tls1 (2)

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:00401013 mov ds:TlsCallbacksEnd, offset loc_401000
CODE:0040101D retn
Who ever heard of a one-line callback?
Peter Ferrie, Microsoft Corporation

It’s about what you write, and where you write it.
By writing to TlsCallbacksEnd, the array is extended in memory.
Now the array contains two entries, not one.
Peter Ferrie, Microsoft Corporation

Peter Ferrie, Microsoft Corporation

The second entry is executed after the first one returns.
The array can be extended infinitely.
Existing entries can be altered at runtime, too.
For example, one entry can decrypt the others.
Peter Ferrie, Microsoft Corporation