Reverse Engineering Malware: Catching 'brbbot' in the Act with x64dbg & CyberChef

Unmasking the Payload: A Hands-On Session with brbbot

There’s a certain thrill to tearing apart malware to see what makes it tick. It’s a cat-and-mouse game between the malware author and the analyst, and today, we are playing the cat.

In this session, we are analyzing a suspicious executable named brbbot.exe. Our goal? Figure out what files this malware is trying to read, intercept the data, and decode its payload. We’ll be relying on a standard but powerful stack: x64dbg, Process Hacker, and CyberChef.

Here is a step-by-step breakdown of how we pull the threads on this binary.

Step 1: Setting the Trap with Breakpoints

To figure out what a piece of malware is reading from the disk, the easiest approach is to intercept the Windows API calls responsible for file I/O.

Inside x64dbg, we want to pause execution right when the malware tries to read a file. We head down to the command bar and type in bp ReadFile (after quickly correcting a little pb typo—happens to the best of us). This sets a breakpoint on the ReadFile function located in kernel32.dll.

Once we hit "Run," the malware executes until it inevitably hits our breakpoint. Execution pauses, giving us total control to inspect the memory and registers.

Step 2: Hunting the Handle

Now that we have the malware paused at ReadFile, we need to know what file it’s actually looking at.

If we look up the ReadFile function on the Windows API documentation (MSDN), we can see that the very first parameter it takes is hFile—the handle to the device or file being read.

Looking at our debugger, we inspect the arguments being passed to the function. We spot a handle ID—first it's 0x11C, and later in the run, we see 0x24. But a handle ID is just a number; it doesn't tell us the file path.

To bridge this gap, we switch over to Process Hacker.

We open the properties for the brbbot.exe process.
We navigate to the Handles tab.
We filter/search for our handle values (11C, 110, or 24).

Instantly, Process Hacker reveals the exact target: a sketchy .tmp configuration file hiding in the user's AppData\Roaming directory.

Step 3: Following the Execution Flow

Knowing the file is great, but we want the contents. We step over the ReadFile execution in the debugger to let the malware pull the file data into memory.

As we step through the subsequent assembly instructions, we notice the program preparing to run a decryption routine. We also catch a glimpse of some very revealing strings in memory—specifically, a URL structure pointing to an ads.php endpoint and a parameter that says encode=5B.

That 5B is a massive clue. It highly suggests the malware is using a simple XOR cipher with the hex key 0x5B to obfuscate its data.

We find the block of hex data the malware just loaded into memory, copy it out, and save it to our desktop as response.brbbot.hex.

Step 4: Cooking the Data in CyberChef

With our encrypted hex dump in hand, we move over to our REMnux environment and fire up CyberChef, the ultimate Swiss Army knife for data decoding.

We drag our hex file into the input window. Knowing our suspected key is 5B, we drop the XOR operation into the recipe and input 5B as the hex key.

Result: Gibberish. This is a classic oversight when moving fast. The data we dropped into CyberChef is literally the text representation of hex (e.g., "73 6F 6D 65..."). We can't XOR text characters; we need to XOR the raw bytes.

We quickly fix the recipe by adding the From Hex operation before the XOR operation. This converts the hex string into raw bytes, which are then perfectly decrypted by the 5B key.

The Reveal: Evasion Tactics Exposed

With the recipe corrected, the output window instantly transforms the gibberish into highly readable, plaintext data.

What did the malware authors hide? A massive blocklist of executable names:

wireshark.exe
processhacker.exe
avast.exe
avgui.exe
...and dozens of other antivirus, debugging, and virtualization tools.

This tells us exactly what the malware was doing: reading an encrypted configuration file to get a list of security tools. It then loops through the running processes on the victim's machine, looking for any of these names to decide if it is being monitored. If it sees our tools, it will likely alter its behavior or shut down to evade detection.

The Takeaway

This session is a perfect example of why dynamic analysis is so powerful. By strategically placing a breakpoint on a common Windows API, tracing the file handles, and having a good eye for hardcoded keys, we turned an opaque, obfuscated executable into an open book.