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Why Did My Security Software Not Detect a Virus on My PC? – Ask Leo!

Why would an exploit not be caught or detected by my antivirus program? If not detectable, how much “damage” can the exploit actually do if users follow prudent operating precautions? Would System Restore be usable if infected? I also follow your advice and routinely image my Dell laptop.

We need to clear up a little terminology, but your question is a very good one: how can malware get past anti-malware programs and infect your PC?

And more importantly, what can you do to protect yourself?

Let’s define some terms with what I’m thinking is my silliest metaphor ever, and then talk about how to stay safe.

Malware can make its way to your machine any of several different ways including poor security software, out of date security software, un-patched and out of date system software, and user error. Avoid malware by using good tools, keeping everything up to date, and practicing safe computing. Once malware gets onto your machine it can do anything.

Vulnerabilities and Exploits

A vulnerability in software isn’t a bad thing in and of itself. It’s kind of like a hole in a bathroom wall: as long as no one’s looking through the hole, there’s no damage done.

Naturally, you’d like to have the problem fixed and the hole repaired (i.e. you’d like your software to be updated so the vulnerability is removed), but as long as the hole hasn’t been found by anyone, it’s not really putting you at imminent risk. It shouldn’t be there, of course, but as long as no one knows about it, all is well.

An exploit is like someone finding the hole and looking in at whatever’s happening in your bathroom. If the hole is big enough, perhaps they can even reach in and steal personal things like your toothbrush, or flush your toilet when you’re not looking.

A software exploit can do things like look at the information on your computer, steal personal things like your passwords, or use your computer to send spam when you’re not looking.

And yes, I just compared spam to whatever you might flush down your toilet.

Now to factor in anti-malware tools.

The term anti-malware is really a catch-all for a couple of different approaches.

  • Anti-spyware tools are kind of like security cops. They don’t know about the holes, but they have a list of about all the other places from which you could be spied on. They monitor the doors and windows and make sure no one has installed a video camera in the medicine cabinet. As soon as they see suspicious activity in those locations, they alert you and attempt to remove the threat.
  • Anti-virus tools are more like security cops with a big book of mug shots of all the people who are known to look in holes in bathroom walls. As soon as they see someone from that book, they kick them out, or at least let you know they’re lurking about.

The problem, of course, is that these cops are only as good as the information they carry. If the anti-spyware cop is unaware of the fact that video cameras can also be placed in the light fixture, they won’t check that. If the anti-virus cop doesn’t have the photo of the Peeping Tom discovered elsewhere this morning, he won’t recognize him.

That’s why I so often insist that you not only have up-to-date security software (cops who know all the important tricks of the trade), but that you also make sure to update their databases of malware (the list of places to look and malcontents to look for) consistently.

The metaphor can be extended even further. Not all cops are the same; some are better at seeing certain kinds of things that others, others get better data from their head office, and so on, meaning that some anti-malware tools are better at catching certain things that others.

And some are just incompetent.

Ultimately, though, not having up-to-date tools with up-to-date information is one way malware makes it into your system.

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Finding Holes

Unlike a bathroom wall, the vulnerabilities (or “holes”) in software are often not obvious or easy to discover. It’s not uncommon for a vulnerability to exist for years before someone stumbles across it and develops a way to exploit it.

To continue the “computer software is like a bathroom” story even further, the holes in your wall are very, very difficult to find. Depending on the quality of the original builder, there may be easier-to-find holes, but those are often found and fixed relatively quickly.

And here’s the scary part: hackers are like someone who spends all day and all night looking at your bathroom wall from the outside, hoping to find a hole no one else has found before. It’s not a new hole – it was there all along — but it is a new discovery, and often termed a new vulnerability.

Or sometimes they’ll find a new way to use a previously known hole that hasn’t been patched yet.

Either way, as soon as they’re successful, they create malware which exploits the fact your bathroom wall (the software on your computer) has an unpatched hole.

Fixing Holes

“So just plug the holes!” I hear you yelling.

Exactly. The problem is, as I mentioned above, the holes can be extremely hard to find.

But once they’re found, the hole is patched by updating the software on your machine to versions without those holes.

Usually. Some holes are fixed more quickly than others, and some may not be fixed at all. Some holes are harder to patch than others.

And some holes are worse than others. A hole that allows someone to see your toothbrush might be less important than a hole that allows someone to actually steal it.

Another issue is that fixing a hole can damage the wall, sometimes to the extent that a new hole is created elsewhere. By that, I mean fixing a bug in software can unintentionally introduce other bugs. Thus the benefit of fixing a known hole has to be weighed against the risk that doing so might create another hole we won’t know about.

The bottom line here, though, is that having out-of-date software — software with known holes fixed by updates you haven’t downloaded yet — is another way malware can find its way onto your machine.

Avoiding holes: extreme version

An experience with Java a few years ago is a great example of a widespread and newly discovered vulnerability.

To continue our now-tortured comparison:

  • Many, many people had this model of “bathroom”. (Many people had Java installed.)
  • Many holes were found and repaired in this bathroom’s walls. (Java has a history of vulnerabilities.)
  • A new hole was discovered, and new people were found looking in before the security cop’s mug book could be updated. (A new “zero day” exploit of a vulnerability in Java was found in the wild.)
  • Until the hole was patched, everyone using this bathroom was vulnerable to having their toothbrush stolen, or worse. (Everyone with Java on their machine was at risk.)

The common advice was to remove the bathroom completely (uninstall Java), use a different bathroom (use alternate tools that don’t use Java), or avoid using a bathroom altogether (don’t do whatever you were doing that required Java).

The metaphor breaks down at this point, because while most of us may not need Java (the advice remains to uninstall it unless you know you need it), we all need to use the bathroom.

Avoiding holes: more common version

The advice for avoiding software exploits is the same as it’s always been:

  • Keep your computer software up to date. (Keeps the holes we know about getting patched.)
  • Keep your anti-malware tools up to date, and keep their databases up to date. (Keeps the security cops sharp, and equip them with current information of what to look for.)
  • In some cases, uninstall software that is known to have issues. (Keeps you from doing things that a Peeping Tom might see or use against you.)
  • And of course, don’t invite a crowd of Peeping Toms onto your computer by opening attachments you’re not certain are safe, running questionable downloads, or visiting questionable sites.

In other words, keep your bathroom clean and don’t invite strangers in.

And, yes, even after doing all that, there’s still the possibility of a hole you don’t know about being found and exploited before all the defenses are updated.

If infected…

What can malware do? ANYTHING IT WANTS

To answer your second question: what can malware do?

Essentially, anything it wants.

Naturally, the specifics depend on the size of the hole being exploited and what’s available on your computer, but it’s safest to assume that once a vulnerability on your machine is exploited and an infection occurs, all bets are off.

That’s one reason that I so strongly recommend regular backups. If your machine is infected today, restore to yesterday’s backup, and the infection is gone. Period.

System Restore can sometimes help, but there are two problems with it:

  • In my experience, it’s extremely unreliable. There’s nothing worse than counting on System Restore to save you only to have it respond with things like “No restore points found” or the like.
  • You’re still not sure the malware is gone. System Restore doesn’t restore everything, and those things it does not restore remain infected if they were, in fact, infected to begin with.

Try System Restore if you like — be sure to run full and updated anti-malware scans thereafter — but it’s not something I feel at all confident relying on.

As for me, I’m moving my toothbrush.

If you found this article helpful, I’m sure you’ll also love Confident Computing! My weekly email newsletter is full of articles that help you solve problems, stay safe, and give you more confidence with technology. Subscribe now and I’ll see you there soon,


This post was written by Leo Notenboom and was first posted to

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