The Current State of the Malware/AntiVirus Arms Race

An anonymous reader writes "An article at Net Security explores how malware has developed self-defense techniques. This evolution is the result of the double-edged sword of the malware arms race. Anti-virus technology is ever more advanced, but as a result surviving viruses are increasingly sophisticated. What Net Security offers is a lengthy look at the current state of that arms race. 'There are many different kinds of malware self-defense techniques and these can be classified in a variety of ways. Some of these technologies are meant to bypass antivirus signature databases, while others are meant to hinder analysis of the malicious code. One malicious program may attempt to conceal itself in the system, while another will not waste valuable processor resources on this, choosing instead to search for and counter specific types of antivirus protection. These different tactics can be classified in different ways and put into various categories.'"

Comments

When programers fight...

[Coraon]

it's the computers that suffer. Wont someone please think of the computers?!

Re:When programers fight...

[Dimentox]

WARNING WORM DETECTED: By reading the parent you have been infected with the new slashdot worm. To remove it Please click here

Re:When programers fight...

[e9th]

I tried that, but it was really expensive. After some tweaking, I discovered that it was better to have my computers execute instructions.

Viruses will never go away

[Rosco P. Coltrane]

not because virus writers are clever, but because A/V companies are always very careful not to make too successful products, otherwise they'd kill the golden goose.

Re:Viruses will never go away

[doti]

And how will they compete with Free software anti-virus?

Re:Viruses will never go away

[42Penguins]

and I will yawn and enjoy a relatively un-penetrated life.
That's pretty much a given here on /.

Re:Viruses will never go away

[HomelessInLaJolla]

Are you saying that every single one of the best AV software authors are too stupid to be able to write malware?

Or are you suggesting that every single one of the best AV software authors are, by some supernatural intervention, of such outstanding moral and ethical calibre that they would never do such a thing?

Or are you implying that every single one of the best AV software authors are so completely, single-mindedly, dry that they would never consider the academic exercise of writing extremely low-level "system administration software"?

Or are you trying to spread the idea that every single one of the best AV software authors are such mindless automatons that they would never brainstorm about new and novel malware methods in their course of duties?

And, if you're going to be rational enough to point out that "every single one of" is a little bit extreme, just what percentage of the global group of AV software authors do you suppose falls into the above categories? Of those, how many of them have family members, friends, social colleagues, or professional associates who have access to their ideas and experimental code and, of those family members, friends, social colleagues, or professional associates, what percentage of them meet the criteria of saintly moral and ethical fibre?

I think it's obvious that you're very wrong to dismiss the idea that a good portion of 0-day exploits and malware comes from inside the professional sector as "conspiracy theory" or "canard".

Re:Viruses will never go away

[kestasjk]

This XP install has been going for over a year and hasn't got malware yet, and I don't use any anti-virus or anti-spyware apps. If you don't download spyware, use some common sense, and run under a user account and not an admin you don't get malware.

Re:Viruses will never go away

[gardyloo]

This XP install has been going for over a year

    Geez, and I thought Gentoo was supposed to take a while.

Re:Viruses will never go away

[MajinBlayze]

This always makes me laugh:

hasn't got malware yet

followed by:

I don't use any anti-virus or anti-spyware apps

Honestly, I used to have the same view; Then one day I was having some hd problems, and started watching traffic. After restarting my computer, it wouldn't boot, as something had corrupted my MBR. After that, I learned not to trust so much, and ultimately got interested in Linux. If for nothing more than the fact that there are fewer viruses/malware for the platform.

Evolution?

[truthsearch]

Malware evolution? That's just theory and conjecture. If god had wanted our computers to be free of viruses he wouldn't have invented Microsoft.

(There goes some karma.)

Re:Evolution?

[geoffrobinson]

The viruses are intelligently designed. I'm not vouching for Microsoft Windows.

From TFA

[Chris Tucker]

"This article will only examine malicious programs written for the Windows operating system (and its predecessor, DOS) due to the rarity and relatively small number of malicious programs for other platforms."

OK, you had to go to the second page of TFA to see this, but at least they came right out and said that Windows is the primary and almost exclusive target of malware.

Unlike almost every other article about viruses and malware in recent years.

Mac OS X: Because it was easier to make UNIX user friendly than it was to fix Windows!

Re:From TFA

[Opportunist]

Hey, there is rather little malware for Vista! For the same reason there is virtually none for Mac or Linux: It doesn't pay.

