There’s a common myth circulating around the web: As long as a file doesn’t have an EXE (“.exe”) extension, then you can open it without having a second look. This argument has a significant flaw and doesn’t account for the behavior of programs that open the files. We will now have a look at a few different file types that viruses can be present in and discuss them in detail.
How Non-EXE Viruses Work
First of all, a file doesn’t have to have the EXE extension to execute. Screensaver (SCR) and batch (BAT) files are good examples of this, and you’ll find plenty of viruses with this extension to this day.
Besides executable files, you can also have a virus that manipulates the program that opens it, like malicious Windows Help (CHM) files. A CHM virus will open the Windows Help program and use some of its features to harm your computer. Depending on how complex the program is, a virus can even collect information from your computer and send it home without any sign of wrongdoing. Every virus needs some form of access to the Windows application programming interface (API). Some programs offer some level of access (like Microsoft Word) and can really put your computer at risk.
1. Word Macro Viruses
One of the most common types of a typical viruses is the Microsoft Word macro virus. It’s grown to a level in which even Microsoft warns about them in its own site. One of the biggest attacks came from a macro virus known as W97M/Melissa.A. This particular virus would deliver itself in an email that contains a Word document attachment. Once the virus executed in Microsoft Word 97/2000, it would first send itself from your computer to all your email contacts. After that, the virus writes itself into your normal template (normal.dot) so that every document you start within Word would be infected. Although most viruses are dangerous, this particular macro virus posed a very small threat. Another variant of the virus (Melissa.V) actually destroys Excel documents after searching for them.
To protect yourself from viruses like these, it’d be wise to disable macros. The most recent versions of MS Word already do this for you and ask if you’d like to enable macros when you open such a document. The threat presented by these viruses shouldn’t be underestimated, though.
2. PDF Viruses
In the year 2001, a new kind of virus was discovered hidden within PDF files. Adobe’s Reader application allows PDFs to run embedded executable files, meaning that there’s a vast amount of potential with this. As far as carnage is concerned, this type of virus can rip your system (and privacy) to shreds. Another type of PDF virus also runs an embedded script, which can also wreak havoc on your system.
One such virus, known only by the name of “Peachy,” entertained you with a game and then ran a VBScript file once you won. The PDF document would then send copies of itself to your Outlook contacts.
The threat of these viruses has been minimized by Outlook’s upgrade in 2002 which detected any external attempt to grab its contact list. The program will warn you when an external source tries to get a hold of email addresses. The best way to defend yourself against PDF viruses is to put them under scrutiny (such as uploading them to VirusTotal) and heeding any warnings from Outlook when unauthorized scripts try to access its contact list.
The Threat Continues
Technology is evolving rapidly. Although PDF and Word macro viruses aren’t as threatening as they used to be, you shouldn’t breathe a sigh of relief just yet. Other viruses will take its place. The best thing you can do is make sure you exercise a certain degree of prudence when opening files sent to you via email