This is proof that an exception handler is registered. Thats normal, because the application is not started yet. Defaulted to export symbols They form a linked list chain on the stack, and sit relatively close to the bottom of the stack.

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Exploit writing tutorial part 3 : SEH Based Exploits Published July 25, By Corelan Team corelanc0d3r In the first 2 parts of the exploit writing tutorial series, I have discussed how a classic stack buffer overflow works and how you can build a reliable exploit by using various techniques to jump to the shellcode. The example we have used allowed us to directly overwrite EIP and we had a pretty large buffer space to host our shellcode.

On top of that, we had the ability to use multiple jump techniques to reach our goal. But not all overflows are that easy. What are exception handlers? An exception handler is a piece of code that is written inside an application, with the purpose of dealing with the fact that the application throws an execption.

This is often the result of the default handler kicking in. It is obvious that, in order to write stable software, one should try to use development language specific exception handlers, and only rely on the windows default SEH as a last resort. So in the event an error or illegal instruction occurs, the application will get a chance to catch the exception and do something with it.

If no exception handler is defined in the application, the OS takes over, catches the exception, shows the popup asking you to Send Error Report to MS. In order for the application to be able to go to the catch code, the pointer to the exception handler code is saved on the stack for each code block. Each code block has its own stack frame, and the pointer to the exception handler is part of this stack frame. This ensures that the exception handler is set up for the thread and will be able to catch errors when they occur.

The opcode for this instruction is 64A This will trigger an improper termination of the program and the OS handler will kick in Quick example : compile the following source code sehtest. This is proof that an exception handler is registered. Defaulted to export symbols for The TEB for the main function is now set up. They form a linked list chain on the stack, and sit relatively close to the bottom of the stack. SEH chain. When an exception occurs, Windows ntdll. The current handler points at 7CAD8.

The handler points at A. This address is part of the executable, so it looks like this is an application handler. This means that this is the last entry in the chain. I will write an entire post on Stack cookies and DEP. In sort, you only need to remember that these two techniques can make it significantly harder to build exploits.

Since you cannot simply jump to a register because registers are xored , a call to a series of instructions in a dll will be needed. You should try to avoid using a call from the memory space of an OS specific dll, but rather use an address from an application dll instead in order to make the exploit reliable assuming that this dll is not compiled with safeSEH.

The theory behind this technique is : If we can overwrite the pointer to the SE handler that will be used to deal with a given exception, and we can cause the application to throw another exception a forced exception , we should be able to get control by forcing the application to jump to your shellcode instead of to the real exception handler function.

You could try to use ntdll. I highly recommend using Immunity Debugger and pvefindaddr. Normally, the pointer to the next SEH record contains an address. But in order to build an exploit, we need to overwrite it with small jumpcode to the shellcode which should sit in the buffer right after overwriting the SE Handler.

The pop pop ret sequence will make sure this code gets executed In other words, the payload must do the following things cause an exception. The shellcode should be directly after the overwritten SE Handler.

See SEH in action — Ollydbg When performing a regular stack based buffer overflow, we overwrite the return address EIP and make the application jump to our shellcode.

When doing a SEH overflow, we will continue overwriting the stack after overwriting EIP, so we can overwrite the default exception handler as well. How this will allow us to exploit a vulnerability, will become clear soon. The application dies silently probably because of the exception handler that has kicked in, and has not been able to find a working SEH address because we have overwritten the address. Shortly after, the application dies and stops at this screen : The application has died at 0xE This address sits in the address space of user A couple of addresses higher on the stack, we can see some other exception handlers, but all of them also belong to the OS ntdll in this case.

So it looks like this application or at least the function that was called and caused the exception does not have its own exception handler routine. We caused an exception by building a malformed ui. Now it becomes interesting. Since we can control the value in the handler, we can have it execute our own code.

The debugger first breaks it puts a breakpoint before executing the file. This will launch the application. Alternatively, press F5 Soritong mp3 player launches, and dies shortly after.

When we run! Following frames may be wrong. Download the package, and put the dll file in the windbg program folder, inside the winext subfolder. So this is not limited to SEH based exploits When applying this module on the Soritong MP3 player, right after the first exception occurs, we see this : This exception may be expected and handled.

Defaulted to export symbols for SoriTong. Great module, nice work Microsoft :- Can I use the shellcode found in the registers to jump to?

Yes and no. But from SP1 and up, a protection mechanism has been plut in place to protect things like that from happening. Before the exception handler takes control, all registers are XOred with each other, so they all point to 0x That way, when SEH kicks in, the registers are useless.

This technique works well, but may cause stability issues if you cannot find a jmp instruction in a dll, or if you need to hardcode addresses , and it may also suffer from buffer size problems, limiting the amount of space available to host your shellcode. Then how can we exploit SEH based vulnerabilities? Next, put your shellcode. When the exception occurs, the application will go to the SE Handler. So you need to put something in the SE Handler so it would go to your shellcode.

This is done by faking a second exception, so the application goes to the next SEH pointer. The shellcode sits after the SE Handler. How can you find the shellcode with SEH based exploits? This will make the application break when the exception occurs, and then you can look for the shellcode. See the sections below on how to do this. Open windbg, open the soritong.

It will start paused, so launch it. The debugger will catch the first chance exception. This corresponds with offset This location is 0xfd64 as shown at the! This will make the flow jump back to next SEH. The opcode for a short jump is eb, followed by the jump distance. In other words, a short jump of 6 bytes corresponds with opcode eb So the next SEH field must be overwritten with 0xeb,0x06,0x90,0x90 How exactly does the pop pop ret function when working with SEH based exploits? When an exception occurs, the exception dispatcher creates its own stack frame.

It will push elements from the EH Handler on to the newly created stack as part of a function prologue. This field points to the address of the exception registration record the next SEH that was pushed onto the program stack. We have overwritten the next SEH with some basic jumpcode instead of an address , so the code gets executed.

In fact, the next SEH field can be considered as the first part of our shellcode jumpcode. Building the exploit — putting all pieces together After having found the important offsets, we only need the the address of a pop pop ret before we can build the exploit. Using findjmp. It may be easier to use msfpescan from Metasploit simply run msfpescan against the dll, with parameter -p look for pop pop ret and output everything to file.


Exploit Writing Tutorial Part 4 _ From Exploit to Metasploit – the Basics _ Corelan Team

Another exploit was released just a little bit later. Nice work. The question is : How do exploit writers build their exploits? What does the process of going from detecting a possible issue to building an actual working exploit look like? How can you use vulnerability information to build your own exploit? This document is purely intended for educational purposes.


Exploit Writing Tutorial Part 3 _ SEH Based Exploits _ Corelan Team

When a buffer on the stack overflows, the term stack based overflow or stack buffer overflow is used. When you are trying to write past the end of the stack frame, the term stack overflow is used. The next step is to determining the buffer size to write exactly into EIP. Metasploit has a nice tool to assist us with calculating the offset.



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