原文如下：
Errors, Vulnerabilities & Exploits explained
2007-3-2 - Mark Vogels
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)公钥 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: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:
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代码:
#include "stdio.h"
exploitfunction()
{
printf("This line will be printed after successfully exploiting the buffer overflow. ");
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(" These lines get printed during normal execution with at least 1 commandline argument.The address of exploitfunction
is 0x%.8X ",&exploitfunction);
}
else
printf("Please provide the program with at least 1 commandline argument. ");
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:
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代码:
#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:
引用:
C:>exploit.exe
This line will be printed after successfully 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.
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