As of this writing, the latest public preview of Windows 10 IoT Core (build 14393) suffers from an elevation of privilege vulnerability via a misconfigured service ACL. The InputService service which run as SYSTEM grants authenticated users (i.e. members of the “NT AUTHORITY\Authenticated Users” group) SERVICE_ALL_ACCESS access rights, allowing an unprivileged, authenticated user to change the binary path of the service and gain elevated code execution upon restarting the service.
For reference, you can validate that InputService runs as SYSTEM with the following commands:
Get-CimInstance -ClassName Win32_Process -Filter "ProcessId=$($InputService.ProcessId)" | Invoke-CimMethod -MethodName GetOwner
This trivial vulnerability was discovered while running the Get-CSVulnerableServicePermission function in CimSweep against an IoT Core instance running on my Raspberry Pi 2. CimSweep is designed to perform incident response and hunt operations entirely over WMI/CIM.
I wrote a proof of concept exploit that simply adds an unprivileged user to the Administrators group.
While this is a classic service misconfiguration vulnerability, there are several caveats to be mindful of when exploiting it on Win 10 IoT Core. First of all, IoT Core is designed to be managed remotely and for that, you are given two remote management options: PowerShell Remoting and SSH. I chose to use PowerShell Remoting in my PoC exploit primarily to point out that the default SDDL for PowerShell Remoting in IoT Core requires that users be members of the “Remote Management Users” group. Additional information on administering IoT Core with PowerShell can be found here. For reference, the default SDDL for PowerShell Remoting can be obtained and interpreted with the following command:
Get-PSSessionConfiguration -Name microsoft.powershell | Select-Object -ExpandProperty Permission
There is no such group membership requirement for SSH. Hopefully, at a future point, SSH endpoints on Windows will have the granular security controls that PowerShell Remoting offers via the PSSessionConfiguration cmdlets. Some additional caveats were that when I remoted in as an unprivileged user, I did not have sufficient privileges to use the Service cmdlets (Get-Service, Set-Service, etc.) or CIM cmdlets (Get-CimInstance, Invoke-CimMethod, etc.) in order to change the service configuration. Fortunately, sc.exe presented no such restrictions.
While this is by no means a “sexy” vulnerability, the fact that such a trivial vulnerability was present in a modern Windows OS tells me that perhaps Win 10 IoT Core isn’t getting the security scrutiny of other Windows operating systems. I hope that many of the same security controls and mitigations will eventually be applied to IoT Core if the plan is for this to be the operating system that drives critical infrastructure.
Lastly, if you’re attending Black Hat USA 2016, you should plan on attending Paul Sabanal’s (@polsab) talk on Windows 10 IoT Core!
May 22, 2016 – Vulnerability reported to MSRC
May 23, 2016 – MSRC opened a case number for the issue.
July 20, 2016 – Follow-up email sent to MSRC asking for a status update. No response received
July 25, 2016 – Decision made to release the vulnerability details