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Windows Azure Guest Agent Privilege Escalation

Low

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

First create a malicious service payload that you wish to be executed. The following command requires a working metasploit installation. It will create a service executable that executes the “calc.exe” command:

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

April 6, 2022 - Tenable discloses to vendor. Automated acknowledgment received from vendor.
April 19, 2022 - Tenable requests status update.
April 28, 2022 - Tenable requests status update via MSRC email due to lack of responses in researcher portal.
May 13, 2022 - MSRC disputes issue as impact is trivially accomplished via other means as an administrator. Tenable confirms their findings.
June 1, 2022 - Tenable requests status update after noticing update to "Develop" stage in researcher portal.
June 1, 2022 - MSRC closes case as by-design.

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-2022-18
Affected Products:
Windows Azure Guest Agent
Risk Factor:
Low

Advisory Timeline

June 1, 2022 - Initial release