tls3

Thread Local Storage callbacks were discovered in 2000.
However, widespread use didn’t occur until 2004.
Now, it should be the first place to look for code,
since it runs before the main entrypoint.
And that can make all the difference…
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation
Entry Point

Peter Ferrie, Microsoft Corporation
C3 RET

So the main file does nothing.
If we assume that the structure is normal,
then we could check the thread local storage table.
Just in case.
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation
TLS is present
(size doesn’t matter)

Peter Ferrie, Microsoft Corporation
Callback pointer
Callback array

So the search moves to the callbacks,
of which there is only one, but it looks peculiar.
It’s not a virtual address.
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation

Imported TLS callbac
We know that the TLS callback array can be altered at runtime.
We know that the TLS callbacks can point outside of the image.
Now we are looking at a new way to achieve that.
Imports are resolved before TLS callbacks are called.
So TLS callbacks can be imported addresses!
Let’s check the import table.
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation
TLS3.DLL

Peter Ferrie, Microsoft Corporation
a

So the search moves to TLS3.DLL,
and the mysterious function called ‘a’.
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation

So that’s how it’s done.
If we let it run…
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation

The code runs.
Peter Ferrie, Microsoft Corporation