Understanding Virus

Keep your friends close and enemies closer.

Contrary to popular belief, hacker’s own machines are often more vulnerable to attacks than the average user. This is especially true of those who are new to the world of hacking because of their more frequent access to remote machines, dodgy forums, open administrative privileges on their own machines and inexperience in covering their tracks etc. Further, those who are experienced often take advantage of the ones who are in the learning stage, trying to get into this field. In this dog-eat-dog world, any kind of safety is an illusion unless proven otherwise. Again, for the newbie hacker, it is of vital importance to learn how to protect themselves to avoid being consumed by their own curiosity.

Viruses are perhaps the oldest form of malicious software. They’ve been around for a long time and are still evolving and causing havoc all over the world. At first sight, they seem to be innocent looking executable files, but if opened they can turn your world upside down in a second. Reliance on antivirus software is fine as a first line of defense, but you need a basic arsenal of skills for securing the executables on your system and coping with viruses on your own. Here we discuss proactive methods you can use to defend yourself against malicious executable code in files, resources, component libraries, scripts and macros, as well as how to avoid a handful of other potential vulnerabilities.

Depending on how destructive their payload, computer viruses can result in a significant loss of data, time, and money. In the best scenario, you may just lose the time it takes to disinfect your computer. At worst, a mission-critical server may be reduced to little more than an expensive door-stop. Unfortunately, antivirus software isn’t perfect, and there’s always some lag between the emergence of a new electronic predator and the availability of virus definitions to protect against it. Moreover, many programmers of today’s more diverse and sophisticated computer viruses are taking the preemptive step of disabling or even removing antivirus software as part of their operation.

I’m not suggesting that you shouldn’t rely on your antivirus programs for day-to-day virus protection, especially since antivirus programs are very good at keeping you safe from known threats. However, if you’ve never had to fight a virus without your antivirus software, then it’s only a matter of time before you will. The main reason for this is new viruses. For an antivirus to be able to guard against new viruses, it must first be reported, the virus scanning code must be modified to include this new virus, and finally the virus database copy on the client side must be updated.
Because virus protection changes every day, those annoying notifications that your antivirus probably gives routinely are there to protect you. You must’ve noticed in the UPDATE menu of any antivirus an option to “Update Virus Database” or something like that. This is evidence of the ongoing war against viruses.

In general, how do viruses work? Well, first, the programmer writes the executable code required to carry out the virus’s activation process and then whatever nasty things it’s meant to do. What does the author want the virus to accomplish? Should it reformat your hard drive? Delete JPG files? Mail copies of itself to your friends and coworkers? Making any of this happen requires “executable” code of some kind. Second, in order for this code to execute, the virus needs to be activated or opened. The usual way a virus’s executable code is run is the direct method: some unwary user receives an e-mail attachment called “Click-Here.exe” or something equally enticing. This runs the program and the virus is unleashed.

As easily avoided as this result seems to be, it still works far more often than it should. Virus writers have discovered a number of other, less obvious techniques for getting a virus to take over your computer. Below we take a look at some of these techniques, beginning with the question of what constitutes executable code, then we’ll examine several sneaky activation methods. These activation methods are particularly important, as this is where you’ll understand how to completely unhook viruses from your system in order to regain control of it following an infection.

What Constitutes Executable Code?
Of course, you know that .exe files are executable, as are other similar file types such as .cmd and .com etc. There are many other file types that may contain executable code, and any executable code can be unsafe. In general, executable code falls into three broad categories: standalone programs, code included within resources or libraries, and script or macro code executed by an interpreter of some kind. In a broad sense, a standalone program is pretty much any file type that relies on the operating system for it to execute. How do you know which ones these are? The answer lies in the Windows® registry. To battle against viruses on their own turf, you’ll have to be very comfortable delving into the registry. So let’s take a look at how executable programs are invoked (A little technical, read slowly).

