Guard Pages

Created the Saturday 23 March 2019. Updated 6 months, 2 weeks ago.

Memory breakpoints are a technique used by malware to detect if a debugger is present. This technique involves setting up a "guard page" in memory, which is a page of memory that is protected by the operating system and cannot be accessed by normal code. If a debugger is present, the malware can use this guard page to detect its presence.

This technique works by putting a return address onto the stack, then accessing the guard page. If the operating system detects that the guard page has been accessed, it will raise a STATUS_GUARD_PAGE_VIOLATION exception. The malware can then check for this exception, and if it is present, it can assume that no debugging is taking place. This allows the malware to evade detection and continue to operate without being interrupted by a debugger.

Technique Identifiers

U0102 B0006.006

Technique Tags

Memory breakpoints Guard pages Debugger detection STATUS_GUARD_PAGE_VIOLATION

Featured Windows API's

Below, you will find a list of the most commonly used Windows API's that are currently utilized by malware authors for current evasion technique. This list is meant to provide an overview of the API's that are commonly used for this purpose. If there are any API's that you feel should be included on this list, please do not hesitate to contact us. We will be happy to update the list and provide any additional information or documentation that may be helpful.

Code Snippets

Description

Source: https://github.com/LordNoteworthy/al-khaser/blob/master/al-khaser/AntiDebug/MemoryBreakpoints_PageGuard.cpp

#include "pch.h"

#include "MemoryBreakpoints_PageGuard.h"

/*
In essence, what occurs is that we allocate a dynamic buffer and write a RET to the buffer.
We then mark the page as a guard page and push a potential return address onto the stack. Next, we jump to our page,
and if we're under a debugger, specifically OllyDBG, then we will hit the RET instruction and return to the address we pushed onto
the stack before we jumped to our page. Otherwise, a STATUS_GUARD_PAGE_VIOLATION exception will occur, and we know we're not being
debugged by OllyDBG.
*/

BOOL MemoryBreakpoints_PageGuard()
{
	UCHAR *pMem = NULL;
	SYSTEM_INFO SystemInfo = { 0 };
	DWORD OldProtect = 0;
	PVOID pAllocation = NULL; // Get the page size for the system 

	// Retrieves information about the current system.
	GetSystemInfo(&SystemInfo);

	// Allocate memory 
	pAllocation = VirtualAlloc(NULL, SystemInfo.dwPageSize, MEM_COMMIT | MEM_RESERVE, PAGE_EXECUTE_READWRITE);
	if (pAllocation == NULL)
		return FALSE;

	// Write a ret to the buffer (opcode 0xc3)
	RtlFillMemory(pAllocation, 1, 0xC3);

	// Make the page a guard page         
	if (VirtualProtect(pAllocation, SystemInfo.dwPageSize, PAGE_EXECUTE_READWRITE | PAGE_GUARD, &OldProtect) == 0)
		return FALSE;

	__try
	{
		((void(*)())pAllocation)(); // Exception or execution, which shall it be :D?
	}
	__except (GetExceptionCode() == STATUS_GUARD_PAGE_VIOLATION ? EXCEPTION_EXECUTE_HANDLER : EXCEPTION_CONTINUE_SEARCH)
	{
		VirtualFree(pAllocation, 0, MEM_RELEASE);
		return FALSE;
	}

	VirtualFree(pAllocation, 0, MEM_RELEASE);
	return TRUE;
}

Detection Rules

                                    rule Detect_GuardPages: AntiDebug {
    meta: 
        description = "Detect Guard Pages as anti-debug"
        author = "Unprotect"
        comment = "Experimental rule"
    strings:
        $1 = "GetSystemInfo" fullword ascii
        $2 = "VirtualAlloc" fullword ascii
        $3 = "RtlFillMemory" fullword ascii
        $4 ="VirtualProtect" fullword ascii
        $5 ="VirtualFree" fullword ascii
    condition:   
       uint16(0) == 0x5A4D and filesize < 1000KB and 4 of them 
}

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.

Anti-debugging Techniques Cheat Sheet - 0xAA - Random notes on security