1. Fun With Thread Local Storage (part 2)
Peter Ferrie
Senior Anti-virus Researcher
26 June, 2008 1

2. You Can Call Me Al
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…
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3. Empty!
Entry Point
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4. Empty!
C3 RET
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5. Empty! 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.
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6. TLS is present (size doesn’t matter) Empty!
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7. Callback pointer Callback array Empty!
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8. Empty! So the search moves to the callbacks,
of which there is only one... or is there?
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9. The One and Only
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10. Maybe DllMain contains something interesting?
Am I Missing Something?
CODE: push offset LibFileName ; "tls2"
CODE: call j_LoadLibraryA
CODE: A mov ds:TlsCallbacksEnd, eax
Hmm, LoadLibrary("tls2")
Maybe DllMain contains something interesting?
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11. I Am Missing Something No, that’s not it.
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12. Let’s revisit the code:
Take 2
Let’s revisit the code:
CODE: push offset LibFileName ; "tls2"
CODE: call j_LoadLibraryA
CODE: A mov ds:TlsCallbacksEnd, eax
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13. 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. …
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14. Surprise!
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15. Not OK Of course it will. You just have to ask nicely.
Or take a cue from a driver.
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16. Chaotic-Evil 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.
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17. Before
So we go from this…
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18. After
To this…
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19. Presto!
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20. Just a little something to add to the workload.
Really Not OK
Just a little something to add to the workload.
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