There are lot of games that catches “cheaters” by checking the return address of a function call. After executing the function, it will return to the location of call. Anti-cheat checks the return address if it’s within the module range and whitelists ranges, else, if it’s not, you will get flagged and will result to ban.

Assembly Macros

Call

Saves procedure linking information on the stack and branches to the procedure (called procedure) specified with the destination (target) operand. The target operand specifies the address of the first instruction in the called procedure. This operand can be an immediate value, a general purpose register, or a memory location.

This instruction can be used to execute four different types of calls:

Near call
A call to a procedure within the current code segment (the segment currently pointed to by the CS register), sometimes referred to as an intrasegment call.

Far call
A call to a procedure located in a different segment than the current code segment, sometimes referred to as an intersegment call. Inter-privilege-level far call. A far call to a procedure in a segment at a different privilege level than that of the currently executing program or procedure.

Task switch
A call to a procedure located in a different task.

The latter two call types (inter-privilege-level call and task switch) can only be executed in protected mode. See the section titled “Calling Procedures Using Call and RET” in Chapter 6 of the IA-32 Intel Architecture Software Developer’s Manual, Volume 1, for additional information on near, far, and inter-privilege-level calls. See Chapter 6, Task Management, in the IA-32 Intel Architecture Software Developer’s Manual, Volume 3, for information on performing task switches with the CALL instruction.
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push ReturnAddress — The address of the next instruction after the call
jmp SomeFunc — Change the EIP/RIP to the address of SomeFunc

Ret

Transfers program control to a return address located on the top of the stack. The address is usually placed on the stack by a CALL instruction, and the return is made to the instruction that follows the CALL instruction.

The optional source operand specifies the number of stack bytes to be released after the return address is popped; the default is none. This operand can be used to release parameters from the stack that were passed to the called procedure and are no longer needed. It must be used when the CALL instruction used to switch to a new procedure uses a call gate with a non-zero word count to access the new procedure. Here, the source operand for the RET instruction must specify the same number of bytes as is specified in the word count field of the call gate.

The RET instruction can be used to execute three different types of returns:

Near return
A return to a calling procedure within the current code segment (the segment currently pointed to by the CS register), sometimes referred to as an intrasegment return.

Far return
A return to a calling procedure located in a different segment than the current code segment, sometimes referred to as an intersegment return.

Inter-privilege-level far return
A far return to a different privilege level than that of the currently executing program or procedure.

The inter-privilege-level return type can only be executed in protected mode. See the section titled “Calling Procedures Using Call and RET” in Chapter 6 of the IA-32 Intel Architecture Software Developer’s Manual, Volume 1, for detailed information on near, far, and inter-privilege- level returns.

When executing a near return, the processor pops the return instruction pointer (offset) from the top of the stack into the EIP register and begins program execution at the new instruction pointer. The CS register is unchanged.

When executing a far return, the processor pops the return instruction pointer from the top of the stack into the EIP register, then pops the segment selector from the top of the stack into the CS register. The processor then begins program execution in the new code segment at the new instruction pointer.

The mechanics of an inter-privilege-level far return are similar to an intersegment return, except that the processor examines the privilege levels and access rights of the code and stack segments being returned to determine if the control transfer is allowed to be made. The DS, ES, FS, and GS segment registers are cleared by the RET instruction during an inter-privilege-level return if they refer to segments that are not allowed to be accessed at the new privilege level. Since a stack switch also occurs on an inter-privilege level return, the ESP and SS registers are loaded from the stack.

If parameters are passed to the called procedure during an inter-privilege level call, the optional source operand must be used with the RET instruction to release the parameters on the return.

Here, the parameters are released both from the called procedure’s stack and the calling procedure’s stack (that is, the stack being returned to).
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add esp, 18h — Increase the stack pointer, decreasing the stack size, usually by the amount of arguments the function takes (that actually got pushed onto the stack and the callee is responsible for cleaning the stack). This is due to the stack “grows” downward.
pop eip — Practically pop the top of the stack into the instruction pointer, effectively “jmp” there.

Push

Decrements the stack pointer and then stores the source operand on the top of the stack. The address-size attribute of the stack segment determines the stack pointer size (16 bits or 32 bits), and the operand-size attribute of the current code segment determines the amount the stack pointer is decremented (2 bytes or 4 bytes). For example, if these address- and operand-size attributes are 32, the 32-bit ESP register (stack pointer) is decremented by 4 and, if they are 16, the 16-bit SP register is decremented by 2. (The B flag in the stack segment’s segment descriptor determines the stack’s address-size attribute, and the D flag in the current code segment’s segment descriptor, along with prefixes, determines the operand-size attribute and also the address-size attribute of the source operand.) Pushing a 16-bit operand when the stack addresssize attribute is 32 can result in a misaligned the stack pointer (that is, the stack pointer is not aligned on a doubleword boundary).

The PUSH ESP instruction pushes the value of the ESP register as it existed before the instruction was executed. Thus, if a PUSH instruction uses a memory operand in which the ESP register is used as a base register for computing the operand address, the effective address of the operand is computed before the ESP register is decremented.

In the real-address mode, if the ESP or SP register is 1 when the PUSH instruction is executed, the processor shuts down due to a lack of stack space. No exception is generated to indicate this condition.
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sub esp, 4 — Subtracting 4 bytes in case of 32 bits from the stack pointer, effectively increasing the stack size.
mov [esp], eax — Moving the item being pushed to where the current stack pointer is located.

Pop

Loads the value from the top of the stack to the location specified with the destination operand and then increments the stack pointer. The destination operand can be a general-purpose register, memory location, or segment register.

