On 32-bit machines, the
NtGlobalFlag field is located at the offset of
PEB (process environment block)
0x68, and the 64-bit machine is at offset '0xBC`. The default value for this field is 0. When debugging This field is set to a specific value while the device is running. Although this value does not quite indicate that a debugger is actually running, this field is often used for this purpose.
This field contains a series of flag bits. The process created by the debugger sets the following flags:
FLG_HEAP_ENABLE_TAIL_CHECK (0x10) FLG_HEAP_ENABLE_FREE_CHECK (0x20) FLG_HEAP_VALIDATE_PARAMETERS (0x40)
Therefore, you can check these flags to detect the presence of the debugger. For example, use the 32-bit code below to detect on a 32-bit machine:
mov eax, fs:[30h] ;Process Environment Block
mov al, [eax+68h] ;NtGlobalFlag
and al, 70h cmp al, 70h je being_debugged
The following is the detection code for 64-bit code on a 64-bit machine: `` `asm push 60h pop rsi gs:lodsq ;Process Environment Block mov al, [rsi*2+rax-14h] ;NtGlobalFlag and al, 70h cmp al, 70h je being_debugged
It should be noted that if a 32-bit program is running on a 64-bit machine, there will actually be two PEBs: one is a 32-bit part and the other is a 64-bit. The corresponding field of the 64-bit PEB will also look like Changed in 32-bit.
So we have the following, using a 32-bit code to detect the 64-bit machine environment:
mov eax, fs:[30h] ; Process Environment Block ;64-bit Process Environment Block ;follows 32-bit Process Environment Block mov al, [eax+10bch] ;NtGlobalFlag and al, 70h cmp al, 70h je being_debugged
Remember not to compare directly without masking other bits, so you won't be able to detect the debugger.
NtGlobalFlag is used to detect the debugger, but the three flags of
NtGlobalFlag are only when the program is
created by the debugger instead of the process attached by the debugger. Will be set.
Change the initial value of NtGlobalFlag¶
Of course, the way to bypass this detection is also very simple, that is, the debugger wants to reset the field to 0. However, this default initial value can be changed in any of the following four ways:
The value of
GlobalFlagof the registry
HKLM\System\CurrentControlSet\Control\SessionManagerwill be replaced by the
NtGlobalFlagfield. Although it may be changed by Windows (described below), the registry key will be on the system. All processes in the process have an impact and take effect after the restart.
Of course, this also produces another way to detect the debugger: If a debugger copies the key values in the registry to the
NtGlobalFlag field in order to hide itself, the key values in the registry have been replaced beforehand and have not yet been The restart takes effect. Then the debugger just copies a fake value, not the one that is really needed. If the program knows the real value instead of the fake value in the registry, then the debugger can be detected.
Of course, the debugger can also run other processes and then query the
NtGlobalFlag field to get the real value.
- It is still
GlobalFlag, but here is
HKLM\Software\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\ <filename>. (Image hijacking), here you need to
Replace the file name of the executable file that needs to be changed (no need to specify the path). After settingGlobalFlag
, the system will overwrite its value to theNtGlobalFlag` field (only covered for the specified process). Can be changed again by Windows (see below).
- Load two fields in the configuration table (
Load Configuration Table):
GlobalFlagsClear lists the flags that need to be cleared, and
GlobalFlagsSet lists the flags that need to be set. These settings will take effect after the
GlobalFlag application, so it can override the value specified by
GlobalFlag. However it Cannot override the flag set by Windows. For example, setting
FLG_USER_STACK_TRACE_DB (0x1000) allows Windows to set the
FLG_HEAP_VALIDATE_PARAMETERS (0x40) flag, even if
FLG_HEAP_VALIDATE_PARAMETERS is cleared in the load configuration table (
Load Configuration Table). Windows will also be reset during subsequent process loading.
- When the debugger creates a process, Windows will make some changes. By setting
_NO_DEBUG_HEAPin the environment variable,
NtGlobalFlagwill not set the flags of the 3 heaps because of the debugger. Of course they are still You can continue to set it by
GlobalFlagor by loading
GlobalFlagsSetin the configuration table.
How to bypass detection?¶
There are 3 ways to bypass the detection of
- Manually modify the value of the flag bit (
- Use the
hide-debugplugin in Ollydbg
- Start the program in the way that Windbg disables the debug heap (
windbg -hd program.exe)
Manual bypass example¶
The following is an example that demonstrates how to manually bypass detection.
.text:00403594 64 A1 30 00 00 00 mov eax, large fs:30h ; PEB struct loaded into EAX
.text:0040359A db 3Eh ; IDA Pro display error (the byte is actually used in the next instruction)
.text:0040359A 3E 8B 40 68 mov eax, [eax+68h] ; NtGlobalFlag (offset 0x68 relative to PEB) saved to EAX
.text:0040359E 83 E8 70 sub eax, 70h ; Value 0x70 corresponds to all flags on (FLG_HEAP_ENABLE_TAIL_CHECK, FLG_HEAP_ENABLE_FREE_CHECK, FLG_HEAP_VALIDATE_PARAMETERS)
.text:004035A1 89 85 D8 E7 FF FF mov [ebp+var_1828], eax
.text:004035A7 83 BD D8 E7 FF FF 00 cmp [ebp+var_1828], 0 ; Check whether 3 debug flags were on (result of substraction should be 0 if debugged)
.text:004035AE 75 05 jnz short loc_4035B5 ; No debugger, program continues...
.text:004035B0 E8 4B DA FF FF call s_selfDelete ; ...else, malware deleted
Set the breakpoint in offset
0x40359A in Ollydbg, run the program to trigger the breakpoint. Then open the
CommandLine plugin with
dump fs:+0x68dump out the contents of
Binary->Fill with 00's and replace the value
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