Why is there very little "commercial" malware for Firefox? Firefox has quite a few security bugs and holes that can be exploited for phishing and identity theft, still, virtually all commercial malware relies on WinXP and IE. Why? Because of the numbers.

Writing malware for IE means that you can infect about 3/4 if not more of possible targets, while malware for FF means you will reach about 1/4 at best. So for which one do you develop if your goal is to infect as many targets as possible?

Since today most malware kits rely on user stupidity rather than system flaws, the system's own security is no deciding factor anymore. I'd rather attribute it to the number of possible targets and, of course, that the malware writers are used to the Windows architecture and can (ab)use it very creatively.

Re:From TFA

[kebes]

Market share is certainly a factor, but I think it's a stretch to say that it's the only factor.

Let's say some nefarious guys are trying to get their malware installed on everyone's computers. So they buy some exploit code that targets IE. They say "Great, this will infect 3/4 of the computers out there!"

Now if these malware distributors are approached by some other guy who says "I can sell you exploit code that targets Firefox"... do you think the malware distributors will say "no thanks" or will they say "Great, that covers the other 1/4 of computers out there!" (Maybe they will pay less for that exploit, but they will surely use it if it's available.)

Since Firefox's market share is not insignificant (10% to 25%?), there should be a market for such exploits. Similarly, there should be a market (perhaps smaller, but still a market) for the 4% Mac users. It appears that despite this, the targeting of Mac and Firefox is very much less than Windows/IE (more than can be accounted for by market share alone).

I'm sure that part of it has to do with market share. However inherent security is also part of the equation. (And frankly I don't know why such a statement is so controversial on Slashdot... why should security be based on only one factor in the first place?)

Re:From TFA

[Opportunist]

Security is by definition the minimum of the system's capabilities and the user's. When the system can't hold its water (or data), the user can be the best security guru in the world and it is insecure. Likewise, the system can be as tight as possible, with a clickmonkey at the helm it is hopeless (provided it's an all purpose machine that doesn't restrict the user's ability to cause havoc).

Still, market share is a key factor when it comes to malware. Malware "kits" cost a wee bit of money, ranging from a few hundred to a few thousand USD, depending on sophistication and "additional services" (let's not get into too much detail, you get the idea). Basically, everyone develops for IE on a WinNT core machine. Why? Market.

Yes, there would be a market for FF exploits. But it's smaller. Development costs are pretty much equal for FF and IE exploits, and you can not really develop a "generic" exploit that targets both, unless you target the OS underneath and not the browser itself (that happens too, but generally requires a lot more knowledge about the OS itself, and it is by far less flexible). Since the cost of spreading malware is roughly equal for whatever you want to land, and doing so is not really cheap, attackers usually try to maximize their efficiency by limiting themselves to the most popular OS/browser combination (provided they want to do ID theft attacks). At the very least, they will limit themselves to the most popular OS.

The limiting factor here isn't that the "kit" itself would be costy. Yes, you might have a FF exploit kit available and you'd sell it for a fraction of the IE kit (but why should you, you could more easily develop an exploit kit for IE (there are effing templates for it in VC!) and cash in). But the spreading cost for either malware stays the same.

Thus the usual exploit targets IE/WinXP. Should the market share of FF rise, I'd wager to about 35-40%, we'll probably see mass spam of FF targeted malware, due to people using FF feeling secure and are thus maybe less wary. It might happen. But generally, you'll never see masses of malware for non-mainstream targets (OS, browser, webserver...). The cost of spreading is the same, no matter what your target is. So why shoot at something but the biggest target?

Re:From TFA

[sid0]

Not to mention the fact that the average Firefox/Linux/OS X user is smarter than the average Windows n00b, and would never open an executable email attachment.

Lack of Specifity on Infection Vectors

[Cr0w T. Trollbot]

My brief overview of the article leads me to believe that it's long on general malware theory, and short on the specifics of current malwear infection vectors as opposed to techniques. I believe that most of the readers of Slashdot are familiar with how a rootkit works. Far more valuable would be a breakdown of the most common infection vectors for rootkits right now. Is it TCP/IP stack overflows, Active-X controls, e-mail trojans, or old-fashioned human error?

