Gepard Shield Analysis:
KiUserExceptionDispatcher Hook Check

When I was looking into Gepard Shield — the anti-cheat running inside Ragnarok Online — one thing caught my attention early: it goes after KiUserExceptionDispatcher. That's a smart target. It's an undocumented function deep in ntdll.dll that Windows calls every time a user-mode exception happens. Breakpoints, single-step traps, access violations — they all flow through it. Debuggers depend on that flow to work. Control that function and you can see a debugger coming before it even gets a chance to respond.

Gepard installs its hook shortly after login. It patches the very start of KiUserExceptionDispatcher — the first five bytes — replacing the normal function prologue with a jump into its own code.

Before the patch, the bytes look like this:

8B FF 55 8B EC

Standard x86 prologue, nothing unusual. After Gepard hooks it:

E9 XX XX XX XX

E9 is a near jump. The four bytes that follow are a relative offset telling the CPU where to land — straight into Gepard's exception handler. From that point on, every exception that fires in the process hits Gepard's code first.

Protecting the Hook

Installing the hook is one thing. Keeping it there is another. Anyone trying to bypass this anti-debugger would just restore those five bytes back to the original prologue. Gepard knows this, so it runs a periodic check to verify the hook hasn't been touched.

The check comes down to a single instruction:

movzx dx, byte ptr [ecx]

Before this runs, Gepard loads ECX with the address of a specific byte inside its installed hook — one of the bytes that would be overwritten if someone patched it back. Then this instruction reads that byte and drops it into DX. That's it. One byte, one register.

What happens next is the actual detection: Gepard compares that value against what it knows the byte should be. If they match, the hook is intact and the check passes quietly. If they don't — meaning something changed that byte — Gepard knows the hook was removed or overwritten, and it treats that as a tamper event.

What Happens on Detection

The response is immediate. Gepard pops a message box saying tampering was detected and shuts the game client down. No grace period, no second chance.

Bypassing It

The check itself isn't hard to get around once you understand what it's looking for — you just need to make sure that byte stays at the expected value while you do whatever you're doing. But that's only one layer. Gepard has other anti-debug tricks on top of this, so removing this particular check doesn't hand you a working debugger session. You still need to deal with syscall-based detection and the usual catalogue of anti-debug techniques before you're actually in a position to do anything useful.