FLIRT Signatures Evasion

Created the Friday 01 July 2022. Updated 5 months ago.

FLIRT (Fast Library Identification and Recognition Technology) is a database that contains pattern of bytes (signatures) used to identify known functions from legit libraries.

Malwares can abuse known FLIRT signatures by replacing or adding specific bytes to hide malicious code inside a function that would be recognised as a legitimate library.

This technique will trick the reverse engineering process if the FLIRT signatures is trusted without further analysis.

Technique Identifier


Technique Tag


Code Snippets



The malicious sample listed below abused variant bytes of __IsNonwritableInCurrentImage signature to add two instructions consisting of an anti-debugging technique referenced as U0114 on Unprotect.

Sha256 : a41ba65405a032f4450ba80882cdd01d715d9d1684f4204050566be29a6dedb0

// Malicious code
.text:00A4264A                 push    0DEADBEEFh
.text:00A4264F                  call    kernel32_CloseHandle

//Full abused function tagged as __IsNonwritableInCurrentImage.
.text:00A42610 __IsNonwritableInCurrentImage proc near ; CODE XREF: sub_A428D0:loc_A429E2↓p
.text:00A42610                 ms_exc          = CPPEH_RECORD ptr -18h
.text:00A42610 ; __unwind { // __except_handler4
.text:00A42610                 push    ebp
.text:00A42611                 mov     ebp, esp
.text:00A42613                 push    0FFFFFFFEh
.text:00A42615                 push    offset stru_A5AE98
.text:00A4261A                 push    offset __except_handler4
.text:00A4261F                 mov     eax, large fs:0
.text:00A42625                 push    eax
.text:00A42626                 sub     esp, 8
.text:00A42629                 push    ebx
.text:00A4262A                 push    esi
.text:00A4262B                 push    edi
.text:00A4262C                 mov     eax, ___security_cookie
.text:00A42631                 xor     [ebp+ms_exc.registration.ScopeTable], eax
.text:00A42634                 xor     eax, ebp
.text:00A42636                 push    eax
.text:00A42637                 lea     eax, [ebp+ms_exc.registration]
.text:00A4263A                 mov     large fs:0, eax
.text:00A42640                 mov     [ebp+ms_exc.old_esp], esp
.text:00A42643 ;   __try { // __except at loc_A42676
.text:00A42643                 mov     [ebp+ms_exc.registration.TryLevel], 0
.text:00A4264A                 push    0DEADBEEFh
.text:00A4264F                 call    kernel32_CloseHandle
.text:00A4264F ;   } // starts at A42643
.text:00A42655                 mov     [ebp+ms_exc.registration.TryLevel], 0FFFFFFFEh
.text:00A4265C                 xor     eax, eax
.text:00A4265E                 mov     ecx, [ebp+ms_exc.registration.Next]
.text:00A42661                 mov     large fs:0, ecx
.text:00A42668                 pop     ecx
.text:00A42669                 pop     edi
.text:00A4266A                 pop     esi
.text:00A4266B                 pop     ebx
.text:00A4266C                 mov     esp, ebp
.text:00A4266E                 pop     ebp
.text:00A4266F                 retn
.text:00A42670 ; ---------------------------------------------------------------------------
.text:00A42670 loc_A42670:                             ; DATA XREF: .rdata:stru_A5AE98↓o
.text:00A42670 ;   __except filter // owned by A42643
.text:00A42670                 mov     eax, 1
.text:00A42675                 retn
.text:00A42676 ; ---------------------------------------------------------------------------
.text:00A42676 loc_A42676:                             ; DATA XREF: .rdata:stru_A5AE98↓o
.text:00A42676 ;   __except(loc_A42670) // owned by A42643
.text:00A42676                 mov     esp, [ebp+ms_exc.old_esp]
.text:00A42679                 mov     [ebp+ms_exc.registration.TryLevel], 0FFFFFFFEh
.text:00A42680                 mov     eax, 2000h
.text:00A42685                 mov     ecx, [ebp+ms_exc.registration.Next]
.text:00A42688                 mov     large fs:0, ecx
.text:00A4268F                 pop     ecx
.text:00A42690                 pop     edi
.text:00A42691                 pop     esi
.text:00A42692                 pop     ebx
.text:00A42693                 mov     esp, ebp
.text:00A42695                 pop     ebp
.text:00A42696                 retn
.text:00A42696 ; } // starts at A42610
.text:00A42696 __IsNonwritableInCurrentImage endp

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