Fruthermore, "trends" in malware construction obscure the reality that certain software packages (Windows, IIS) are otrders of magnitude more vulnerable than others (OS X, Linux, Apache). The unstated elephant in the room is that 95-99% of malware attacks are due to Microsoft vulnerabilities.

Crow T. Trollbot

Re:Lack of Specifity on Infection Vectors

[another_fanboy]

The unstated elephant in the room is that 95-99% of malware attacks are due to Microsoft vulnerabilities.
Microsoft's dominance over the market makes it more enticing to malware writers, regardless of how many vulnerabilities it has. If damage is their desire, they want the most damage; if it is a zombie network, they want the biggest zombie network.
If linux ever manages to overtake windows, it will become the primary target.

No mention of the effect of whitelisting?

[Anonymous Coward]

There doesnt seem to be any mention of whitelisting in the arms race between malware and desktop management systems in this article. Companies like Trinamo are championing the approach of designating only a handful of applications as being "approved" for execution, denying viruses, trojans, malware, and other junk like toolbars a chance to run before they can do any harm. They have a bunch of free information on the subject online. http://www.trinamo-solutions.com/downloads/downloa d.html
This story is all over industry security portals at the moment, and has appeared in theregister, securityfocus, and others.
Jack

Re:No mention of the effect of whitelisting?

[Control Group]

Your idea boils down to making the computer no longer a general-purpose device. This, obviously, defeats the purpose of having a computer in the first place.

An awful lot of modern malware doesn't comprise "viruses" in the classical sense, it comprises trojans. The only way to absolutely prevent a trojan from running is by preventing the user from running arbitrary software. This may fly in a corporate environment, but never for home use.

Basically, it comes down to either being vulnerable to malware, or not letting the computer do what the user tells it to.

(The latter, of course, being the driving force behind so-called "trusted computing"...which is pretty much exactly what you're advocating)

Re:No mention of the effect of whitelisting?

[Brad1138]

Maybe Microsoft could have a pop up for every process that tries to run, then YOU would have controll. Ya, that sound likes a great idea.

Re:They forgot one!

[syntaxeater]

http://www.microsoft.com/technet/sysinternals/util ities/ProcessExplorer.mspx

It's essentially a beefed up task manager that allows you to suspend and kill specific threads and processes.

Oh please...

[Opportunist]

This conspiracy is about as old as the AV industry. At least you spared us this time the drivel about AV vendors first of all creating malware so they can sell their stuff.

Basically it's impossible to write the perfect AV software. It simply does not work. The perfect AV software could, of course, exist: Simply disallowing ANY kind of user interaction and installation of additional products. Perfect computer. Useless, but perfectly safe.

The problem is that malware does not use anything "special" that makes it easy to say "something that uses function X or accesses Y is malware". Doesn't work that way. What malware does it usually not much different from normal program activity. They access the windows registry, create keys there, they create and alter files (not necessarily system files, which would be "suspicious" behaviour to say the least), they plug into Internet Explorer, they open ports for incoming connections, they transfer data to and from the computer.

It's not anything that is by defintion "bad". How'd you want to create the "perfect" AV product?

Re: Oh Please...

[a-zarkon!]

It seems to me that the malware authors are putting at least if not more effort into research, development, and quality assurance than the major OS and AV vendors expend on improving their products. I wonder if that is a function of the malware authors being compensated more directly as a result of their efficiency? They don't appear to be trying to bundle a "malware suite" or get additional revenue from licensing and support.

I wonder if AV vendors would be able to deliver a better product if they cut overhead and simply focused on developing and maintaining a product that worked efficiently and effectively for a decent price. I know I would prefer an AV solution that just did anti-virus very well and didn't involve a hard-press sales call every other week to evaluate their "security suite."

Re:Oh please...

[Vitriol Angst]

Wow.

Thanks for the usual post about; "there aren't any conspiracies" -- now THAT is a pretty flimsy theory. People get together in groups to figure out how to profit from others, or do something that they don't want people to know about. Wow, that NEVER happens. What was I thinking?

I think the almost PERFECT AV software can be made. You basically TRUST the applications and processes already running on a system. Any NEW process that enters the system, but be acting in a defined way and only allowed access to what it has permissions for.

So you need tokens, permissions, and a AV software that looks at what viruses DO -- rather than this mickey-mouse "signature" technique, that I'm sure has done a great job in creating a market for moving a few "1's" and "0's" around to roll out the next virus.

