Surprise Exception Handlers

Peter FerrieSenior Anti-virus Researcher

11 June, 2008

1

Corrupted!

A program that causes this message to appear:

would probably be considered corrupted and not worthy of attention.

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Empty!

Especially if it looks like this…

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Empty!

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Entry Point

Empty!

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C3 RET

Empty!

So the main file does nothing.If we assume that the structure is normal,

then we could check the import table.Just in case.

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Empty!

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SEH.DLL

Empty!

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a

Empty!

So the search moves to SEH.DLL,and the mysterious function called ‘a’.

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‘A’ function

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Failure To Launch

CODE:00401000 push esiCODE:00401001 xor esi, esiCODE:00401003 lods dword ptr fs:[esi]CODE:00401005 inc eaxCODE:00401006CODE:00401006 loc_401006:CODE:00401006 dec eaxCODE:00401007 xchg eax, esiCODE:00401008 lodsdCODE:00401009 inc eaxCODE:0040100A jnz short loc_401006CODE:0040100C mov dword ptr [esi], offset sub_401014CODE:00401012 pop esi

At this point, eax is zero, which means a load failure.A DLL that fails to load causes the message to appear.

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I’m OK, You’re OK

But what happens when we click on ‘OK’?

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Surprise!

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Not OK

The code runs.

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How Did That Happen?

Let’s revisit the code:

CODE:00401001 xor esi, esiCODE:00401003 lods dword ptr fs:[esi]CODE:00401005 inc eaxCODE:00401006CODE:00401006 loc_401006:CODE:00401006 dec eaxCODE:00401007 xchg eax, esiCODE:00401008 lodsdCODE:00401009 inc eaxCODE:0040100A jnz short loc_401006CODE:0040100C mov dword ptr [esi], offset sub_401014CODE:00401012 pop esi

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Not OK

A standard search and replace of the topmost SEH handler.Why does it work?

The secret is in what Windows does after the DLL refuses to load.First comes the call to NtRaiseHardError() to display the message.

However, next comes a called to RtlRaiseStatus().This is intended to notify a debugger of the problem.

RtlRaiseStatus() calls NtRaiseException().Which raises an exception.

Which, without a debugger, calls the topmost SEH handler.Which is now inside the DLL that was supposed to have terminated.

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Not OK

Nothing significant has changed in the process environment.So the DLL is free to run normally.

So is the EXE, if it wants to.This technique works only for statically-linked DLLs.LoadLibrary() failures do not call the SEH handler.

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Really Not OK

Just a little something to add to the workload.

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