Hiện nay Windows XP vẫn đang là hệ điều hành được sử dụng rộng rãi. Một tính năng rất hay của nó (và cũng là cái để virus lợi dụng để kích hoạt) đó là tính năng “cắm là chạy” (plug and play). Với việc USB được sử dụng phổ biến như hiện nay thì chúng ta hẳn đã từng “dính virus” autorun.
Cơ chế hoạt động của loại virus mang tên autorun này đúng như cái tên của nó: tạo 1 file “autorun.inf” chứa nội dung kích hoạt virus khi ta cắm USB vào máy tính. Hãng bảo mật Panda đã ra mắt 1 công cụ giúp phòng ngừa loại virus này đó là: Panda USB and AutoRun Vaccine (PUAV). khi cài vào hệ thống, PUAV sẽ thực hiện 2 việc:
- Tiêm vaccine cho hệ thống: tắt tính năng autoplay.
- Tiêm vaccine cho USB: khi có USB được cắm vào, nó sẽ tạo ra 1 file autorun.inf “đặc biệt” có tính chất không thể xóa, sửa, đổi tên…vì vậy nó loại bỏ điều kiện cần để kích hoạt virus autorun. phiên bản mới nất hiện nay là 1.0.1.4 đã hỗ trợ USB format NTFS.
Ưu điểm của PUAV so với các công cụ như: USB Disk Security, Autorun Remover…
- Miễn phí
- Không tốn tài nguyên: file cài đặt chỉ hơn 800Kb, khi chạy thường trú cũng chỉ chiếm khoảng 800Kb bộ nhớ RAM.
- Virus không thể xóa, sửa, thay đổi…..fiel autorun.inf do PUAV tạo ra.
Nhược điểm
- Nếu USB không có file autorun hoặc có file autorun “lành tính” thí không có vấn đề gì. Nhưng nếu USB có file autonrun.inf do virus tạo ra và máy bạn có 1 antivirus chạy thì khi đó PUAV sẽ đổi tên file autorun này, tạo 1 file autorun khác (theo cách của PUAV) nên sẽ bị antivirus chặn vào tạo ra 1 lỗi ngớ ngẩn làm treo PUAV.
- PUAV chỉ cắt bỏ điều kiện cần đề kích hoạt virus autorun.
Nguyên lý làm việc của PUAV như sau:
//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 FThe 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]
Đây là công cụ thực sự tốt mà lại miễn phí, hơn nữa nó lại có hiệu quả lâu dài khi sự hoành hoàch của các loại virut autorun khó chịu như hiện nay.