Launch the Registry Editor by opening “Run” and type “regedit”, and expand the HKEY_CLASSES_ROOT (HKCR) node.It’s the operating system’s repository for information on file associations and commands. Under HKCR, you’ll find nodes representing all the file type extensions registered on your computer. Navigate down the tree until you locate the key named .exe. Select this node and observe that its default value (shown in the right-hand pane) is exefile. This is a pointer to another node under HKCR (the exefile key). Scroll down and find the exefile folder. The exefile key contains a shell subkey. This is where a file type’s available actions are defined. In OS terminology, these actions are known as verbs. For example, a Microsoft® Word document might have a “print” verb defined, which allows you to right-click the file in Windows and choose Print from the context menu. Expand the shell subkey for the exefile node to view the available verbs for EXE files. You’ll probably see two or three different subkeys, depending on your system. The one to be concerned with is “open.”(HCKR ->exefile ->shell ->open) Expand this node and select its command subkey. Each verb has its own subkey, and each of those keys in turn has its own command subkey. The default value in this subkey dictates exactly what happens when that verb is executed.

Double-clicking the file icon in Explorer has the same effect—it executes the default verb’s command (open for EXE files). As you can see, for EXE files, the open command verb has a value of: %1 %* This is used in MS-DOS® batch file language. The basic idea is that the path and file name of the EXE file you activated are substituted for the “%1″ parameter, while any switches or command-line parameters that go along with it are passed through the “%*” parameter. So it would stand to reason that any other file types whose open verb evaluates to some flavor of “%1″ would tend to pose a risk. There are a number of these, and they’re all potentially dangerous. Considering that the virus writer knows that most people won’t double-click a file with a .exe extension or a .bat file, there are several other options including : .cmd .com, .pif, .vbs – All of these file types have a default open verb of %1. A virus writer could simply change the .exe extension of his virus executable to ,say .com, and s/he probably just increased the chances that the unsuspecting masses will run it. Particularly dangerous is (was, actually) the humble screen saver file type (.scr extension). Close to the start of the 21st century, .scr viruses were literally everywhere. They were widely believed to be power-saving, which is so not true (Stand-by mode is much better). People thought it couldn’t hurt, right? so what’s the harm. So Naive.. Again, extension hardly matters, everything a .exe virus can do, a .scr one can do equally well. The days of .scr might be gone, but viruses are probably here to stay, Whatever the type, extension, purpose and payload – The core concepts are always the same. So, let’s take a deeper look at the working of .scr viruses. The key to the future lies in the past.

Between the shadows.. In your Registry Editor, compare the open verb’s command default value for EXE and SCR files, respectively. As you will notice, they’re pretty much identical— “%1″ %* for EXEs and “%1″/s for Screen Saver files. Screen savers are, as it turns out, standalone executables. The only difference between these two default verbs is a /S switch for the SCR file type. The intended purpose of the screen saver’s “open” verb is to allow for testing a screen saver, and the screen saver executable interprets the /S switch accordingly. There’s nothing to stop a virus writer from giving their application a .scr extension and then simply ignoring the /S switch passed to it when the user invokes the program. Exploiting the popularity of screen savers was even easier because the caption of the screen saver’s open verb is shown as “Test” in the right-click menu. A user thinks he’s just testing a screen saver, but what he’s actually doing is activating a virus. A particularly clever virus might even display an actual screen saver, preoccupying you with pretty flowers while it destroys files on your hard drive in the background. This caption is stored in the default value for the open key itself. Meaning, just like you can simply find and change your “Recycle Bin”s name to say “Dumpster” or “TrashCan”, you can also change the “Test” option to anything you like. On an unrelated note, you may also change the “Open” option for a word document to maybe “Do Belly Dance” or anything you like. Of course, this only changes the string and not what it does.(Try using the CTRL + F, find function to poke around and feel the power of regedit. Be warned, don’t change anything that you don’t know about. Changing text strings like “My Computer” is harmless, but changing some core functions code may wreck your computer before you can undo it.