On the Mac, you not only have to use an Admin password to install a new application -- even running as an Admin, you have to "approve" the application opening a file the first time. The only weakness in this system is that it goes by name -- and a virus could be called "Photoshop." But with all of the reasonable actions set up on the Mac, and the fact that there is no ECONOMY for viruses -- there are few viruses.

There could be a lot more done, to protect an OS -- other than hope that every exploit like a buffer overrun on whatever the next function added to an application will be.

As long as devices communicate -- there is an opportunity for viruses -- just like in our own immune systems. But with computers, there is an opportunity to do a better job of "white listing" SAFE sources, and letting things run for a bit in a sand box, and only allow them to do certain things. It's that last bit that, even permissions don't effectively address. Should all applications be able to write ANYWHERE that the permissions allow? Perhaps not. Perhaps the permissions of WHAT an application can DO are more important than setting that on directories and files.

But the "perfect AV product" isn't the issue -- there isn't even a serious attempt to get rid of Malware in the first place. A product that could do that would kill the market.

Re:Oh please...

[Opportunist]

Know what? Sit down and write it. Yes, it's gonna put me out of work, and I'll probably have to do something sensible instead of prodding at malware all day, but that would be worth it. It's no fun to dig through disassembled trojans and learn every day a new flaw about Windows. And to make matters worse, I can't even talk about it.

What you suggest first of all requires a sensible distinction between system and user space. Which doesn't exist in Windows, at least until Vista. Be aware that you're dealing with a system where the normal user usually has full access to the full system, down to installing drivers and injecting code into running processes, even system processes.

Your model trusts the system. Which is allright, until someone finds a way to compromise it. Which is no big deal in Windows, since it is possible to manipulate even loaded system core dlls. The files, not only the copy in ram. And if that fails, it's no problem at all to inject code into the copies in ram (they're conveniently at the same address space in all programs, even in Vista. Yes, Vista randomizes. Well, chooses from 256 possible locations, and only ONCE per reboot...), all you have to do is make sure the program to manipulate it is loaded at reboot (which is conveniently supported by Windows through about 10 or 20 different ways, depending on just when you'd like your malware to load).

And of course that you have debug privileges. Which is, as mentioned, no issue in any Windows version up to XP, since it's near impossible to use it sensibly without. Too many programs rely on the nonexistant distinction between user and system.

So if you can write that perfect AV program, please do. I've tried for years, I failed. I admit it. Please take the torch and run with it.

It's violent territory...

[cromar]

What's funny is that virus writers fight with each other too.

I don't get it...

[disasm]

I thought these problems ended years ago when the year of the linux desktop came and everyone stopped using windows... You mean there are still poor souls out there that don't use linux or mac?

Sam

God I hate articles w/o print links!!!

[taosk8r]

Please, there oughtta be a law that multi-page articles with text squeezed between massive, obnoxious graphics, have a PRINT FRIENDLY LINK!! ARGH!

And the collateral damage of this' war'...

[rickb928]

...extends beyond poor performance, spam, cost of software, etc.

We got hit here with a collateral listing of one of our tools as 'spyware'.. It shut down our software across the U.S.

We used a toolkit from a vendor to encrypt and compress files for transmission and for patch distribution. It was slick, lightweight, and sufficiently secure. it was also a commercial product, and was sold to another publisher who used it in their software.

One of their packages is an IM logging and monitoring tool. Good for AOL IM, and others. You have to either download it as shareware, or buy it outright, and then you have to install it, with the usual requirement that you actually have access to the PC. It's not and has never been distributed as 'spyware' in the sense of an unexpected or unsolicited install, nor was it ever distributed from a website or as part of another package - unless you repackaged it yourself. The biggest users were corporate IT departments monitoring IMs for compliance, and parents/spouses/etc snooping on others.

Not what I think of as 'spyware'. But someone else thought differently.

The IM logger got reported to either Trend Micro or McAfee as 'spyware' more than a year ago. Sporadic reports continued, until the latest (?) release came out and got popular. Then the flood of reports ensued. And when I say 'flood', I mean 'dozens'. I suspect some HijackThis logs started showing it, and after a few more reports, it was assumed by someone that this application was part of other kits. Listing the application by one anti- company leads to everyone else listing it. No one wants to be left behind, and none of the 'security' companies wants to be the one that lets bad stuff in, just because they actually evaluated the listing. No, it got listed by everyone.

