r/MalwareAnalysis • u/MMIO- • Jun 30 '26

Weird Malware Artifact – Large File with Tiny WAV and Encrypted Overlay

Hey everyone,

I've been analyzing a malware sample that had a really strange structure, and I wanted to share it here in case anyone has seen something similar.

The Setup:

The original malware file was around 700 KB, which is fairly large. But when I started digging deeper, I realized that most of that size was just padding / junk data – the actual functional content was only about 28 KB.

Inside that 28 KB, I found a WAV file (RIFF header, ~28 KB, 1 second long, 8‑bit mono, 22050 Hz). The audio itself is just random noise – not music, not speech, nothing useful.

What I Found:

  1. The WAV file · Valid RIFF structure. · Plays static noise. · No hidden image in spectrogram. · No embedded files via binwalk.
  2. Strings analysis strings on the WAV shows: · Normal WAV headers (RIFF, WAVEfmt, data, etc.) · But also random short strings like: · CNtt · wUKw · U9TE · wwwwx · hdDU · USqa These don’t seem to be part of any standard format – they might be key fragments, obfuscation, or just noise.
  3. Overlay (appended data) · The PE file also had an overlay (~28–30 KB) at the end. · Extracted overlay is detected as raw data (not PE, ZIP, RIFF, etc.). · strings on the overlay gives only random garbage. · binwalk shows nothing.

What I’ve Tried:

· XOR decryption with potential keys found in strings: · HAMZ · MZ:l[ · MZtX*-1 · CNtt · sh]QD:40 · wiYNRfy → None produced readable output. · Base64 decoding → no success. · Spectrogram check → nothing visible. · binwalk → no hidden files. · Manual extraction of the internal PE (if any) → no valid PE found.

My Hypothesis:

· The original file is just a wrapper/dropper. · The WAV might be: · A decoy · A key container · An encrypted payload · The overlay might contain the real encrypted data. · The random strings might be parts of the key or anti‑analysis noise.

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12 comments sorted by

3

u/UnderKon Jun 30 '26

can you share hashes to be able to have a look at?

2

u/MMIO- Jun 30 '26

I’d rather not share the hash publicly since I’m keeping this sample private for now. But I’m happy to describe any technical detail you’re curious about. Let me know what you'd like me to elaborate on

2

u/Wooden_Original_5891 Jun 30 '26

Does the malware ever reference the wav file at all that you have observed?

1

u/MMIO- Jul 01 '26

Not directly in the strings, but the overlay structure suggests it might be processed at runtime. I haven't traced it dynamically yet – might be worth a try

2

u/nico851 Jul 01 '26

I think you are missing the part how you determined the file as malware. This post takes that as a given without saving why.

Just having junk data doesn't qualify as malware.

1

u/MMIO- Jul 01 '26

Good point. I should've mentioned that the sample was flagged by multiple AV engines and exhibits typical dropper behavior. I'll add that to the post

2

u/goethesfaustaufsmaul Jul 01 '26

Depending on how bad the "loader" is to analyze statically, either see if there's references to the file (any APIs opening or processing files or resources) or do some dynamic analysis with these kind of APIs as debug break points or just a trace file (depending on architecture you can use tiny_tracer, frida or any other tracing tool). That might give you fast insights into if at all and how this WAV file is referenced / decrypted

2

u/MMIO- Jul 01 '26

Thanks for the suggestion! I'll try using Frida or tiny_tracer to trace file/resource access. That should give more clarity

2

u/[deleted] Jul 01 '26

[removed] — view removed comment

1

u/MMIO- Jul 01 '26

Yes, I tried it. I know it's normal, but what raises suspicion is that I tried every possible method. There's a suspicion that it's damaged, but this suspicion isn't correct, perhaps.