Computer viruses are code written to steal and destroy. They keep systems from functioning normally and they replicate relentlessly. Once inside your computers and devices, they can control programs, corrupt files and even permanently shut down your hardware. Once upon a time, the hackers who wrote viruses were amateurs playing pranks, just to prove their abilities. Today, malware coders are often organized criminals, sometimes even working directly for a foreign government or intelligence agency. The stakes are high, so it’s important to be on your guard and secure your network. 

Firewalls are an essential partner for antivirus software. They watch incoming and outcoming traffic for suspicious activity. While they don’t directly find and clean viruses – that’s what antivirus is for – they can protect your system from being exploited by the effects of an active virus. For example, if you accidentally click on a phishing email attachment and malicious code immediately (or worse – secretly) starts to initiate a download, your firewall will alert you and prevent that download traffic from proceeding.

Here are five major viruses that could have been stopped by a firewall: 

#1. Mydoom

AKA: Novarg

Estimated damages: $38 billion (inflation adjusted to $52.2 billion)

Number of users affected: millions - up to 25% of all emails sent at its peak

When: 2004 to present

Well-known as the worst virus in history, Mydoom infects machines, finds email addresses, and then sends copies of itself to those addresses. It enslaves machines to create a botnet and performs distributed denial of service (DDoS) attacks, with the ability to shut down a targeted website or server. It’s still active today, sending 1.2 billion copies of itself every year.

#2. Klez

Estimated damages: $38 billion (inflation adjusted to $52.2 billion)

Number of users affected: 7 million 

When: 2001 to present

This email-based worm uses text, which consists of some HTML code to open and execute an attached file. The virus searches for email addresses to send itself to, sometimes including private data it finds along the way. Every version of it becomes more destructive.

#3. ILOVEYOU

AKA: Love Bug, Love Letter for You 

Estimated damages: $15 billion 

Number of users affected: 10 million 

When: 2000

An email message with the subject line “ILOVEYOU” hid an attached file extension that was disguised as a normal text file. When users opened the file it would overwrite random file types: office software, images, and audio files, etc. It would then use Outlook to send copies of itself to all addresses in the Windows Address Book. Its author wrote the virus to steal passwords so he could log into online services for free – claiming to not know how fast and far it would spread.

#4. Slammer Worm

AKA: SQL Worm, Sapphire Worm

Estimated damages: $1.2 billion

Number of users affected: millions 

When: 2003

Through random IP scanning and selection, the Slammer exponentially spread by exploiting vulnerabilities and sending itself to other random machines. It used victims’ machines to launch DDoS attacks, dramatically slowing down internet traffic and crashing the internet in only 15 minutes. It affected everything from ATM machines and cell phones to airlines and 911 dispatchers.

#5. Sasser

Estimated damages: $500 million (but potentially $18 billion)

Number of users affected: millions

When: 2004

An 18-year-old German college student was turned in by friends to collect Microsoft’s $250,000 bounty for the creator of a malware worm that crashed millions of PC’s. The code used a security vulnerability in operating systems to infect computers just by turning the machine on. Once infected, the virus caused machines to sputter and spontaneously reboot.

Where can I find a good firewall?

Even for those who aren’t tech-savvy, Avast Advanced Firewall can be quickly customized to your needs with simple controls. A smart profile setting automatically detects the number of devices on your network and sets your profile to private or public mode to protect you from intruders.

Installing an efficient and strong firewall around your network is essential. It’s a great way to secure your system from the effects of virus invasion and prevent data theft. Install Avast Advanced Firewall today.