Introduction to IDAPython for Vulnerability Hunting - Part 2

Overview

In our last post we reviewed some basic techniques for hunting vulnerabilities in binaries using IDAPython. In this post we will expand upon that work and extend our IDAPython script to help detect a real Microsoft Office vulnerability that was recently found in the wild. The vulnerability that will be discussed is a remote code execution vulnerability that existed in the Microsoft Office EQNEDT32.exe component which was also known as the Microsoft Office Equation Editor. This program was in the news in January when, due to a number of discovered vulnerabilities. Microsoft completely removed the application (and all of its functionality) from Microsoft Office in a security update. The vulnerabilities that ended up resulting in Equation Editor being killed are of the exact type that we attempted to identify with the script written in the previous blog post. However, calls to strcpy in the Equation Editor application were optimized out and inlined by the compiler, rendering the script that we wrote previously ineffective at locating the vulnerable strcpy usages.

While the techniques that we reviewed in the previous post are useful in finding a wide range of dangerous function calls, there are certain situations where the script as written in the previous blog post will not detect the dangerous programming constructs that we intend and would expect it to detect. This most commonly occurs when compiler optimizations are used which replace function calls to string manipulation functions (such as strcpy and strcat) with inline assembly to improve program performance. Since this optimization removes the call instruction that we relied upon in our previous blog post, our previous detection method does not work in this scenario. In this post we will cover how to identify dangerous function calls even when the function call itself has been optimized out of the program and inlined.

Understanding the Inlined strcpy()

Before we are able to find and detect inlined calls to strcpy, we first need to understand what an inlined strcpy would look like. Let us take a look at an IDA Pro screenshot below that shows the disassembly view side-by-side with the HexRays decompiler output of an instance where a call to strcpy is inlined.

 Example of inlined strcpy() - Disassembly on left, HexRays decompiled version on right

Example of inlined strcpy() - Disassembly on left, HexRays decompiled version on right

In the above screenshot we can observe that the decompiler output on the right side shows that a call to strcpy in made, but when we look to the left side disassembly there is not a corresponding call to strcpy. When looking for inlined string functions, a common feature of the inlined assembly to watch for is the use of the “repeat” assembly instructions (rep, repnz, repz) in performing the string operations. With this in mind, let’s dig into the disassembly to see what the compiler has used to replace strcpy in the disassembly above.

 

First, we observe the instruction at 0x411646: `repne scasb`. This instruction is commonly used to get the length of a string (and is often used when strlen() is inlined by the compiler). Looking at the arguments used to set up the `repne scasb` call we can see that it is getting the length of the string “arg_0”. Since performing a strcpy requires knowing the length of the source string (and consequently the number of bytes/characters to copy), this is typically the first step in performing a strcpy.

 

Next we continue down and see two similar looking instructions in `rep movsd` and `rep movsb`. These instructions copy a string located in the esi register into the edi register. The difference between these two instructions being the `rep movsd` instruction moves DWORDs from esi to edi, while `rep movsb` copies bytes. Both instructions repeat the copy instruction a number of times based upon the value in the ecx register.

 

Viewing the above code we can observe that the code uses the string length found by `repne scasb` instruction in order to determine the size of the string that is being copied. We can observe this by viewing seeing that following instruction 0x41164C the length of the string is stored in both eax and ecx. Prior to executing the `rep movsd` instruction we can see that ecx is shifted right by two. This results in only full DWORDs from the source string being copied to the destination. Next we observe that at instruction 0x41165A that the stored string length is moved back into ecx and then bitwise-AND’d with 3. This results in any remaining bytes that were not copied in the `rep movsd` instruction to be copied to the destination.  

 

Automating Vuln Hunting with the Inlined strcpy()

Now that we understand how the compiler optimized strcpy function calls we are able to enhance our vulnerability hunting scripts to allow us to find instances where they an inlined strcpy occurs. In order to help us do this, we will use the IDAPython API and the search functionality that it provides us with. Looking at the above section the primary instructions that are more or less unique to strcpy are the `rep movsd` with the `rep movsb` instruction following shortly thereafter to copy any remaining uncopied bytes.

 

So, using the IDAPython API to search for all instances of `rep movsd` followed by `rep movsb` 7 bytes after it gives us the following code snippet:

 

ea = 0
while ea != BADADDR:
   addr = FindText(ea+2,SEARCH_DOWN|SEARCH_NEXT, 0, 0, "rep movsd");
   ea = addr
   if "movsb" in GetDisasm(addr+7):
       print "strcpy found at 0x%X"%addr

 

If we run this against the EQNEDT32.exe, then we get the following output in IDA Pro:

 Output of Script Against EQNEDT32.exe

Output of Script Against EQNEDT32.exe

 

If we begin to look at all of the instances where the script reports detected instances of inlined strcpy we find several instances where this script picks up other functions that are not strcpy but are similar. The most common function (other than strcpy) that this script detects is strcat(). Once we think about the similarities in functionality between those functions, it makes a lot of sense. Both strcat and strcpy are dangerous string copying functions that copy the entire length of a source string into a destination string regardless of the size of the destination buffer. Additionally, strcat introduces the same dangers as strcpy to an application, finding both with the same script is a way to kill two birds with one stone.

