static.md

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Go Back to Reverse Engineering Malware 101

Section 5: Static Analysis

Static analysis is like reading a map for directions on where to go. As you follow through this map you capture notes on what things might look interesting when you actually begin your journey.

This section will teach you how to jump into code in static disassembly then rename and comment on interesting assembly routines that we will debug in Section 6.

LAB 2

Possible Packer?

Notice in CFF explorer that there is UPX in the header.

When you open the executable in IDA, you will notice large section of non-disassembled code.

Because UPX is a common packer, there are many tools that offer unpacking for UPX. Open the executable in PE Explorer which will unpack the binary automatically. Save the file with a name to identify it as unpacked.

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Reopen the executable in IDA.

The next step is getting a sense as to what the program is doing. So far we can assume:

• This exe is connecting to the internet somehow
• This exe is using a string encryption function
• This exe might be spawning a shell

Most windows programs start at address 004010000.

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Jumping in!

Navigate to the Strings window.

Here is an interesting string that we should start with:

This string is a typical registry key path to allow programs to autorun/startup on reboot. This is considered a persistence mechanism. Double Click the string.

Using the X key we can jump to the reference of that string in the assembly code.

This function is offset 00401340. Notice in that function is setting a registry key using Window API RegOpenKeyEx.

We should rename this function SetRegkey.

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Jump up to the calling function using X on SetRegkey. Scroll up until you see some interesting API.

Notice it's calling InternetOpen which opens a HTTP session.

This function call has the following arguments:

C++

HINTERNET InternetOpen(
  _In_ LPCTSTR lpszAgent,      // Arg 1 = URL
  _In_ DWORD   dwAccessType,   // Arg 2
  _In_ LPCTSTR lpszProxyName,  // Arg 3
  _In_ LPCTSTR lpszProxyBypass,// Arg 4
  _In_ DWORD   dwFlags         // Arg 5
);

We need to figure out what register esi is because it contains the URL we are looking for.

Assembly x86

push 0    ; Arg 5
push 0    ; Arg 4
push 0    ; Arg 3
push 1    ; Arg 2
push esi  ; Arg 1 URL
call ds: InternetOpenA

Right before the first push 0 there is a mov esi,eax which means esi = eax.

When a function returns, the return value is stored in eax. So let's look into the function that is being called. It takes a string as the first argument (that is a wicked string), while the second argument might be the string length. Press Enter to jump to the function.

Scroll down until you find xor al, 5Ah. Eventually you will be able to recognize when a string loop is being processed in assembly. In this case, it is xor a byte with 5Ah which is Z in ascii.

We can assume that this function is doing some kind of Xor encoding. So let's rename this function as XorDecode. We will need this information later when we debug in Section 6.

Let's use the tool XORSearch to see if we can find some interesting xor decoded strings. Open the terminal cmd.exe from the start bar, and navigate to the XORSearch.exe

XORSearch.exe <Path to Unknown.exe> "A string to test"

"Yo this is dope!" How weird.

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Getting the bigger picture

Let's navigate to the start of the program using the X key. Use the spacebar to toggle between graph view and text view.

It's easy to trace back through the program disassembly, but let's look at some control flow assembly instructions. Remember jmp, jne, jnz, jnb are control flow functions.

Jump Examples

jz loc_401962 ; jump too offset loc_401962 if the previous condition is zero

jle short loc_401634 ; jump to relative offset 401634 if the previous condition is less than or equal to

Next scroll down through and find the order of API function calls in the program. You should make note of all the function offsets.

Some of the more interesting API Calls from the image above. Look up what each function does, many are self explanatory.

• GetEnvironmentVariable
• CopyFile
• DeleteFile
• InternetOpen
• InternetConnect
• HttpOpenRequest
• HttpSendRequest
• MessageBox
• FindResource
• CryptStringToBinary
• CreateFile
• ShellExecute
• CreateProcess

Now you know how to navigate the disassembly forward and backwards to get to interesting routines. The next step is making a rough path to follow for deeper analysis in Section 6.

Section 4 <- Back | Next -> Section 6