Event ID 7039 – out…pid a pid

February 26, 2021 in Compromise Detection, Sysmon, threat hunting

This event is not very well explained on the internet, so I took a liberty of describing it below:

The event message is as follows:

A service process other than the one launched by the Service Control Manager connected when starting the [SERVICE_NAME] service. The Service Control Manager launched process [PID1] and process [PID2] connected instead.

Note that if this service is configured to start under a debugger, this behavior is expected.

The message kinda tells us what happened – two different processes talk to SCM instead of one. It doesn’t really tell us WHY this happens.

Example from a case I looked at in response to a query on Twitter:

In this particular case the c:\windows\sysmon.exe was registered as a program that service process starts from. I believe this file was later manually replaced with a newer version of sysmon.exe. The little-known fact about distributable version of Sysmon (sysmon.exe from the sysinternals page) is that it is built as a 32-bit executable with an embedded 64-bit executable inside its resources. When launched on a 64-bit system the 32-bit version extracts and spawns that 64-bit version executable (note the PIDs and compare them against the Event Log):

Looking at it in general terms: when you register a service its configuration in Registry points to an executable file. This executable is then used to launch a service. Some services are not designed in a very good way. Once such programs are launched as a service, they spawn other processes, sometimes even batch files that may as well launch other programs. If one of these spawn programs talks to SCM the latter immediately recognizes that it’s not the same executable as the service process the service configuration points to. Such design is in general poor and could be a subject to possible privilege escalation (in a lolbinish way). And since this is a security concern the event 7039 is being logged.

And this leads me to the key reason I wanted to write an article. The Event 7309 tells you two things:

  • Whoever designed the service didn’t do the best job, OR, more importantly,
  • A bad guy may be using a badly designed service to escalate privileges.

Hence, you should be looking at these.

And last, but not least – does it mean Sysmon is designed badly? Nope. It’s designed in a clever way to use a single portable executable for 32-bit and 64-bit systems. The problem arises from a corner case in a way it was manually upgraded, instead of using the “-u” switch.

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