 

Now that we have code to find inlined strcpy and strcat in the code, we can add that together with our previous code in order to search specifically for inline strcpy and strcat that copy the data into stack buffers. This gives us the code snippet shown below:

 

# Check inline functions
info = idaapi.get_inf_structure()
ea = 0

while ea != BADADDR:
   addr = FindText(ea+2,SEARCH_DOWN|SEARCH_NEXT, 0, 0, "rep movsd");
   ea = addr
   _addr = ea

   if "movsb" in GetDisasm(addr+7):
       opnd = "edi" # Make variable based on architecture
       if info.is_64bit():
           opnd = "rdi"

      
       val = None
       function_head = GetFunctionAttr(_addr, idc.FUNCATTR_START)
       while True:
           _addr = idc.PrevHead(_addr)
           _op = GetMnem(_addr).lower()

           if _op in ("ret", "retn", "jmp", "b") or _addr < function_head:
               break

           elif _op == "lea" and GetOpnd(_addr, 0) == opnd:
               # We found the origin of the destination, check to see if it is in the stack
               if is_stack_buffer(_addr, 1):
                   print "0x%X"%_addr
                   break
               else: break

           elif _op == "mov" and GetOpnd(_addr, 0) == opnd:
               op_type = GetOpType(_addr, 1)

               if op_type == o_reg:
                   opnd = GetOpnd(_addr, 1)
                   addr = _addr
               else:
                   break

Running the above script and analyzing the results gives us a list of 32 locations in the code where a strcpy() or strcat() call was inlined by the compiler and used to copy a string into a stack buffer.

 

Improving Upon Previous Stack Buffer Check

Additionally, now that we have some additional experience with IDA Python, let us improve our previous scripts in order to write scripts that are compatible with all recent versions of IDA Pro. Having scripts that function on varying versions of the IDA API can be extremely useful as currently many IDA Pro users are still using IDA 6 API while many others have upgraded to the newer IDA 7.

 

When IDA 7 was released, it came with a large number of changes to the API that were not backwards compatible. As a result, we need to perform a bit of a hack in order to make our is_stack_buffer() function compatible with both IDA 6 and IDA 7 versions of the IDA Python API. To make matters worse, not only has IDA modified the get_stkvar() function signature between IDA 6 and IDA 7, it also appears that they introduced a bug (or removed functionality) so that the get_stkvar() function no longer automatically handles stack variables with negative offsets.

 

As a refresher, I have included the is_stack_buffer() function from the previous blog post below:

def is_stack_buffer(addr, idx):
   inst = DecodeInstruction(addr)
   return get_stkvar(inst[idx], inst[idx].addr) != None

First, we begin to introduce this functionality by adding a try-catch to surround our call to get_stkvar() as well as introduce a variable to hold the returned value from get_stkvar(). Since our previous blog post worked for the IDA 6 family API our “try” block will handle IDA 6 and will throw an exception causing us to handle the IDA 7 API within our “catch” block.

 

Now in order to properly handle the IDA 7 API, in the catch block for the we must pass an additional “instruction” argument to the get_stkvar() call as well as perform a check on the value of inst[idx].addr. We can think of “inst[idx].addr” as a signed integer that has been cast into an unsigned integer. Unfortunately, due to a bug in the IDA 7 API, get_stkvar() no longer performs the needed conversion of this value and, as a result, does not function properly on negative values of “inst[idx].addr” out of the box. This bug has been reported to the Hex-Rays team but, as of when this was written, has not been patched and requires us to convert negative numbers into the correct Python representation prior to passing them to the function. To do this we check to see if the signed bit of the value is set and, if it is, then we convert it to the proper negative representation using two’s complement.

 

def twos_compl(val, bits=32):
   """compute the 2's complement of int value val"""
   
   # if sign bit is set e.g., 8bit: 128-255 
   if (val & (1 << (bits - 1))) != 0: 
       val = val - (1 << bits)        # compute negative value

   return val                             # return positive value as is

   
def is_stack_buffer(addr, idx):
   inst = DecodeInstruction(addr)

   # IDA < 7.0
   try:
       ret = get_stkvar(inst[idx], inst[idx].addr) != None

   # IDA >= 7.0
   except:
       from ida_frame import *
       v = twos_compl(inst[idx].addr)
       ret = get_stkvar(inst, inst[idx], v)

   return ret

 

Microsoft Office Vulnerability

The Equation Editor application makes a great example program since it was until very recently, a widely distributed real-world application that we are able to use to test our IDAPython scripts. This application was an extremely attractive target for attackers since, in addition to it being widely distributed, it also lacks common exploit mitigations including DEP, ASLR, and stack cookies.

 

Running the IDAPython script that we have just written finds and flags a number of addresses including the address, 0x411658. Performing some further analysis reveals that this is the exact piece in the code that caused CVE-2017-11882, the initial remote code execution vulnerability found in Equation Editor.

 

Additionally, in the time that followed the public release of CVE-2017-11882, security researchers began to turn their focus to EQNEDT32.exe due to the creative work done by Microsoft to manually patch the assembly (which prompted rumors that Microsoft had somehow lost the source code to EQNEDT32.exe). This increased interest within the security community led to a number of additional vulnerabilities being subsequently found in EQNEDT32.exe (with most of them being stack-buffer overflows). These vulnerabilities include: CVE-2018-0802, CVE-2018-0804, CVE-2018-0805, CVE-2018-0806, CVE-2018-0807, CVE-2018-0845, and CVE-2018-0862. While there are relatively scarce details surrounding most of those vulnerabilities, given the results of the IDAPython scripting that we have performed, we should not be surprised that a number of additional vulnerabilities were found in this application.

 

Posted on November 20, 2018 .