Signed binaries can be used to do a lot of funny, unexpected stuff – today I will cover a simple proxy execution technique that can be used as a possible EDR evasion, and who knows, perhaps could throw some sandboxes off as well.

For a quick demo I chose Autoruns as a culprit, because it’s popular, but bear in mind that it can be any application really that registers its own file type in the Registry.

So…

When you run the Autoruns program it will, apart from showing you the startup entries, register a file type .ARN:

HKCR\.ARN

that points to

HKCR\Autoruns.Logfile.1

which tells the system how to open the file:

HKCR\Autoruns.Logfile.1\shell\open\command=

"<PATH>\Autoruns.exe" "%1"

Since the entry is created using a signed binary, one could drop autoruns on the system, and execute it (GUI is not such a big deal as it could be either partially, or fully hidden e.g. using ‘start /min’, launching on a different desktop, etc.).

One could now replace the autoruns in the path where it was executed from with the malicious autoruns.exe, create a dummy foo.arn file, and

• launch the foo.arn file – this will execute the payload
• add the foo.arn file to e.g. Run key; since the file is non-malicious, it won’t trigger an alert from a typical AV; next time the user logs in, the dummy .arn file will be ‘opened’ i.e. launched via Explorer leading to malicious autoruns.exe being executed (Update: I recently learnt that Kovter is using similar trick since at least 2016)

It is really trivial, but as I explained in the first post in this series, the idea is to delegate atomic actions, especially ones that are easy to spot by monitors, to signed binaries. Combining various functional bits and bobs offered by various tools one can build a decent chain of commands that will hide, or at least obfuscate the real purpose of the whole activity.