Book Review: Practical Malware Analysis

I’ve been dying to get this review out for a while now. There’s so much good and deep content in this book, that reading it on nights after work and weekends took longer than expected! I’ll tell you now that if you’re into computers and computer security, this book won’t let you down. This book is like having your very own personal malware analysis teacher without the expensive training costs.

About the Book

The book material is exhaustingly complete with 21 chapters + appendices covering everything from static analysis, environment setup, x86 assembly to anti-disassembly and anti-virtual machine practices. Total book content, minus lab solutions comes in at an enormous 475 pages (with lab solutions, 732 pages) . Let’s just say that you better be prepared to eat, breathe, and live malware analysis for quite some time.

The skill level for the book is targeted at someone with experience in programming and security although an ambitious beginner should do fine.

My Review & Recommendation

The authors, Michael Sikorski and Andrew Honig, do a great job of teaching the concepts and not just the tools. They want you to develop the skills necessary to think on your own as a malware analyst so that when new techniques come out that aren’t in the book, you’ll have the mental tools to figure out the challenges. Don’t worry though, this book isn’t filled with boring theory like those books you read back in school, the concepts taught have actual practical uses.

Better yet, the book gives you the opportunity to apply the concepts with labs at the end of each chapter. You’ll actually be dissecting “real malware” written by the authors for the purposes of this book.

Equally as awesome is that each lab comes with a “quick solution” and a “detailed solution.” I learn best when I can fight through a tough problem and check with the solution when I’m stuck.

The book is entirely centered around Windows based malware, particularly malware written for Windows XP. This was a good learning experience for me because I’m not familiar with the internal Windows APIs and features. It’d actually be very interesting if the authors included a section on Linux-based and/or Mac-based malware. On that note, I did actually try to run some of the lab malware on Windows 7 32 and 64 bit thinking that it would be no big deal but I received an APPCRASH error every time. I spoke with one of the authors over email and he was very helpful. He said that the malware was designed for Windows XP for teaching purposes that will be revealed when reading the book. With this slight limitation comes some positive: it leaves room for a 2nd edition of the book focused on the newer Vista/7 features as malware becomes more prominent on these machines.

Book content aside, the physical paperback book itself is a pleasant surprise. NoStarch Press is one of my favorite publishers because they use the “lay-flat” type binding (they also published one of my other favorite books: Hacking the Art of Exploitation. You’ll be praising this when you want to set the book down and copy some code.

The book does also come in digital formats. I used a combination of both for the review. You won’t be an expert in malware analysis when you’re done with this book but it sure as hell will give you the information you need to get there.

This book is broad, covering a ton of topics. Each Chapter could have likely been a book in and of itself. As I’ve said in a previous post, this book will become the de facto standard for learning about malware analysis.

Thanks to No Starch Press for the review copy, this book is awesome!

Book Review: Malware Analyst's Cookbook and DVD

Here is my book review of the Malware Analyst’s Cookbook and DVD: Tools and Techniques for Fighting Malicious Code by Michael Hale Ligh, Steven Adair, Blake Hartstein, and Matthew Richard.

About the Book

The book is a huge compilation of short how-to articles called recipes on the “tools and techniques for fighting malicious code.” In addition, the book comes with a number of very useful custom written tools for automating or speeding up the process. The book is divided into several chapters which specialize on a specific topic. Some of the book’s topics include: Honeypots, Malware Classification, Malware Labs, Malware Forensics, Debugging Malware, Kernel Debugging, Memory Forensics (4 chapters of this)

Initial Impression

I had very high expectations for this book based on the fact that there aren’t many books out there on this subject and it’s something I’m particularly interested in. When I first received the book, I was pleasantly surprised at the literal size and the amount of content: this book is LOADED with information coming in at right close to 700 pages! A quick flip through the book told me that this book covers everything from very basic topics (e.g. using dig) to very advance topics (e.g. kernel debugging). I couldn’t wait to start the book!

The Audience/Skill Level

In the introductory part of the book, it has a small break down of “Who Should Read This Book.” Generally, I would sum it up as anyone and everyone that is interested in security would find this book interesting and entertaining. The book supports a wide range of skills levels from beginners to advanced. A basic knowledge of C/C++ and some Windows API’s is helpful but not required. Likewise, a basic knowledge of Python is not required but would help if you’d like to better understand the scripts that the book provides.

