full-disclosure - Windows covert channel

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Message-ID: <20031020043626.GA12978@miyazaki>
From: kain at kain.org (Kain)
Subject: Windows covert channel

On Sun, Oct 19, 2003 at 10:23:37PM -0400, Karl DeBisschop wrote:
> On Sun, 2003-10-19 at 19:04, James Kelly wrote:
> > I seem to remember in the dim reaches of my memory a covert channel in 
> > the Windows file system where you could paste one file at the end of 
> > another without it being detectible when you edited the orginal file.
> > 
> > 
> > can someone aim me at the right "buzz phrase" that describes this so I 
> > can Google it further?
> 
> Many people have mentioned data streams. But since you said 'end of
> file' I wonder if you are referring to the DOS idea that ^Z is an end of
> file marker, and many apps won't look beyond it.

I don't know enough about NTFS to know if the same concept applies, but in
FAT/FAT32, your files are allocated in clusters. Therefore, given a file of
size X, and a cluster size of Y, you will have X mod Y bytes in the last
allocated cluster that won't be visible through the filesystem that you can
directly write and hide information in.  Of course, it's possible (likely) that
processes that truncate, grow, or defragment/reallocate the file on disk will
lose the information, so it is spotty at best.

This sort of fun is also not too hard to pursue with other filesystems.  For
example, ISO9660 (cdromfs) has multiple directory and file tables, and there's
nothing stopping you from hiding data all over an ISO that just browsing the
filesystem would show.  This sort of thing lets you create structures like
hybrid ISO9660/HFS/HFS+ images for Macs.

For NTFS, I would suggest starting at http://linux-ntfs.sourceforge.net which
has plenty of pointers to NTFS utilities and documentation.

If I were desining a covert file storage system for Win*, I would probably
write an installable file-system driver that would mangle rarely-modified NTFS
files, adding additional streams to them to store data.  That would however,
produce massive forensic evidence to an informed observer.  I would also
consider writing a filesystem driver that could use a (read-only/unmounted)
NTFS partition as it's storage, hiding its filesystem in the unallocated space
of the disk.)
-- 
Bryon Roche
Professional {Developer,Guru,Mad Scientist}
<kain@...n.org>
PGP Key Fingerprint: FE0D EC23 6464 726A CD54  48D3 04AD 86FE 6878 ABD5
Success, recognition, and conformity are the bywords of the modern world where
everyone seems to crave the anesthetizing security of being identified with the
majority...Human salvation lies in the hands of the creatively maladjusted.
  -- Martin Luther King, Jr.
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