A Zip Bomb to Bypass Security Controls and Sandboxes

The file is a zip archive.
The archive contains an ISO file that, once mounted, discloses a classic PE file.

After checking carefully the size of the different files, it was clear that the ZIP archive is 2M but the PE file is much bigger: 400MB! Reminds the “Zip Bomb”?
A malicious very small archive that, once decompressed, is very big and consumes a lot of resources to be unpacked.
From the analysis of the PE file using static analysis techniques (PEstudio) something suspicious comes up
The file has an “overlay” that uses 99% of the file size! And the first bytes are all zeroes.
This overlay starts at offset 0x1B9C00.

Microsoft Windows is very permissive regarding data appended to files.
For example, it’s common to see Word documents containing a macro that extracts a payload located at the end of the file.
Here, the PE file has been altered by appending a lot of zeroes to the code.
That’s the reason why the archive is small.
Packing zeroes is very efficient and produces a small file.

This is a very nice technique to bypass many security controls.
For performance reasons, big files are often skipped or can generate timeouts due to the huge amount of data to analyze.

The PE file is a bitrat sample using the following configuration:

{
“family”: “bitrat”,
“rule”: “Bitrat”,
“c2”: [
“kot-pandora[.]duckdns[.]org:24993”
],
“version”: “1.38”,
“attr”: {
“tor_process”: “tor”,
“communication_password”: “d6723e7cd6735df68d1ce4c704c29a04”
}
}

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