security issue. By default, ec2-user has sudo access and can already operate with root permissions whenever they please. In cases where the macOS EC2 instance is shared and has been configured to restrict sudo privileges, this could be a potential avenue for bypass. Obviously in such a case, admins could easily re-configure things and apply relevant permissions and mitigations themselves. We do, however, believe it'd be good to align with the permissions used for the binaries related to the other Launch Daemons (amazon-ena-ethernet and amazon-ssm-agent) by changing ownership of the binaries being used by ec2-macos-init and ec2-macos-system-monitor to root rather than the current ec2-user. Amazon has modified these permissions in the latest available AMIs.