And the controls along with it. Including the one we used for everyday, legitimate encryption and compression.

Our customers started reporting failed installs and reinstallations. One reported they got a virus alert. We looked things over. Why now? We hadn't changed anything substantial in years.

Then, on a whim, I Googled for it. BAM! Our control was listed as malware. WHA?

We figured it out an an hour. I asked around some of the contacts I knew at Symantec, etc. Their advice was simple - give up. Go get a new tool, recode, and move on. Surrender. Even though the module we used was by itself harmless, it was guilty by association. So we did. So far as I know, the company that produced these tools & modules is struggling with this. After all, their code signatures are now officially 'malware'. Kinda like banning drills 'cause someone drilled a hole in their finger by accident. Pretty soon, nothing gets drilled. Not a good state of affairs for the drillmakers.

And not a good state of affairs for drill users, either.

That IM logger that started all this? It was commercial software, and other than being highly annoying for kids who value hiding their IMS from snooping parents ("Hey, who's paying the Internet bill around here?"), or spouses caught on dating sites, the businesses forced by law to treat IMs as if they were business correspondence found this to be a good tool. Not so good any more. About the only way to use this is to keep writing exceptions to your anti- software. If you can. And keep re-writing these exceptions every damned update. Maybe more than twice a day.

It looks like this application is dead. Kinda sad.

We survived, though some of our customers did get concerned. In our business, being labelled as 'spyware' could cause massive problems, beyond the usual. It could be front-page of the fishwrap stuff.

In the midst of the virus/spyware/malware/anti- battle, this is one small story of how unintended consequences have real costs. We had to scurry to buy new stuff, re-code, and distribute. Our original tool vendor has had to give up on a good product, through no fault of their own. The application vendor that 'st

Problem is getting harder to fix

[sherriw]

Cleaning out a virus/trojan problem has become close to impossible for the average person. Most people and even actual computer service shops just format and re-install.

I have only moderate PC service skills and this weekend my family's computer popped up a AVG warning that a Trojan was detected. This is not my computer but it shares my net connection via wireless. When I saw that detection warning I pulled the plug on it's net connection and then investigated. My brother had been downloading wma to mp4 converters. And bingo! On top of that, no one was keeping the AVG up to date or doing regular scans. Apparently everyone assumes I'll clean up their messes for them. Pisses me off.

So, guess how hard it is to clean out a Trojan these days? Guess what, your anti-virus is useless! It may detect the virus, and clean it, but it re-installs itself.

Get ready for a loooooong process involving:

-Disable system restore and remove all restore points.
-Reboot in safe mode, run anti-virus /spyware scan.
-Use Autoruns or any other startup/running processes program.
-Write down what is being run on startup and what is currently running.
-Hop on Google to find out which of those are legitimate processes.
-Remove the bad-uns.
-Look for a cleaner program for your specific Trojan/Virus. Careful to get it from a reputable site.
-Run the special cleaner program in safe mode and regular mode.
-Grab output from HijackThis and use google to research any suspicious entries.
-Do all this without connecting the infected computer to the net. (PAIN!!!)
-Profit!!!! (I couldn't resist saying that)

So, then you cross your fingers for a few weeks waiting to see if your AV pops up another warning. All the while doing manual updates of your anti virus. Keep it in quarantine a while longer. Then, cautiously re-connect to the web and HOPE it's clean. Then YELL at your family to stop downloading crap, and make a "nice" desktop wallpaper in msPaint to drive home the rules.

*sigh* it's a huge pain, especially for people like me that need to research every process because they don't know what's legit or not. Not to mention that my sister does her online banking on that computer, and I've had to tell her to go change her passwords, get a new CC number, and inform her bank to put a watch on her account for any suspicious activity.

I really wish these virus writers would fry.

No wonder people just format and re-install.

A question about diminishing returns

[Bombula]

At what point is it simply not worth the effort to write a new virus?

I assume it's getting more and more difficult to write viruses as time goes by - is that correct? If this is indeed an arms race, then one side or the other is going to run out of time and energy and money sooner or later, and I'm guessing it won't be the AV companies since there's so much at stake.

Re:A question about diminishing returns

[Opportunist]

Actually, surprisingly it's getting easier. Think game development. With the advent of DirectX, you needn't know too much math anymore to get some cool looking 3D graphics on the screen. The same applies to malware. Back in the good ol' days of DOS, you had to know quite a bit about the inner workings of the system to get your virus in. You had to redirect software resets, trap a few interrupts, essentially you had to write a driver. Today, most of the malware that circulates could be written in VB. Some is.