The address-size attribute of the stack segment determines the stack pointer size (16 bits or 32 bits-the source address size), and the operand-size attribute of the current code segment determines the amount the stack pointer is incremented (2 bytes or 4 bytes). For example, if these address- and operand-size attributes are 32, the 32-bit ESP register (stack pointer) is incremented by 4 and, if they are 16, the 16-bit SP register is incremented by 2. (The B flag in the stack segment’s segment descriptor determines the stack’s address-size attribute, and the D flag in the current code segment’s segment descriptor, along with prefixes, determines the operandsize attribute and also the address-size attribute of the destination operand.) If the destination operand is one of the segment registers DS, ES, FS, GS, or SS, the value loaded into the register must be a valid segment selector. In protected mode, popping a segment selector into a segment register automatically causes the descriptor information associated with that segment selector to be loaded into the hidden (shadow) part of the segment register and causes the selector and the descriptor information to be validated (see the “Operation” section below).

A null value (0000-0003) may be popped into the DS, ES, FS, or GS register without causing a general protection fault. However, any subsequent attempt to reference a segment whose corresponding segment register is loaded with a null value causes a general protection exception (#GP). In this situation, no memory reference occurs and the saved value of the segment register is null.

The POP instruction cannot pop a value into the CS register. To load the CS register from the stack, use the RET instruction.

If the ESP register is used as a base register for addressing a destination operand in memory, the POP instruction computes the effective address of the operand after it increments the ESP register. For the case of a 16-bit stack where ESP wraps to 0h as a result of the POP instruction, the resulting location of the memory write is processor-family-specific.

The POP ESP instruction increments the stack pointer (ESP) before data at the old top of stack is written into the destination.

A POP SS instruction inhibits all interrupts, including the NMI interrupt, until after execution of the next instruction. This action allows sequential execution of POP SS and MOV ESP, EBP instructions without the danger of having an invalid stack during an interrupt1. However, use of the LSS instruction is the preferred method of loading the SS and ESP registers.
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mov eax, [esp] — Move the value on top of the stack into whatever is being pop into.
add esp, 4 — To increase the esp, reducing the size of the stack.

Gadget/ROP Chaining

Let’s get our hands dirty!

WARNING: ALL DETAILS BELOW ARE FOR EDUCATIONAL PURPOSES ONLY.

Now, our goal is to spoof the return address so we will not be having troubles with the return checks, thus, we will not get our account banned.

As you can see in the example image, we have our application module that ranges from 0x500000 until 0x600000. The only valid return address should be in this range, otherwise the application will know that we are calling the function from different module.

Now to get things complicated, what if our function call is outside of the application module? Say, it was from an injected DLL?

As you can see above, we are calling the function somewhere from 0x700000 ~ 0x800000 which is not a valid range for return check, and would result our account to being banned.

Hands-on: Our target application (Game)

As we check the function we want to call, there is a return check inside of it.

The Solution

	static void Engine::CastSpellSelf(int SlotID) {
		if (me->IsAlive()) {
			DWORD spellbook = (DWORD)me + (DWORD)oObjSpellBook;
			auto spellslot = me->GetSpellSlotByID(SlotID);
			Vector* objPos = &me->GetPos();
			Vector* mePos = &me->GetPos();
			DWORD objNetworkID = 0;
			DWORD SpoofAddress = (DWORD)GetModuleHandle(NULL) + (DWORD)oRetAddr; //retn instruction
			DWORD CastSpellAddr = (DWORD)GetModuleHandle(NULL) + (DWORD)oCastSpell;//CastSpell


			if (((*(DWORD*)SpoofAddress) & 0xFF) != 0xC3)
				return; //This isn't the instruction we're looking for

			__asm
			{
				push retnHere //address of our function,  
					mov ecx, spellbook //If the function is a __thiscall don't forget to set ECX
					push objNetworkID
					push mePos
					push objPos
					push SlotID
					push spellslot
					push SpoofAddress
					jmp CastSpellAddr
				retnHere :
			}
		}
	}

As you can see above, from the line 18 to line 23, that is our original function parameters. In line 24, I also pushed the SpoofAddress, which is our gadget.

When the function has finished executing, it will pop to our gadget, then it will hit the return instruction back where we originally called the function (outside of the application). The return address will be our gadget, which is inside the application module, thus successfully bypassing the return check.

Additional Note (Another example)

The function above is a __thiscall function. As per microsoft documentation, the function will clean the passed parameters itself that’s why our gadget has only retn instruction. On other case, if it does not clean the passed parameters, then you might want to find a gadget inside the application module that does pop the passed parameters before the retn.

The above function will be our target and we want to spoof the return address when we call it. Since its __cdecl, we want to clean our own parameters after executing the function. Just find a gadget inside the module that has the ff instructions:

add esp, 28 
ret

We need to clean the esp stack by size of 28, which comes from the parameters. We have 7 parameters so the formula will be 7 x 4bytes = 28, then return.

Thankfully, there is a site where you can easily transform instructions to opcodes so you can easily search the module.

Testing if our spoof works

It’s easy to tell if your spoof works. Just run the application and see if you will get banned after a few days ???

Or just write a code that gets the value of variable where the flag is being stored.

If you are lucky enough in bypassing the check, then you are now safe from bans (or you just think so).

Conclusion

First of all, I want to say thank you to the people of UC for giving some quite good materials and resources. Second, I want to thank PITSF for inspiring a lot of people who’s interested in ethical hacking and security. Mabuhay po kayo. And last but not the least, I want to thank the readers who finished reading this post. I am sorry if there are grammatical/terminology errors, English is not my mother tongue.

ROP Chaining Attack is easy to execute but having additional layer of security is enough to catch intruders to the system. Some anti-cheat enumerate the modules, some implements whitelist of modules, some hook the system functions for them to have advantage on control of system, and etc.

Once again, thank you so much!