Synopsis
The Windows Azure Guest Agent service runs automatically and with SYSTEM level privileges. Incorrect file permissions on the service’s executable (C:\WindowsAzure\GuestAgent_2.7.41491.1044_2022-04-05_193045/WindowsAzureGuestAgent) allow it to be modified by any administrative user by default.
This allows an attacker to elevate from Admin to SYSTEM and is an easy target for achieving persistence on a given system. While low severity because it already requires Administrative privileges, we still believe this warrants attention because of how common the agent is.
Solution
The flaw described above requires administrative access. Under normal circumstances, the impact achieved by this flaw is trivially accomplished with these privileges via other means.
As such, MSRC has stated that this issue does not meet the severity requirements to warrant a patch.
Tenable understands and agrees with their conclusion. Given the prevalence of this service in the Azure environment, however, we are presenting this advisory as a means to inform others that this flaw could potentially be used to bypass more hardened configurations, such as those that disallow the creation or modification of system services.
Proof of Concept
msfvenom -p windows/exec CMD="calc.exe" --platform windows --outagent.exe --service-name "Windows Azure Guest Agent" -f exe-service
By replacing the WindowsAzureGuestAgent executable with this newly generated binary and restarting the Windows Azure Guest Agent service, we can see “calc.exe” spawn as SYSTEM. Please note that this instance will not appear within the user’s active GUI session as it is non-interactive, though any arbitrary command could be used here.
Disclosure Timeline
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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.
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