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AWS EC2 macOS Local Privilege Escalation

Medium
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Synopsis

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:

ec2-user@Mac-mini ~ % ls -1 /Library/LaunchDaemons
com.amazon.aws.ssm.plist
com.amazon.ec2.ena-ethernet.plist
com.amazon.ec2.macos-init.plist
com.amazon.ec2.monitoring.agents.cpuutilization.plist

Of these, com.amazon.ec2.macos-init.plist and com.amazon.ec2.monitoring.agents.cpuutilization.plist point to the following executables:

ec2-user@Mac-mini ~ % 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

ec2-user@Mac-mini ~ % 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.

Solution

Updated to latest AMI revision available.

Additional References

https://github.com/aws/homebrew-aws/commit/ecbdf2e27254be4687dc4227634ebed7b8717d57

Disclosure Timeline

April 13, 2021 - Tenable discloses to vendor.
April 13, 2021 - Vendor acknowledges receipt.
April 21, 2021 - Vendor sends status update that issue is still being investigated.
April 27, 2021 - Vendor sends status update that issue is still being investigated.
April 30, 2021 - Vendor states that fixes are in progress. Tenable acknowledges.
May 7, 2021 - Vendor sends status update that issue is still being investigated.
May 14, 2021 - Vendor sends status update that issue is still being investigated.
May 21, 2021 - Vendor states that fixes should be complete by 5/28. A coordinated disclosure will occur in next AMI release cycle.
June 10, 2021 - Vendor sends status update and states that fix is still on track to be released in June AMI release cycle.
June 30, 2021 - Tenable requests status update.
July 7, 2021 - Vendor states that updates are ready to be pushed.
July 7, 2021 - Tenable requests information about planned publication.

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

Risk Information

Tenable Advisory ID: TRA-2021-27
Credit:
Jimi Sebree
Affected Products:
AWS macOS EC2 Instances
Risk Factor:
Medium

Advisory Timeline

July 12, 2021 - Initial release
July 19, 2021 - Added reference to commit log
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