!epthook (hidden hook with EPT - stealth breakpoints)

Description of the '!epthook' command in HyperDbg.

Command

!epthook

Syntax

!epthook [Address (hex)] [pid ProcessId (hex)] [core CoreId (hex)] [imm IsImmediate (yesno)] [buffer PreAllocatedBuffer (hex)] [script { Script (string) }] [asm condition { Condition (assembly/hex) }] [asm code { Code (assembly/hex) }] [output {OutputName (string)}]

Description

Puts a hidden breakpoint (0xcc) on the target function in user-mode and kernel-mode without modifying the content of memory in the case of reading/writing.

This implementation of the hidden hook causes vm-exit when it triggers. A faster implementation of EPT hidden hooks is !epthook2, which is without vm-exits. Still, it has some limitations, as described in the documentation.

If you want to keep the current context without continuing the debuggee, you should use the 'bp' command instead.

Parameters

[Address (hex)]

The Virtual address of where we want to put the hook.

[pid ProcessId (hex)] (optional)

Optional value to trigger the event in just a specific process. Add pid xx to your command; thus, the command will be executed if the process id is equal to xx. If you don't specify this option, then by default, you receive events on all processes.

Still, in the case of user-mode debugging, HyperDbg will apply it only to the current active debugging process (not all the processes). In that case, you can specify pid all to intercept events from the entire system.

[core CoreId (hex)] (optional)

Optional value to trigger the event in just a specific core. Add core xx to your command thus command will be executed if core id is equal to xx. If you don't specify this option, then by default, you receive events on all cores.

[imm IsImmediate (yesno)] (optional)

Optional value in which yes means the results (printed texts in scripts) should be delivered immediately to the debugger. no means that the results can be accumulated and delivered as a couple of messages when the buffer is full; thus, it's substantially faster, but it's not real-time. By default, this value is set to yes.

[buffer PreAllocatedBuffer (hex)] (optional)

Optional value which reserves a safe pre-allocated buffer to be accessed within the event codes.

[script { Script (string) }] (optional)

A HyperDbg script will be executed each time the event is triggered.

[asm condition { Condition (assembly/hex) }] (optional)

Optional assembly codes which check for conditions in assembly.

[asm code { Code (assembly/hex) }] (optional)

Optional assembly codes will be executed each time the event is triggered.

[output {OutputName (string)}] (optional)

Optional output resource name for forwarding events.

Context

As the Context ($context pseudo-register in the event's script, r8 in custom code, and rdx in condition code register) to the event trigger, HyperDbg sends the virtual address of where put the hidden hook's breakpoint.

Short-circuiting

This event does not support 'event short-circuiting', as this mechanism won't make sense for the function hooks. If you want to change the execution path (e.g., ignoring a function call), you can directly manipulate the RIP register.

!epthook nt!ExAllocatePoolWithTag script {
    @rip = poi(@rsp); // return to the stack
    @rsp = @rsp + 8; // pop from the stack
    printf("ignoring calls to the target function\n");
}

Calling Stages

This event does not support calling stages. Due to the nature of function hooking implementing calling stages in this context wouldn't be meaningful.

Debugger

This event supports three debugging mechanisms.

• Break

• Script

• Custom Code

Please read "How to create a condition?" if you need a conditional event, a conditional event can be used in all "Break", "Script", and "Custom Code".

Break

Imagine we want to put a hook on fffff800`4ed6f010, this will break into the debugger when the target address hits and gives the control back to you.

HyperDbg> !epthook fffff800`4ed6f010

Alternatively, you can use nt!ExAllocatePoolWithTag too.

HyperDbg> !epthook nt!ExAllocatePoolWithTag

You can also use an expression like nt!ExAllocatePoolWithTag+@rcx+5 too.

HyperDbg> !epthook nt!ExAllocatePoolWithTag+@rcx+5

Script

Using the following command, you can use HyperDbg's Script Engine. You should replace the string between braces (HyperDbg Script Here) with your script. You can find script examples here.

HyperDbg> !epthook fffff800`4ed6f010 script { HyperDbg Script Here }

The above command when messages don't need to be delivered immediately.

HyperDbg> !epthook fffff800`4ed6f010 script { HyperDbg Script Here } imm no

Script (From File)

If you saved your script into a file, then you can add file: instead of a script and append the file path to it. For example, the following examples show how you can run a script from file:c:\users\sina\desktop\script.txt.

HyperDbg> !epthook fffff800`4ed6f010 script {file:c:\users\sina\desktop\script.txt}

You can use event forwarding to forward the event monitoring results from this event and other events to an external source, e.g., File, NamedPipe, or TCP Socket. This way, you can use HyperDbg as a monitoring tool and gather your target system's behavior and use it later or analyze it on other systems.

Custom Code

Please read "How to create an action?" to get an idea about running a custom buffer code in HyperDbg.

Your custom code will be executed in vmx-root mode. Take a look at this topic for more information. Running code in vmx-root is considered "unsafe".

Run Custom Code (Unconditional)

Putting a hook on fffff801deadbeef and run 3 nops whenever the hook is triggered. Take a look at Run Custom Code for more information.

HyperDbg> !epthook fffff801deadbeef code {90 90 90}

Or if you want to use assembly codes directly, you can add an asm before the code.

HyperDbg> !epthook fffff801deadbeef asm code {nop; nop; nop}

Run Custom Code (Conditional)

Putting a hook on fffff801deadbeef and run 3 nops whenever the hook is triggered and also 3 nops condition. Take a look at Run Custom Code and how to create a condition for more information.

HyperDbg> !epthook fffff801deadbeef code {90 90 90} condition {90 90 90}

Or if you want to use assembly codes directly, you can add an asm before the condition and also before the code.

HyperDbg> !epthook fffff801deadbeef asm code {nop; nop; nop} asm condition {nop; nop; nop}

Keep in mind that a conditional event can be used in Breaking to Debugger and Running Script too.

IOCTL

This command uses the same method to send IOCTL for regular events.

Use HIDDEN_HOOK_EXEC_CC as EventType, and send the address of where you want to hook in OptionalParam1in DEBUGGER_GENERAL_EVENT_DETAIL.

Design

Take a look at "Design of !epthook" to see how does it work.

Remarks

This command is much slower than !epthook2, because it cause vm-exits, but on the other hand, this implementation doesn't have any limitation. For example, you can use this command for hooking user-mode while you can't use !epthook2 on user-mode.

You shouldn't use any of !monitor, !epthook, and !epthook2 commands on the same page (4KB) simultaneously. For example, when you put a hidden hook (!epthook2) on 0x10000005, you shouldn't use any of !monitor or !epthook commands on the address starting from 0x10000000 to 0x10000fff.

You can use !epthook (just !epthook not !epthook2 and not !monitor) on two or more addresses on the same page (means that you can use the !epthook multiple times for addresses between a single page or putting multiple hidden breakpoints on a single page). But you can't use !monitor or !epthook2 twice on the same page.

This command cannot be used simultaneously with the '!mode' command.

This command creates an event. Starting from HyperDbg v0.7, events are guaranteed to keep the debuggee in a halt state (in the Debugger Mode); thus, nothing will change during its execution and the context (registers and memory) remain untouched. You can visit instant events for more information.

Requirements

Post-Nehalem Processor (EPT)

Processor with Execute-only Pages Support

Related

bp (set breakpoint)

!epthook2 (hidden hook with EPT - detours)