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Using Microsoft Message Analyzer to Find New ETW Sources

In order to make use of krabsetw to consume an ETW provider, you'll first need to identify the schema of the events therein. This can be achieved with several different tools but we'll focus on Microsoft Message Analyzer for now. If you haven't already installed it, please download and install it from the link.

When you open Message Analyzer, you should see a screen similar to the following:

Click the New Session button, as highlighted in the image above. You should then be presented with a dialog that looks like this:

Click the Live Trace button, as highlighted in the image above. You should then be presented with a dialog that looks like this:

Click the Add Providers button, as highlighted in the image above. You should then be presented with a dialog that looks like this:

This dialog allows you to select the provider you're interested in. Typically, when searching for new sources of ETW events, we'll enter in keywords like "wininet" or "powershell" and see what providers are available. In this example, we'll use the PowerShell ETW provider. Once you've found the Microsoft-Windows-PowerShell provider, select it and click the "Add To" button.

After clicking ok, you should then have a provider added and have a window that looks as follows:

In the window above, there's an option to configure the provider. This is a bit of a tricky area with providers. There is no predefined standard on how the configuration of a provider works but it is comprised of two flags: "All" and "Any". In other words, the flags are implementation specific. These flags will allow you to configure what events are actually emit and you will have to play with them for each provider in order to properly pair down the events emitted.

For now, let's not configure any settings on the "Any" and "All" flags. Click the "start" button in Message Analyzer

Now open up a powershell window and define a function named "invoke-mimikatz" in there, similar to the following: function invoke-mimikatz {}. Type invoke-mimikatz and hit enter, then switch back to the Message Analyzer window.

You should roughly have something that looks like the following:

Look for events in the Message Analyzer window where the summary says "Command invoke-mimikatz is Started". These are the events generated that we're interested.

When you select that line in Message Analyzer, you should see a window like this:

Let's talk about the three sections at the bottom now:

  • Bottom Left: This is the "message stack" which allows you to see what "messages" are associated with the event. In general, this is usually just the event metadata (the second message, "Etw") and the provider specific information (the first message, "Microsoft_Windows_PowerShell").

  • Bottom Center: This window shows the properties associated with the event. In particular, when you select one of the two messages from the left window, it will show the properties as associated with that message. The "Etw" message's relevant fields never change on a given OS - that is, the "Etw" message will always have the same properties such as: Id, Opcode, Flags, and so on. A good place to see how these properties are exposed with krabsetw is in the EventRecordMetadata class.

  • Bottom Left: This windows shows the "field data" which is simply the relevant value of the property on the event message. If the event property is a string then it's the string value in there and so on.

Now that we've discussed the three panes on the bottom, let's discuss this particular event that we've selected. This event (id 7937) only contains one interesting property called ContextInfo. This is a reflection of poor event schema design by the provider authors but that's a discussion for a different place. You will find that there are other providers that are better at providing distinct properties in better types than a large string blob.

Now select the ContextInfo property and note the content of the bottom right pane. In there, you'll see Host Application, Command Name, and User which are relatively self-explanatory and provide us with some particularly interesting pieces of info to parse:

  • We can use Host Application to identify suspect processes hosting the PowerShell runtime. In general, we'd only expect to see powershell.exe and wsmprovhost.exe hosting the PowerShell runtime so any other value would be suspect.

  • The Command Name gives us the name of the function invoked. If we have a prohibited or permitted list we can check whether the command name is expected.

  • The User is self-explanatory and gives us the user that started the hosting process.

Conclusion

In order to make use of the information above, we'll need to parse out these lines from the ContextInfo string blob. If you're interested in a sample project to start doing this, please check out the PowerShellMethodAuditor project.

Hopefully this will help you explore ETW providers for interesting information. Please file an issue on Github if you run into any problems with this guide.