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Introduction to Buffer Overflow by Ghost_Rider
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Introduction to Buffer Overflow by Ghost_Rider

Hello, here I am again, this time I'll let you know what is in fact buffer overflow and how you can detect if some program is vulnerable to buffer overflow exploits. This tutorial has C source code, so if you don't know C you can have some problems in this tutorial, you also need to have some notions on ASM and how to use gdb. I tried to do the easiest I could, but still this tutorial isn't one of those where you really don't know **censored** about nothing and when you end it you know all this. This one takes some work to understand, hey it took huge work to write! A little inside note, like everyone that is reading this lines I like to learn, so some weeks ago I said to myself "Hey what the heck, why not to start reading some texts about buffer overflows, I know how everything work but just superficially", so I just started learning and now I'm trying to pass the knowledge that I gained, to everyone that is interested. So this won't be one of those texts where you'll learn everything, this will be like a walkthrough, like the title says an Introduction, (In the end I'll give you some nice texts). If you have any questions concerning this tutorial post in our message board, if you find any "bug" in this tutorial please email me and I'll correct it. Enjoy.

Exploit?

Well probably everyone knows what an exploit is. But you still got to see that the ones that are entering the security world for the first time probably don't have the idea of what that is, that's why I wrote this tinny section. So for the ones that don't know an exploit is a program, usually written in C, that exploits some problem that another program have. The exploit will allow you to run arbitrary code that will let you do something that you shouldn't be able to do in your normal status on the system.

Nowadays, most of the exploits are what we call Buffer Overflow Exploits. What's that you ask. Wait because we'll get there. After all, this is the subject of this tutorial. Another thing you should know is that everyone knows how to use them(how do you think that most of the websites that are defaced?), the script kiddies just go to sites like security focus, packetstorm or fyodor's exploit world, download it and run it, and then got busted. But why doesn't everybody write exploits? Well the problem is that many people doesn't know how to spot some vulnerability in the source code, or even if they can they aren't able to write a exploit. So now that you have an idea of what an exploit is, let's go ahead to the buffer overflow section.

Buffer Overflow after all what's that?

Like I said before most of the exploits are Buffer Overflow exploits.

You are probably now thinking "Bah..this guy is bull**censored**ing around, but still didn't said what buffer overflow is". So let's just talk about it. A buffer overflow problem is based in the memory where the program stores it's data. Why's that, you ask. Well because what buffer overflow do is overwrite expecific memory places where should be something you want, that will make the program do something that you want.

Well some of you right now are thinking "WOW, I know how buffer overflow works", but you still don't know how to spot them.

Let's follow a program and try to find and fix the buffer overflow

------ Partial code below--------

main(int argc, char **argv) {

char *somevar;

char *important;

somevar = (char *)malloc(sizeof(char)*4);

important = (char *)malloc(sizeof(char)*14);

strcpy(important, "command"); /*This one is the important variable*/

stcrpy(somevar, argv[1]);

..... Code here ....

}

.... Other functions here ....

------- End Of Partial Code ------

So let's say that important variable stores some system command like, let's say "chmod o-r file", and since that file is owned by root the program is run under root user too, this means that if you can send commands to it, you can execute ANY system command. So you start thinking. How the hell can I put something that I want in the important variable. Well the way is to overflow the memory so we can reach it. But let's see variables memory addresses.

To do that you need to re-written the code. Check the following code.

--------- Partial Code ------------

main (int argc, char **argv) {

char *somevar;

char *important;

somevar=(char *)malloc(sizeof(char)*4);

important=(char *)malloc(sizeof(char)*14);

printf("%p %p", somevar, important);

exit(0);

rest of code here

}

--------- End of Partial Code --------

Well we added 2 lines in the source code and left the rest unchanged. Let's see what does two lines do.

The printf("%p %p", somevar, important); line will print the memory addresses for somevar and important variables. The exit(0); will just keep the rest of the program running after all you don't want it for nothing, your goal was to know where is the variables are stored.

