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The str instruction retrieves the segment selector from the task register, which points to the task state segment (TSS) of the currently executing task.

Because the value returned by this instruction will differ depending on whether it is run on a host or on a virtual machine, it is sometimes used by malware as a virtualization detection / anti-VM technique.

                         TSS matching VMware
TSS (hex)   │   ????   │   ????   │   0x400x00   │
TSS (bin)   │ ???????? │ ???????? │ 0000000001000000 │
            ¦          ¦          ¦          ¦          ¦
Byte offset ¦    0x3   ¦    0x2   ¦    0x1   ¦   0x0    ¦

For VMware, the 1st and second byte of TSS will be respectively 0x00 and 0x40.

Example 1

.text:00401204                 str     word ptr [ebp+var_418]   ; Load TSS into 4-byte local variable var_418
.text:00401229                 mov     edx, [ebp+var_418]       ; var_418 saved ti EDX
.text:0040122F                 and     edx, 0FFh                ; Get 1st byte of EDX
.text:00401235                 test    edx, edx                 ; Test whether 1st byte is 0x00
.text:00401237                 jnz     short loc_40124E         ; Test failed (native OS detected). Program continues...
.text:00401239                 mov     eax, [ebp+var_418+1]     ; ...else, 2nd byte saved to EAX
.text:0040123F                 and     eax, 0FFh
.text:00401244                 cmp     eax, 40h                 ; Is the 2nd byte equal to 0x40?
.text:00401247                 jnz     short loc_40124E         ; Test failed (native OS detected). Program continues...
.text:00401249                 jmp     loc_401336               ; ...else: malware self deletion

Example 2

C source

// Alfredo Andrés Omella's (S21sec) STR technique
test4 (void)
	unsigned char	mem[4] = {0, 0, 0, 0};
	__asm str mem;
	printf ("\n[+] Test 4: STR\n");
	printf ("STR base: 0x%02x%02x%02x%02x\n", mem[0], mem[1], mem[2], mem[3]);
	if ((mem[0] == 0x00) && (mem[1] == 0x40))
		printf ("Result  : VMware detected\n\n");
		printf ("Result  : Native OS\n\n");


.text:00401210 str_test        proc near
.text:00401210 var_4           = word ptr -4
.text:00401210 var_2           = byte ptr -2
.text:00401210 var_1           = byte ptr -1
.text:00401210                 push    ebp
.text:00401211                 mov     ebp, esp
.text:00401213                 push    ecx
.text:00401214                 mov     byte ptr [ebp+var_4], 0
.text:00401218                 mov     byte ptr [ebp+var_4+1], 0
.text:0040121C                 mov     [ebp+var_2], 0
.text:00401220                 mov     [ebp+var_1], 0
.text:00401224                 str     [ebp+var_4]
.text:00401228                 push    offset aTest4Str ; "\n[+] Test 4: STR\n"
.text:0040122D                 call    _printf
.text:00401232                 add     esp, 4
.text:00401235                 movzx   eax, [ebp+var_1]
.text:00401239                 push    eax
.text:0040123A                 movzx   ecx, [ebp+var_2]
.text:0040123E                 push    ecx
.text:0040123F                 movzx   edx, byte ptr [ebp+var_4+1]
.text:00401243                 push    edx
.text:00401244                 movzx   eax, byte ptr [ebp+var_4]
.text:00401248                 push    eax
.text:00401249                 push    offset aStrBase0x02x02 ; "STR base: 0x%02x%02x%02x%02x\n"
.text:0040124E                 call    _printf
.text:00401253                 add     esp, 14h
.text:00401256                 movzx   ecx, byte ptr [ebp+var_4]
.text:0040125A                 test    ecx, ecx
.text:0040125C                 jnz     short loc_401276
.text:0040125E                 movzx   edx, byte ptr [ebp+var_4+1]
.text:00401262                 cmp     edx, 40h
.text:00401265                 jnz     short loc_401276
.text:00401267                 push    offset aResultVmware_2 ; "Result  : VMware detected\n\n"
.text:0040126C                 call    _printf
.text:00401271                 add     esp, 4
.text:00401274                 jmp     short loc_401283
.text:00401276 ; ---------------------------------------------------------------------------
.text:00401276 loc_401276:
.text:00401276                 push    offset aResultNative_2 ; "Result  : Native OS\n\n"
.text:0040127B                 call    _printf
.text:00401280                 add     esp, 4
.text:00401283 loc_401283:
.text:00401283                 mov     esp, ebp
.text:00401285                 pop     ebp
.text:00401286                 retn
.text:00401286 str_test        endp


Keywords: str anti-vm tss