It looks reasonably easy - the main thing you're looking for local file headers that are referenced multiple times by the central directory. This is not invali

It looks reasonably easy - the main thing you're looking for local file headers that are referenced multiple times by the central directory. This is not invalid itself of course - some compression programs likely deduplicate files with this technique. But if it seems excessive, or it's the only thing in the archive then you've got a zip bomb. You could probably come up with some techniques to obfuscate this of course but it'll increase the size of the archive.