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Instead of calling IsDebuggerPresent(), some packers manually check the PEB (Process Environment Block) for the BeingDebugged flag.

Field Size Offset relative to PEB
IsDebuggerPresent BYTE 0x2

In the below example, fs:30h represents the PEB. It is saved to the EAX register at offset 0x4010CD. At offset 0x4010D3, the value 1 is compared to the offset 2 of the pointer to EAX:

.text:004010CD                 mov     eax, large fs:30h   ; PEB
.text:004010D3                 db      3Eh                 ; IDA Pro display error (byte is actually used in next instruction)
.text:004010D3                 cmp     byte ptr [eax+2], 1 ; PEB.BeingDebugged
.text:004010D8                 jz      short loc_4010E1

As depicted on the following extract showing the PEB structure, BeingDebugged lies at offset 0x002:


If byte ptr [eax+2] returns 1, it means the the program is being debugged and the jump at offset 0x4010D8 won't be taken.

Here is the full function:


Thwart the BeingDebugged chek

To thwart this check, you can:

  • Control the jump by modifying the ZeroFlag (ZF) value before the jump instruction is executed
  • Manually change the BeingDebugged flag value to zero
  • In OllyDbg, use specific plugins (Hide Debugger, PhantOm, ...)

Below is an example that depicts how to manually change the BeingDebugged flag. We have the following code:

.text:00403554                 mov     eax, large fs:30h   ; PEB struct loaded into EAX
.text:0040355A                 mov     bl, [eax+2]         ; BeingDebugged (offset 0x2 relative to PEB) moved to BL
.text:0040355D                 mov     [ebp+var_1820], bl
.text:00403563                 movsx   eax, [ebp+var_1820]
.text:0040356A                 test    eax, eax            ; BeingDebugged check
.text:0040356C                 jz      short loc_403573    ; If flag set to 0, program continues normally...
.text:0040356E                 call    s_selfDelete        ; ...else, program is deleted

Let's open the malware in OllyDbg and set a breakpoint at 0x40355A. The PEB structure has been loaded into the EAX register. Right click on EAX and select "Follow in dump":


Notice that at offset 0x2 in the PEB structure, the value is 0x1. It corresponds to the BeingDebugged flag. Right click on it and select Binary > Fill with 00's.

We could also have used the OllyDbg CommandLine plugin to directly reach the BeingDebugged flag: