Re: [Full-Disclosure] Mydoom

[Nick FitzGerald <nick@virus-l.demon.co.uk>]

Vlad Galu <dudu@diaspar.rdsnet.ro> wrote:

> "Ferris, Robin" <R.Ferris@napier.ac.uk> writes:
> | 
> |Does any one know what the size of the attachment is when is comes in
> |as a zip file?
> 
>       It's the <compressed size>/<ratio>.

Admit it -- you're just guessing!

Mydoom makes its zip form by gluing together a .ZIP header for a 
single, stored (i.e. not compressed) file, the whole of the virus 
binary and a suitable one-file .ZIP central directory.  As the size of 
the first and third components depends on the length of the filename, 
the .ZIP is not of constant size because Mydoom uses different length 
filenames.

And, as I explained earlier, even the size of the .EXE can vary, adding 
yet another inconstancy to the equation.


-- 
Nick FitzGerald
Computer Virus Consulting Ltd.
Ph/FAX: +64 3 3529854

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