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Comment by newgre | Hacker News Reader

A debugger sees exceptions before the application does, and it knows which exceptions it should handle by itself (e.g. breakpoints set by the user) vs passing them to the application. I don't have a windows machine rn to verify but I'd expect your assumption that it's impossible to debug an INT3 VEH to be incorrect.

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Yes, however you can easily detect when a debugger is attached and avoid placing the VEH. There are only two mechanisms for a debugger to operate: by placing its own VEH (or UHE), or by populating one of the four hardware breakpoint registers, and both are very easy to detect.

Sure there's a gazillion ways of detecting a debugger (specifically on windows), but then we are back to detecting debuggers. My point was that VEH (alone) doesn't prevent any sort of debugging, specifically not the VEH handler itself.

Btw, debuggers (on windows) won't usually install VEH to support BPs, they'll use the win32 debugger infrastructure where the OS manages exceptions and delivers them to the attached debugger object (which again can be detected in several ways). They also do not technically need HW bp registers, although often they will. A simple way to implement BPs is to write 0xCC (INT3) to the text section, then restore original bytes when the INT3 fires.

I don't disagree that it's possible, just in my experience the debugger VEH was fired after my application's own VEH and not before it, so the debugger did not step into the VEH. And if you want to hide something from static analysis it's not a bad way, even if it is meta to "security through obscurity". The barrier to entry for implementation is pretty low, like ~50 LOCs so it's not going to be on the level of like state sponsored malware or something like that.

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