After running the program you would get an output like, you will probably not get the same memory addresses:

0x8049700 <----- This is the address of somevar

0x8049710 <----- This is the address of important

As we can see, the important variable is next somevar, this will let us use our buffer overflow skills, since somevar is got from argv[1]. Now, we know that one follow the other, but let's check each memory address so we can have the precise notion of the data storage. To do this let's re-write the code again.
-------- Partial code ---------

main(int argc, char **argv) {

char *somevar;

char *important;

char *temp; /* will need another variable */

somevar=(char *)malloc(sizeof(char)*4);

important=(char *)malloc(sizeof(char)*14);

strcpy(important, "command"); /*This one is the important variable*/

stcrpy(str, argv[1]);

printf("%p %p ", somevar, important);

printf("Starting To Print memory address: ");

temp = somevar; /* this will put temp at the first memory address we want */

while(temp < important + 14) {

/* this loop will be broken when we get to the last memory address we want, last memory address of important variable */

printf("%p: %c (0x%x) ", temp, *temp, *(unsigned int*)temp);

temp++;

}

exit(0);

rest of code here

}

------ End Of partial Code ------

Now let's say that the argv[1] should be in normal use send. So you just type in your prompt:

$ program_name send

You'll get an output like:

0x8049700

0x8049710

Starting To Print memory address:

0x8049700: s (0x616c62)

0x8049701: e (0x616c)

0x8049702: n (0x61) <---- each of this lines represent a memory address

0x8049703: d (0x0)

0x8049704: (0x0)

0x8049705: (0x0)

0x8049706: (0x0)

0x8049707: (0x0)

0x8049708: (0x0)

0x8049709: (0x19000000)

0x804970a: (0x190000)

0x804970b: (0x1900)

0x804970c: (0x19)

0x804970d: (0x63000000)

0x804970e: (0x6f630000)

0x804970f: (0x6d6f6300)

0x8049710: c (0x6d6d6f63)

0x8049711: o (0x616d6d6f)

0x8049712: m (0x6e616d6d)

0x8049713: m (0x646e616d)

0x8049714: a (0x646e61)

0x8049715: n (0x646e)

0x8049716: d (0x64)

0x8049717: (0x0)

0x8049718: (0x0)

0x8049719: (0x0)

0x804971a: (0x0)

0x804971b: (0x0)

0x804971c: (0x0)

0x804971d: (0x0)

$

Nice isn't it? You can now see that there exist 12 memory address empty between somevar and important. So let's say that you run the program with a command line like:

$ program_name send------------newcommand

You'll get an output like:

0x8049700

0x8049710

Starting To Print memory address:

0x8049700: s (0x646e6573)

0x8049701: e (0x2d646e65)

0x8049702: n (0x2d2d646e)

0x8049703: d (0x2d2d2d64)

0x8049704: - (0x2d2d2d2d)

0x8049705: - (0x2d2d2d2d)

0x8049706: - (0x2d2d2d2d)

0x8049707: - (0x2d2d2d2d)

0x8049708: - (0x2d2d2d2d)

0x8049709: - (0x2d2d2d2d)

0x804970a: - (0x2d2d2d2d)

0x804970b: - (0x2d2d2d2d)

0x804970c: - (0x2d2d2d2d)

0x804970d: - (0x6e2d2d2d)

0x804970e: - (0x656e2d2d)

0x804970f: - (0x77656e2d)

0x8049710: n (0x6377656e) <--- memory address where important variable starts

0x8049711: e (0x6f637765)

0x8049712: w (0x6d6f6377)

0x8049713: c (0x6d6d6f63)

0x8049714: o (0x616d6d6f)

0x8049715: m (0x6e616d6d)

0x8049716: m (0x646e616d)

0x8049717: a (0x646e61)

0x8049718: n (0x646e)

0x8049719: d (0x64)

0x804971a: (0x0)

0x804971b: (0x0)

0x804971c: (0x0)

0x804971d: (0x0)

Hey cool, newcommand got over command. Now it does something you want, instead of something he was supposed to do. NOTE: Remember sometimes those spaces between somevar and important can have other variables instead of being empty, so check their values and send them to the same address, or the program can crash before

getting to the variable that you modified. Now let's think a little. Why does this happen? As you can see in the source code somevar is declared before important, this will make, most of the times, that somevar will be first in memory. Now, let's check how each one is got. Somevar gets it's value from argv[1], and important gets it from strcpy() function, but the real problem is that important value is assign first so when you assign value to somevar that is before it important can be overwritten.

This program could be patched against this buffer overflow switching those two lines, becoming :

strcpy(somevar, argv[1]);

strcpy(important, "command");

If this was the way that the program was done even if you give an argument that would get into the memory address of important, it will be overwritten by the true command, since after getting somevar, is assign the value command to important.

This kind of buffer overflow, is a heap buffer overflow. Like you probably has seen they are really easy to do in theory but, in the real world, it's not really easy to do them, after all the example I gave was a really dumb program right? It's a real pain in the ass to find those important variables, and also to overflow that variable you need to be able to write to one that is in a lower memory address, most of times all this conditions.