Re: --++[Preventing the spread of USB malware]++--

[Shreyas Zare <shreyas () technitium com>]

Hi,

I use a simpler solution. I format the USB  drive with NTFS (you need
to set the device policy as "optimize for performance" in hardware
details for formatting with NTFS, after the format u can revert back
to "optimize for quick removal" if you wish to).

I configure the NTFS file permissions for the entire drive such that
only my trusted machine users (the user a/c on machines I fully trust
to be non infected) have write access, and Everyone user has only read
& execute access. Remove all other users from the file permissions.

Then the most important thing, create a folder which I generally name
"DMZ" and set file permission Everyone Full Control. This folder thus
can be accesses to save files on untrusted machines. Thus *only* this
folder may contain infected files if used on infected machine.

This idea makes creating "autorun.inf" files not possible unless the
malware author write code to take ownership & change permissions
(which I have not seen yet). So this works quite well and I have been
using this without any issue since 1yr or so.

On the side note, I am coding a application (which is in testing
phase, and will be commercial soon) to tackle this problem effectively
without doing any such things like formatting with NTFS or editing
file table in HEX and would catch most malware that spread through
USB.

Regards,

-- 
("Computers have a strange habit of doing what you say, not what you
mean." - SANS Top 25 Most Dangerous Programming Errors)

Shreyas Zare
Co-Founder, Technitium
eMail: shreyas () technitium com

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On Tue, Apr 14, 2009 at 5:36 PM, Marcus Vinicius
<marcovvinicius () gmail com> wrote:
> Hello guys. one nice text.
>
>
>
> //Author – Robin Bailey
> //Date – 05/04/2009
> //Email - rbailey.security<0x40>googlemail.com
>
> //Contents
> [1] Introduction
> [2] The problem
> [3] Solution
> [4] Conclusion
>
>
> //Introduction [1]
>
> As the use of memory sticks has become more and more widespread, so malware
> has
> began to use them as a way to spread from machine to machine. While this is
> a
> problem for end users, the real danger is with IT professionals, who might
> use
> the same USB stick in dozens of computers in a single day, will often be
> logged
> in with administrative privileges, and will have access to important
> machines.
> This paper is aimed at those professionals, and how they can mitigate the
> risk
> of passing an infection onto other machines.
>
>
>
> //The Problem [2]
>
> Malware uses two main techniques to spread through memory sticks. The first,
> and less serious, is infecting executable files on the memory stick, so that
> when they are run on another machine, the infection moves with them.
>
> The more common, and more dangerous, is to spread via the `autorun.inf`
> file,
> which Windows automatically executes when the drive is connected, meaning
> that
> no user interaction is needed. Conficker has been getting a lot of attention
> recently, and this was one of the methods it used to spread itself, but many
> other malicious programs used the same technique.
>
> It is possible to disable the autorun feature from Windows, but this
> requires
> that the client machine has done this, which is not always the case, as most
> users will not have the technical knowledge to do this.
>
>
>
> //The Solution [3]
>
> Since we cannot rely on the computer to prevent the execution of the
> autorun.inf file, we must do this from the memory stick. It is possible to
> buy
> memory sticks with read-only switches, so that they can be locked to prevent
> the computer writing to them, but this can cause problems, is easily
> forgotten,
> and doesn't help once the memory stick has been infected.
>
> However, if the memory stick is FAT32, which most are, with the exception of
> some of the new 8GB+ drives, we can create a quick fix using a hex editor,
> and
> a basic knowledge of the FAT32 directory table.
>
> First, we create a blank `autorun.inf` file on the memory stick, then open
> up
> the disk in a hex editor. It doesn't matter if you open the physical disk,
> or
> the logical partition, but if the disk has more than one partition, it is
> better to do the latter. Make sure that the disk is opened with read/write
> permissions, and that you haven't got anything accessing it at the time. HxD
> for Windows is a small, portable hex editor, if you don't already have one.
>
>
>
> While this can be done to a disk with data on, it is safer to do it to a
> blank
> one, just in case there is a problem. If not, make sure that you have a copy
> of
> any data on the stick, if you don't, the you are liable to any loss of data
> that might occur.
>
> Next, run a search in the disk for the string `AUTORUN`, as a non-Unicode
> text
> string. It should find it near the beginning of the disk. The area we are
> interested in is as follows.
>
> 41 55 54 4F 52 55 4E 20 49 4E 46 20
> A  U  T  O  R  U  N     I  N  F
>
>
> The first 8 bytes are the filename (with a space at the end, because autorun
> is
> only 7 characters), followed by a 3 bytes file extension (INF), followed by
> one
> byte for the file attributes. It is this final byte that is relevant.
>
> The current value of the byte (0x20) has just the archive bit set. What we
> want
> to do, is to change this byte to 0x40, which sets the device bit, which is
> never normally found on a disk. The block will now look like this.
>
> 41 55 54 4F 52 55 4E 20 49 4E 46 40
> A  U  T  O  R  U  N     I  N  F  @
>
> Once this has been saved to disk, ignoring any warning that this might
> corrupt
> the disk, we then unmount and remount the volume. Now, when you browse to
> the
> disk, the autorun.inf file can be seen, but it cannot be deleted, opened,
> edited, overwritten, or have its attributes changed.
>
> When this memory stick is connected to an infected machine, which will try
> to
> create an autorun.inf file on it, it will fail with an error, (Cannot create
> file), meaning that this memory stick cannot be infected, and thus cannot
> pass
> an infection on to any other computers.
>
>
>
> //Conclusion [4]
>
> As stated before, this is not a guide aimed at end users, it is aimed at IT
> professionals, or other power users, who will use the same USB stick on
> multiple computers on a day to day basis.
>
> Should this technique become widely used, we will almost certainly see
> malware
> that can bypass it, but until that happens, it can provide a simple but
> effective defense against USB spreading malware.
>
>
> If you have any comments/questions/suggestions send me an email.
>
> # milw0rm.com [2009-04-06]
>
> # EOF
>
> --
> LPIC-1 -- Linux Certified
> http://bi0os.blogspot.com
> "I like when the my box said:
> All ports Are filtred =:)"

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