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.