What is actions in HyperDbg & how to use them?

What are the actions?

Actions define the operation that the debugger should do in the case of triggering an event. In other words, events are containers of actions.

Each event can have zero or many actions. In an unconditional event, all actions are performed one by one with the insertion order, and in conditional events, actions are performed only and only if the condition is met.

Types of Actions

HyperDbg is not a classic debugger, so we can have multiple kinds of operations in events.

Actions can be defined in 3 different types, as demonstrated in the following enum.

  1. Break : Exactly like other classic debuggers, this type of action halts the system and makes the system available for future user commands. This type of action is only available in debugging a remote machine. It is because you cannot halt your current local system.

  2. Script : This action type is a special feature that creates a log from the registers, memory, and special details (pseudo-registers) without halting the system and transfers the logs from kernel mode and vmx-root mode, safely to the debugger user mode and also you can call predefined functions and change the state of the system directly. You can use this type of action in both debugging a remote machine and debugging a local machine.

  3. Custom Code : Running a custom code is a special feature that will allow you to execute your custom assembly codes in the case of triggering an event. This means that your assembly codes will be executed, and the results will be returned safely to the debugger user mode. You can use this type of action in both debugging a remote machine and debugging a local machine.


By default and if you don't specify any parameters like script { } or code { } then HyperDbg interprets events as a break. It means that every time this event is triggered, then the system or the target process is completely halted, and now you can control the system. It is exactly like other debuggers like Windbg.


Script-engine is a powerful feature of HyperDbg that makes you able to create easy statements to create logs from the system state, change the system state, and call pre-defined functions and even halt the system and give its control to the debugger.

This feature never halts the system, and HyperDbg's script engine is vmx-root compatible; however, you should avoid unsafe behavior.

When you use script { } in your events, then you are using the script-engine.

Script-engine is a different project in HyperDbg's solution. There is a file, called "ScriptEngineCommon.h". This file contains the HyperDbg script execution engine's implementation in both user-mode and kernel-mode, and it executes the scripts that were previously interpreted in user-mode.

In order to call the execution engine, you should call ScriptEngineExecute function.

If you want to interpret a script, you should call ScriptEngineParseWrapper which is a wrapper for ScriptEngineParse. This function gives a stack (memory) that can be executed in both user-mode and kernel-mode.

By using the following structure, ScriptBufferSize and ScriptBufferPointer we pass the script buffer to the kernel.

UINT64 EventTag;
BOOLEAN ImmediateMessagePassing;
UINT32 PreAllocatedBuffer;
UINT32 CustomCodeBufferSize;
UINT32 ScriptBufferSize;
UINT32 ScriptBufferPointer;

Read Scripting Language for more information and examples about script-engine and read here for more information about the script engine's design and internals.

Custom Code

Running custom codes gives you a fast and reliable way to execute your codes in the case of triggering events without breaking the whole system, so it's super fast.

When you use code { } in your events, then you are using custom codes.

This powerful feature can optionally give you a non-paged pool buffer with your specific size and gives the address of the buffer to your assembly code in RCX. You can safely use this buffer in your assembly code, and if you want, HyperDbg will safely transfer this buffer to user mode for you.

First, you should create a buffer of bytes that performs your task. For example, the following code is some nops that a custom buffer provides to the debugger. You can change it to whatever assembly bytes that you want without any limitation in size.

char CustomCodeBuffer[8];
CustomCodeBuffer[0] = 0x90; //nop
CustomCodeBuffer[1] = 0x90; //nop
CustomCodeBuffer[2] = 0x90; //nop
CustomCodeBuffer[3] = 0x90; //nop
CustomCodeBuffer[4] = 0x90; //nop
CustomCodeBuffer[5] = 0x90; //nop
CustomCodeBuffer[6] = 0x90; //nop
CustomCodeBuffer[7] = 0xc3; //ret

Don't forget to put a 0xc3 or ret instruction at the end of your custom code buffer. This way, you give the program's execution back to the debugger, and HyperDbg can continue normally. Otherwise, the HyperDbg won't get a chance to get back the execution and cause a crash.