Obfuscation is also easier than ever, with a lot of runtime packers and scramblers existing. It's easy to repack a file in batch mode that ensures that no two samples an AV company could get are the same. Thus the simple "signature" approach someone suggested earlier won't find a thing anymore.

It's also not getting trickier to hook into the system. Since there are still the majority of crates running with users having admin access, the same ol' tactics that worked 5 years ago still work. It's also not simple to track the use of "suspicious" calls, since Windows itself makes quite liberal use of functions that e.g. hook keyboard input or inject code into other processes.

Writing malware is also no longer the pastime of bored adolescent geeks. It's business. We're talking organized crime cartels here and that a "virtual" bank robbery (by hijacking online banking sessions) is more profitable and less risky than the real counterpart is a given. When I see the figures, I sometimes wonder why I stay on this side of the fence...

It still is an arms race, but with the AV companies in the defense. Constantly. An AV company can only react to a development, anticipation is pretty much impossible. There are far too many roads the next attack can come from that it's not feasible to develop in a certain direction without anything warranting it.

A few years ago, malware authors started to obfuscate their code. AV companies reacted by developing ways to crack that obfuscation. Then malware attacked certain AV software directly, as mentioned in TFA. The software was adapted to thwart such attempts. Malware started to contain rootkit functionality to hide itself. AV tools started to come with their own file system drivers to read the HD directly instead of relying on system calls.

You cannot anticipate that sensibly. What will be next? I don't know. I can only see trends and development in the malware that runs through my fingers. Which is a very tiny amount of the malware that gets written every day. It's a bit like trying to sieve a beach with a toy sieve. The big thing in malware today is (and has been for about a year or two) remote controlling, setting up servers somewhere and making the malware phone home. Yes, it's no longer IRC. It's a server in Belarus, Kazakhstan or Brazil (or some other country where the police has better things to do than being bothered by a server that doesn't really do any damage in their own country). So some malware packages started implementing tools that can monitor traffic and find "suspicious" traffic, just in case they can't find the corresponding malware. Possibly because the malware itself doesn't exist anymore, it was only an installer that manipulated some system file in such a way to send that info... and so on.

The current thing is (aside of what's been here for ages) id theft. Your amazon or your ebay account, your online banking information, your credit card information, and of course your machine, as a place to spew malware from, as a spambot or simply as a relay to route traffic through to obfuscate the real destination. With broadband becoming the norm and computers running 24/7 to download .torrents, they turn into the ideal dead drop.

There's much at stake. For both sides. I don't see a winner on either side too soon. Well, it's good for my job security, that's a given, but I didn't go into this venue just to make money (it's not THAT well paid). If I wanted that, I'd have learned ABAP.

ClamAV

[DrYak]

And how will they compete with Free software anti-virus?

Actually, by cheating ;-)

Funny little anecdote in the world of virus scanning (harmless although dishonnest).

CalmAV is such an open-source virus engine (with ClamWin as a Windows port).

There have been several studies done about it (links on ClamAV's site) which reported that ClamAV, despite not being a non-commercial project, has among the fastest response time when new threats emerges.

The studies also surprisingly uncovered a small cheating : some companies did small update that didn't bump up the signature release number, but that included the new virus detection. Normally such non-upped releases should be reserved for modification of the sig library that don't affect the number of detected viruses (like repacking the data more efficiently or whatever). But the companies nonetheless tried to slip in newer sigs, hoping that users would not notice it. When doing a retrospective study, unsuspecting users will read that virus XYZ is detected since Sig-file release A.B.C and they will see that Sig-file release A.B.C was released on YYYY-MM-DD HH:mm, thus will come to the conclusion that the virus was detected earlier than the concurrene. (Source, paragraph A dirty little secret).

But anecdote aside, ClamAV is a nice anti-virus engine, that has plugins (either bundled in or 3rd party) that enables on-the fly scanning of data at usual entry points (ClamAV is popular for mail filters in Unix. ClamWin has plugins for mail clients and FireFox's downloader, etc.) and is a nice stuff to put in the "post-download script" of your usual peer-2-peer software. Please note that ClamWin still lacks a on-access scanning mode (although some 3rd party application like Winpooch can start scanner before executing or reading files).