The Book

This isn’t your typical “take-a-seat-and-read” type book. Get your laptop, your desktop, and even some old machines and be prepared to dive right in.

The book focuses mainly on investigating Windows-based malware using tools mainly on Unix/Linux-based OS’es (Ubuntu, Mac OS X, etc) but there are some equivalent Windows based tools which the authors mention if available.

The recipe style of the book makes it very flexible to read and supports a wide range of audiences without confusing the newcomers and boring the advanced. Each recipe is self contained, well written, and easy to read. If you’re not interested in a specific recipe, you can never read it and you’ll have no problems following along in the rest of the book. Where applicable, a recipe provides links to additional information if you would like to take a deeper dive on the topic.

There are basically two approaches to reading this book.

  1. If you’re new to malware analysis, you can start from the beginning and progress to the end, skipping anything you already know or are not interested in, just like you would with any other book.
  2. The other approach would to be use the book as a shelf reference using the table of contents and index to search for what you’re trying to do. The progression of the book is from basic to advanced, so if you’re intermediate or advanced, you can easily skip to the later sections right from the beginning, although in my case I did find some new information, tips, and tools in the basic section that I wasn’t aware of so the basic sections may be worth a quick skim.

The included DVD does prove to be useful unlike other books, not just for following along and understanding a concept, but more importantly, it contains a number of custom Python scripts all geared towards improving and easing your malware analysis. You can easily add these scripts to your toolkits.

No book review would be complete without listing some of it’s downsides. Luckily for this book, there are very few downsides. The first downside is completely unrelated to the content and has to do with the actual book itself. The soft cover binding of the book is somewhat cheap and wears pretty quickly due to the size and weight of the book. A hardcover edition with a solid, strong binding would be a great enhancement. The other downside has to do with the content. While I think the authors make a great effort to minimize the specifics of a tool and focus more generally on the purpose of the tool, there are a few sections of the book which might get outdated quickly if a tool changes. However, I think this is the nature of the beast with technical books so it shouldn’t be something to worry about or prevent you from buying the book!

The Punchline

Whatever topic it is your looking for related to analyzing malware, with The Malware Analyst’s Cookbook: “There’s a recipe for that.”

Interested in analyzing the memory of a rootkit? There’s a recipe for that!

Interested in setting up a malware lab? There’s a recipe for that!

All in all, I would highly recommend this book to anyone interested in security as well as those who want to learn more about malware analysis. I’d also highly recommend this book to professionals in the security field - keep a copy of this book right next to your computer, I guarantee you’ll find it useful!

Mac OS X 64 bit Assembly System Calls

After reading about shellcode in Chapter 5 of Hacking: The Art of Exploitation, I wanted to go back through some of the examples and try them out. The first example was a simple Hello World program in Intel assembly. I followed along in the book and had no problems reproducing results on a 32 bit Linux VM using nasm with elf file format and ld for linking.

Then I decided I wanted to try something similar but with a little bit of a challenge: write a Mac OS X 64 bit “hello world” program using the new fast ‘syscall’ instruction instead of the software interrupt based (int 0x80) system call, this is where things got interesting. First and foremost, the version of Nasm that comes with Mac OS X is a really old version. If you want to assemble macho64 code, you’ll need to download the lastest version.

[email protected]:~$ nasm -v
NASM version 2.09.03 compiled on Oct 27 2010

I figured I could replace the extended registers with the 64 bit registers and the int 0x80 call with a syscall instruction so my first attempt was something like this

section .data
hello_world     db      "Hello World!", 0x0a

section .text
global _start

_start:
mov rax, 4              ; System call write = 4
mov rbx, 1              ; Write to standard out = 1
mov rcx, hello_world    ; The address of hello_world string
mov rdx, 14             ; The size to write
syscall                 ; Invoke the kernel
mov rax, 1              ; System call number for exit = 1
mov rbx, 0              ; Exit success = 0
syscall                 ; Invoke the kernel

After assembling and linking, I got this

[email protected]:~$ nasm -f macho64 helloworld.s
[email protected]:~$ ld helloworld.o 
ld: could not find entry point "start" (perhaps missing crt1.o) for inferred architecture x86_64