You should fill the following structure, which gives the details of your custom code to the debugger.

UINT32 CustomCodeBufferSize;
PVOID CustomCodeBufferAddress;
UINT32 OptionalRequestedBufferSize;

For example, the following code shows that we use CustomCodeBufferas the custom assembly code, and also, we set the size of the buffer. OptionalRequestedBufferSizeis used to request a non-paged pool buffer. If this field is zero, then it means that you don't need a non-paged buffer, but if it's not zero, then HyperDbg will allocate a non-paged pool for you and pass the address of the buffer each time as the RCX to you assembly code.

// Add action for RUN_CUSTOM_CODE
CustomCode.CustomCodeBufferSize = sizeof(CustomCodeBuffer);
CustomCode.CustomCodeBufferAddress = CustomCodeBuffer;
CustomCode.OptionalRequestedBufferSize = 0x100;

Finally, you have to register the action to the event using DebuggerAddActionToEvent.

Here's the prototype of DebuggerAddActionToEvent.


Event is the event in which you want to register this action on it.

ActionType is the type of action (described above).

SendTheResultsImmediately this field shows whether the buffer should be sent immediately to the user-mode or not.

It is because HyperDbg holds a queue of messages to be delivered to user mode. When the queue has multiple messages (the queue is full), it sends all of them in an IRP packet to the user mode (IRP Pending). This makes the HyperDbg messaging more efficient as we're not going to send each message separately in one IRP packet.

If you set this field to TRUE, the buffer will be delivered to the user -ode immediately, and if you set it to FALSE, then the buffers will be accumulated and delivered when the queue has multiple messages.

You should set it to FALSE in most cases, but if you need immediate results the choose TRUE and it makes your computer substantially slower in high rates of data delivery but at low rates TRUEmakes more scene.

InTheCaseOfCustomCode : you should fill it as described above.

InTheCaseOfRunScript : is used for script engine, should be null in custom code.

The following example shows how to use the DebuggerAddActionToEvent.

DebuggerAddActionToEvent(Event1, RUN_CUSTOM_CODE, TRUE, &CustomCode, NULL);

Please note that DebuggerAddActionToEvent should not be called in vmx-root mode.

How to send buffers back to user-mode?

If you didn't request a safe buffer or even request a safe buffer, then your assembly will be called in the following form.

typedef PVOID
DebuggerRunCustomCodeFunc(PVOID PreAllocatedBufferAddress, PGUEST_REGS Regs, PVOID Context);

If you request a safe non-paged pool buffer, then your assembly will be called in the following form, and as we're calling it with fastcall calling convention, then you can expect buffer address in RCX.

ReturnBufferToUsermodeAddress = Func(Action->RequestedBuffer.RequstBufferAddress, Regs, Context);

Otherwise, RCX is null (in the case, you didn't need a safe buffer).

Func(NULL, Regs, Context);

In the above calls, RDXis the structure of the guest's general-purpose registers, you can modify them directly, and these registers will apply to the guest when it wants to continue its normal execution.

R8 (Context) is an optional parameter that describes the state, and it's different for each event. You have to check each event's documentation to see what it is in that event.

The following structure shows the state of registers in Regs parameter. You can modify or read the general-purpose registers based on this structure as a pointer to this structure is available in RDX.

typedef struct _GUEST_REGS
ULONG64 rax; // 0x00
ULONG64 rcx;
ULONG64 rdx; // 0x10
ULONG64 rbx;
ULONG64 rsp; // 0x20
ULONG64 rbp;
ULONG64 rsi; // 0x30
ULONG64 rdi;
ULONG64 r8; // 0x40
ULONG64 r9;
ULONG64 r10; // 0x50
ULONG64 r11;
ULONG64 r12; // 0x60
ULONG64 r13;
ULONG64 r14; // 0x70
ULONG64 r15;

You can read other registers (non-general purpose registers) directly and modify them. We're not changing them or using them in debugger and hypervisor routines, so reading and changing them will directly apply to the guests' registers and apply to normal execution.