Hot answers tagged

34

TL;DR the first 3 lines set an exception handler (an 'error catcher') the int3 generates an exception execution resumes at next Explanation this trick is (ab)using Structured Exception Handling, a mechanism to define exception handlers, typically by compilers when try/catch blocks are used. In 32bits versions of Windows, they can be set on the fly, ...


6

First of all, read this: https://www.blackhat.com/presentations/bh-asia-03/bh-asia-03-litchfield.pdf That's pretty much how all this started. A SEH buffer overflow is a specific stack overflow that targets the EXCEPTION_REGISTRATION_RECORD sitting some arbitrary distance down the stack. Why does this not happen for every program then since there should ...


6

No, the Windows loader doesn't care about the name of the .pdata section. It doesn't find the RUNTIME_FUNCTION structs based on the section name, but rather based on the content of NtHeader->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXCEPTION]. Furthermore, the RUNTIME_FUNCTION structs don't need to be in "a separate COFF section".


5

You need to check the PE headers of the modules, the offset is dependent on the OS version (the settings can be different for the executable and each loaded DLL). You can look at the mona.py script for examples on how to do that. The script is very large but you should find what you are looking for in the class MnModule, for example the flag for ALSR is ...


5

Regarding your original question: Upon ntdll.dll loading each PE image, the list of exception handler addresses in the PE image is parsed and each address is inserted into a sorted list that ntdll.dll internally uses. When an exception hits, ntdll.dll!KiUserExceptionDispatcher will try to figure out which exception in the SEH chain to use. In turn, each ...


4

I post my own answer based on my own research. I am not sure about correctness of the parts in bold. Comments correcting my answer or fuller and better answers are welcome. I will edit out the boldness after I receive comments confirming or refuting my assumptions. Assume that your malicious buffer overwrote the _EXCEPTION_REGISTRATION_RECORD and you found ...


4

the __ehstate_t type is defined as int. toState is the index of the next handler in the array that needs to be unwind. Action is the operation that needs to be done in order to reach the next state (the value of toState). This happens until toState reaches -1, which is the "blank" state that ends the unwinding process. The state is incremented by one every ...


3

A more flexible solution using x64dbg In addition to ollydbg2's built-in experimental support, it's relatively easy to achieve something similar in x64dbg, although some manual scripting is required. Using x64dbg's SetExceptionBPX function, you can define any exception to be treated as a breakpoint by x64dbg instead of being treated by regular exception ...


3

if you were on windows then you can try ollydbg v2


3

In case of the compiler-level SEH, the callback invoked by the OS is the compiler-provided function, usually __except_handler3 or similar. Once called, it inspects the stack, retrieves the trylevel, looks up the corresponding scopetable entry and calls the exception filter (lpfnFilter). If the filter returns non-zero, the handler (lpfnHandler) is invoked - ...


2

So, EHFlags bits are defined as follows (see ehdata_values.h): #define FI_EHS_FLAG 0x00000001 #define FI_DYNSTKALIGN_FLAG 0x00000002 #define FI_EHNOEXCEPT_FLAG 0x00000004 i.e. 5 is FI_EHS_FLAG|FI_EHNOEXCEPT_FLAG In crt\src\vcruntime\frame.cpp we can find this snippet: auto tryBlockMap = T::TryBlockMap(pFuncInfo, pDC); if (...


2

I think this is a great moment to introduce you to this great article written back in 1997, but which still holds up. I really recommend that you read it because it will explain to you everything about SEH. To answer your question about continuing execution, I will quote one paragraph from the article: When the operating system sees that ...


2

First, your platform is very important ( mine is Windows ) In Windows WinDbg + !exploitable is one of fast analyze options. it is here Additionally I use WinDbg + !analyze to determine standard name of bug... it is default WinDbg extension. Finally, as the nature of bugs is unknown (in your case) it is not an easy way to detect root cause.


2

Standard exploitation of SEH based BoF will try to point the SE Handler to a POP/POP/RET instruction. This is not always the case depending on enabled memory protections. (use Mona plugin for Immunity to check this using the command !mona mod, or simply inspect the vulnerable binary characteristics) If your SE Handler is pointing to 0x384c5c36, you should ...


2

38 4c 5c 36 is the value of the EIP when dbg cannot continue. Based on your comment above, in response to your questions: The exploit's payload. It's the location in memory where the exploit's payload is loaded. If you're only looking to analyze the payload (and not the exploit/trigger), set a breakpoint on that address and debug from there.


2

To add to @jvoisin's answer, you can have a look at checksec - python implementation of checksec.sh Both these scripts are simple (compared to mona.py) and should help you get started.


2

If you are under GNU/Linux, you may want to look at checksec.sh. If you are under Windows, you could take at look at it; maybe you'll find some interesting stuff that you could backport in Python.


2

just modified your source a bit to print Rip prior and post in handler added another exception and eliminated a warning (empty handler block) and tested with +1,+2,+3 +4 +5 on the handler Hardware breaks doesn't get hit I haven't checked by single stepping inside RtlpExecuteHandlerForxxx calls here is a modified src #include <windows.h> #...


1

Well the comment: "Install new handler" is a bit misleading. What you are installing is EXCEPTION_REGISTRATION_RECORD and if you would check how this structure looks like it would be more obvious that this is the way: typedef struct _EXCEPTION_REGISTRATION_RECORD { PEXCEPTION_REGISTRATION_RECORD Next; PEXCEPTION_DISPOSITION Handler; } ...


1

There are two things you must check: You overwritten exception handler but, is there any exception occurs? Are there any protection mechanism active for exception handlers? For first one, most easy exception which you can trigger and not caught until your handler is obviously access violation. You can try to trigger an exception with overwrite a value ...


1

The SEH overwrite attack requires that the function has registered an SEH handler. The SEH handler address exists below the stack cookie and return address. If you've overwritten the return address then you've written too many data. The chain's integrity will be checked, and there must be executable memory at the handler address (i.e. 0x41414141) for the ...


Only top voted, non community-wiki answers of a minimum length are eligible