Libraries Can Be Dangerous Executable code can live inside resources or component libraries of many different varieties. These may not seem like obvious candidates for viruses, but they can certainly be exploited in that way. These file types include Dynamic Link Libraries (DLL), Control Panel Applets (CPL), various Type Libraries (TLB, OLB, and so on). This code isn’t directly executable with a %1 command verb like .exe, but this doesn’t mean that the code can’t be run. Just about any function exported from a DLL can be invoked using a helper application called RUNDLL32.exe.

A virus could employ two possible attacks. One would be to replace an existing DLL with a compromised version, in which a particular function is replaced by one of the same name but with altered functionality. Then, whenever the system invokes this function, instead of having the desired result, the virus is activated instead. The second approach is simply to write a DLL from scratch and invoke its functions using RUNDLL32.EXE when needed. This isn’t quite as straightforward as invoking the code in an EXE file, but a DLL, OCX, TLB or other library file is more likely to be accepted by an unsuspecting user or to be overlooked by an antivirus program, so it may well be worth the greater effort on the virus author’s part.

Scripts and Macros—Increased Flexibility Brings Increased Risk Script code requires a script engine to interpret and run, but it can still be exploited. Scripts come in several forms, they may be used to perform a repetitive task, modify documents, pretty much everything that a .exe file can do. Microsoft has done a lot to tighten the security of these macros and scripts, but it’s still easy for a macro virus to do a lot of damage. A popular script category is Windows Script Host (WSH) files. These files, usually with .wsf, .js, or .vbs extensions, carry a default file association which causes them to be executed, no questions asked, when users double-click them. As you might imagine, this can be disastrous.

Web applications may also carry dangerous scripts. Client-side scripts, for example are fairly limited in their access to the host system for security reasons, but there’s a little-known file type, the HTML Application (with an HTA extension), which works like a client-side Web application without the same security restrictions. Its purpose is to allow developers to use their Web development skills to build rich applications using the Web browser metaphor. But again, the unsuspecting user can unleash all sorts of chaos by downloading and executing such a file without first examining its contents. All of these scripts and macros can be readily examined before they’re executed, but clearly an average user won’t bother reading some weird code.

Virus Activation Methods The most common way for a virus to be activated is for a user to directly execute, say an e-mail attachment. Virus writers do just about anything to make you open the attachment using this action, but most developers are savvy enough not to just run an unknown executable. Of course, just about all of us have done it at one time or another—particularly now that viruses can access address books and can assemble a credible-looking e-mail message, ostensibly from someone you know and trust. Be that as it may, as users have become more aware, virus creators have gotten more devious, and there is now a host of new methods for activating a virus on a computer that don’t require any code to be explicitly executed (E-mail attachment viruses are a rare sight nowadays, since most email services themselves check emails for malicious files).

Registration Files Files with a REG extension, are system registration files that hold information to be integrated into the system registry. The problem with them is that they carry a default verb of “open”. This means that if any registration file is double-clicked, it immediately dumps its contents directly into the system registry, without any confirmation required (depending on your OS). Since it has access to the registry, it is free to modify anything it pleases. It may delete vital keys required for the system to startup or even modify existing ones to make further file types vulnerable. Also, it may create a new entry for a previously unknown extension. A general antivirus may not recognize and hence ignore a file extension of for example, .bobo. But in the registry there could be an entry to make the file’s default action to “Open” or “Run” – giving it all the powers of a .exe file. This could be potentially devastating. Luckily, most antivirus softwares nowadays are adopting the “sandboxing” technique to run untested files. Basically the antivirus gives the file a test run in a secure and sealed environment (sandbox) and checks if it tries to get out by for example, trying to access something that it’s not supposed to or changing system settings and variables without asking the users explicitly. Inside the sandbox, it is denied anything remotely suspicious and the antivirus raises a big red flag and moves the file to the quarantine section (virus vault) or maybe even delete it straightaway.

