tls2

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... or is there?
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation

Am I Missing Somethi
CODE:00401010 push offset LibFileName ; "tls2"
CODE:00401005 call j_LoadLibraryA
CODE:0040100A mov ds:TlsCallbacksEnd, eax
Hmm, LoadLibrary("tls2")
Maybe DllMain contains something interesting?
Peter Ferrie, Microsoft Corporation

I Am Missing Somethi
No, that’s not it.
Peter Ferrie, Microsoft Corporation

Let’s revisit the code:
CODE:00401010 push offset LibFileName ; "tls2"
CODE:00401005 call j_LoadLibraryA
CODE:0040100A mov ds:TlsCallbacksEnd, eax
Peter Ferrie, Microsoft Corporation

It’s All About Image
It’s the TlsCallBacks extended array trick again.
Q. What value does the new callback contain?
A. The DLL’s imagebase.
Q. DEP won’t let that run, right?
A. …
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation

Of course it will.
You just have to ask nicely.
Or take a cue from a driver.
Peter Ferrie, Microsoft Corporation

When the SectionAlignment value is less than 4kb,
the file header is marked Writable and Executable
(unless the last section characteristics override it).
That turns the ImageBase into code.
Peter Ferrie, Microsoft Corporation

So we go from this…
Peter Ferrie, Microsoft Corporation

To this…
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation