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A virus runs when the user launches an infected program or boots from an infected disk.
Viruses keep a low profile, so as to spread widely without being detected. Most of the time, the
virus code simply infects new programs or disks. Eventually, often at a predefined date and time,
the virus payload kicks in. Early virus payloads were often mindlessly destructive; these days
they're more likely to steal information or implement a DDoS (Distributed Denial of Service)
attack against a major web site.
Worms are similar to viruses, but they don't require the user to launch an infected program.
Simply put, the worm copies itself to another computer and then launches that copy. In 1988 the
Morris worm, intended as a simple proof of concept, caused serious damage to the budding
Internet. While it wasn't meant to be malicious, its over-enthusiastic self-replication sucked up a
huge amount of bandwidth.
Just as Greek forces fooled the people of Troy by concealing warriors inside the Trojan Horse,
Trojan programs conceal malicious code within a seemingly useful application. The game,
utility, or other application typically performs its stated task, but sooner or later it does
something harmful. This type of threat spreads when users or Web sites inadvertently share it
with others.
Threats Defined by Behavior
Viruses, worms, and Trojans are defined by the way they spread. Other malicious programs take
their names from what they do. Spyware, not surprisingly, refers to software that spies on your
computer and steals your passwords or other personal information. Adware pops up unwanted
advertisements, possibly targeted to your interests by using information stolen by a spyware
component.
Rootkit technology hooks into the operating system to hide a malicious program's components.
When a security program queries Windows to get a list of files, the rootkit removes its own files
from the list. Rootkits can also hide entries in the Registry.
A bot infestation doesn't actively harm your computer, but it makes your system complicit in
harming others. It quietly hides itself until the owner, or "bot herder", broadcasts a command.
Then, along with hundreds or thousands of others, it does whatever it's told. Bots are often used
to send spam, so the spammer's own systems aren't implicated.
Some malicious programs exist specifically to aid in distribution of other malware. These
dropper programs tend to be tiny and unobtrusive themselves, but they can funnel a steady
stream of other malware onto your computer. A dropper may receive instructions from its remote
owner, as a bot does, to determine which malware it will distribute. The owner gets paid by other
malware writers for this distribution service.
As the name suggests, ransomware holds your computer or your data for ransom. In the most
common form a ransomware threat will encrypt your documents and demand payment before it
will decrypt them. This type of malware is relatively uncommon simply because the perpetrator
must stay visible enough to receive that payment.
Scareware
Not all antivirus programs are what they seem. Some are actually fakes, rogue programs that
don't protect your security and do harm your bank balance. At best these programs offer no real
protection; at worst they include actively harmful elements. They work hard to scare you into
paying for registration, so they're often called scareware. If you do register, you've both wasted
your money and handed your credit card information to crooks. Avoiding scareware gets more
and more difficult as the programs get more refined.
These categories aren't mutually exclusive. For example, a single threat might virus-style, steal
your personal information like spyware, and use rootkit technology to hide itself from your
antivirus. A scareware program is a kind of Trojan, and it might also steal private data.
The term malware encompasses all of these types of malicious software. Any program whose
purpose is harmful is a malware program, pure and simple. Industry groups like the AntiMalware Testing Standards Organization