Path Vulnerabilities Another hazard is something called the PATH environment variable. Anyone who uses a computer is bound to have used a “shortcut” file at least once. Whether it’s the “Google Chrome” icon on your desktop or maybe “WinZip” in your taskbar these shortcut files are simply links to the actual files stored in the folder in which they were installed. It’s clearly a little tedious to open “My Computer” -> C: Drive -> Program Files -> Mozilla .. To open “Mozilla Firefox” every time. Hence, we have these shortcut files which simply store the “Path” to the actual application. These files don’t exactly use the Path environment variable, but the concept is exactly the same.

The computer has certain files (example: explorer.exe) that it may routinely need to open (example: at startup). So, in the Registry it has simply stored the name of the file it needs to open, and it’s path (in a path variable). Whenever the OS needs to open a particular system file it simply looks it up in the registry, follows the path in the memory and opens it. What does this has to do with viruses? With everything from phones to cars to houses getting smarter every moment, our computer viruses are not behind. It’s just a little too obvious and literally “on the nose” when a user clicks a file and his computer goes berserk. Once infected, our victim, although compromised is now a little wiser for he knows exactly where it went wrong. If the victim downloaded something from a website that caused damage to his system, s/he may report the website to the police. Clearly jail time is not so appealing to anyone, let alone virus programmers.

This brings us back to path vulnerabilities. Get this. The attacker needs to infect a system without making it too obvious. To do so, s/he needs a time gap between the actual infection and the attack, so that it hits the victim out of the blue. We have two ways to do this by exploiting the path variable. Average users don’t really bother to sniff around system files like in the “Windows” folder (you may find this in your C: drive) and this much understandable carelessness can be exploited. The attacker can simply take two systems, one with Windows 7 and the other with XP and search for the location of (example) “winlogon.exe” – A file that is used by the OS to maintain a user session. (You may find this running in the task manager). Now in the virus file, the attacker can simply write up code to create two copies of virus and send one to the path for Windows 7, and the other one to the path for Windows XP. (The path for the “winlogon.exe” file, which s/he searched earlier). After this, the virus can be coded to delete the file “winlogon.exe” and simply rename itself “winlogon.exe”. So, on the next startup when the OS looks up this file, it instead unleashes the virus and we’ve got BOOM. Total stealth, total annihilation and the victim has got no clue of what the hell just happened. The other way to do this is to code the virus to edit the path in the registry from it’s default value to where it (the virus) is stored. It can then rename itself and in this case on the next startup the OS doesn’t even go to the actual file but instead to the virus and starts it up. Equally effective.

The Best Offense is a Strong Defense Understanding how viruses take hold is the first step in knowing how and where to untangle them from your system once it has been compromised. As viruses become more sophisticated, you can expect them to become more aggressive toward your antivirus software. So what can we do about these nefarious little beasties? We’ve come a long way. Even though the end is not in sight, the roads that lie ahead of us are shorter than the ones behind. Cyber Security today, is tighter than ever but that doesn’t mean we are immune to attacks. If you do have the latest antivirus software and keep your OS updated, you probably don’t need to worry about 99% of all the malcious software out there, but the remaining 1% is the reason why cyber security is a $100 billion market today. Successful attacks are rarer today than ten years ago, but they certainly make up for it in sheer intensity. More and more people each day are relying more and more on their computers, not knowing that simply entering their credit card information on a secure website could be sending all those details to a hacker. Attacks and infections are decreasing in number, but rising in desctructiveness. Today, almost nobody bothers to spread around a virus that simply causes reboots or wipes data, since there’s no real gain for the hacker in it. The malicious softwares today are much more targeted. Clearly, obtaining someone’s social security number or bank pin is much more valuable to a hacker than deleting some random person’s movies and pictures.

The only thing we can do is keep our eyes open and turn around at the slightest hint of trouble. Avoid downloading from unconfirmed sources and make sure your antivirus software and OS is updated. Even if your system is compromised you may never actually know it, but the hacker could have identified that you are an easy target. If just once he could get valuable information from your system, it’s very likely that he will keep the system compromised, silently (make it a slave). Further, make sure to never ever store sensitive or financial information on your computer. Keeping it on an offline system or an external hard drive is much safer