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


//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.