Apparently Mac OS X doesn’t use _start for linking, instead it just uses start. After removing the underscore prefix from start, I was able to link but after running, I got this

[email protected]:~$ ./a.out
Bus error

I was pretty stumped at this point so I headed off to Google to figure out how I was supposed to use the syscall instruction. After a bunch of confusion, I stumbled upon the documentation and realized that x86_64 uses entirely different registers for passing arguments. From the documentation:

 
The number of the syscall has to be passed in register %rax.
    
rdi - used to pass 1st argument to functions
rsi - used to pass 2nd argument to functions
rdx - used to pass 3rd argument to functions
rcx - used to pass 4th argument to functions
r8 - used to pass 5th argument to functions
r9 - used to pass 6th argument to functions
    
A system-call is done via the syscall instruction. The kernel destroys registers rcx and r11.

So I tweaked the code with this new information

...
mov rax, 4              ; System call write = 4
mov rdi, 1              ; Write to standard out = 1
mov rsi, hello_world    ; The address of hello_world string
mov rdx, 14             ; The size to write
syscall                 ; Invoke the kernel
mov rax, 1              ; System call number for exit = 1
mov rdi, 0              ; Exit success = 0
syscall                 ; Invoke the kernel
...

And with high hopes that I’d see “Hello World!” on the console, I still got the exact same ‘Bus error’ after assembling and linking.

Back to Google to see if others had tried a write syscall on Mac OS X. I found a few posts of people having success with the syscall number 0x2000004 so I thought I’d give it a try. Similarly, the exit syscall number was 0x2000001. I tweaked the code and BINGO! I was now able to see “Hello World” output on my console but I was seriously confused at this point; what was this magic number 0x200000 that is being added to the standard syscall numbers?

I looked in syscall.h to see if this was some sort of padding (for security?) I greped all of /usr/include for 0x2000000 with no hints what-so-ever. I looked into the Mach-o file format to see if it was related to that with no luck.

After about an hour and a half of looking, I spotted what I was looking for in ‘syscall_sw.h’

/*
 * Syscall classes for 64-bit system call entry.
 * For 64-bit users, the 32-bit syscall number is partitioned
 * with the high-order bits representing the class and low-order
 * bits being the syscall number within that class.
 * The high-order 32-bits of the 64-bit syscall number are unused.
 * All system classes enter the kernel via the syscall instruction.
 *
 * These are not #ifdef'd for x86-64 because they might be used for
 * 32-bit someday and so the 64-bit comm page in a 32-bit kernel
 * can use them.
 */
#define SYSCALL_CLASS_SHIFT	24
#define SYSCALL_CLASS_MASK	(0xFF << SYSCALL_CLASS_SHIFT)
#define SYSCALL_NUMBER_MASK	(~SYSCALL_CLASS_MASK)

#define SYSCALL_CLASS_NONE	0	/* Invalid */
#define SYSCALL_CLASS_MACH	1	/* Mach */	
#define SYSCALL_CLASS_UNIX	2	/* Unix/BSD */
#define SYSCALL_CLASS_MDEP	3	/* Machine-dependent */
#define SYSCALL_CLASS_DIAG	4	/* Diagnostics */

Mac OS X or likely BSD has split up the system call numbers into several different “classes.” The upper order bits of the syscall number represent the class of the system call, in the case of write and exit, it’s SYSCALL_CLASS_UNIX and hence the upper order bits are 2! Thus, every Unix system call will be (0×2000000 + unix syscall #).

Armed with this information, here’s the final x86_64 Mach-o “Hello World”

section .data
hello_world     db      "Hello World!", 0x0a

section .text
global start

start:
mov rax, 0x2000004      ; System call write = 4
mov rdi, 1              ; Write to standard out = 1
mov rsi, hello_world    ; The address of hello_world string
mov rdx, 14             ; The size to write
syscall                 ; Invoke the kernel
mov rax, 0x2000001      ; System call number for exit = 1
mov rdi, 0              ; Exit success = 0
syscall                 ; Invoke the kernel

And here’s the output

[email protected]:~$ nasm -f macho64 helloworld.s
[email protected]:~$ ld helloworld.o 
[email protected]:~$ ./a.out
Hello World!