Are malicious programs that spread from system to system via network connections?

This information gives an introduction to viruses and ways to avoid them.

Frequently Asked Questions

• What is a virus?
• What do viruses do?
• What is a worm?
• What is a Trojan horse?
• Can I get a virus by reading my email messages?
• How can I avoid a virus infection from email?
• What are some general tips to avoid viruses and lessen their impact?

For More Information

US-CERT offers many resources to help you create a more secure home computing environment. These documents may be of particular interest if you have concerns about viruses and Trojan horses:

• Understanding Anti-virus Software
• Using Caution with Email Attachments
• Good Security Habits
• Recovering from a Trojan Horse or Virus

Frequently Asked Questions

What is a virus?
A computer virus is a program that spreads by first infecting files or the system areas of a computer or network router's hard drive and then making copies of itself. Some viruses are harmless, others may damage data files, and some may destroy files. Viruses used to be spread when people shared floppy disks and other portable media, now viruses are primarily spread through email messages.

Unlike worms, viruses often require some sort of user action [e.g., opening an email attachment or visiting a malicious web page] to spread.

What do viruses do?
A virus is simply a computer program--it can do anything that any other program you run on your computer can do. Some viruses are designed to deliberately damage files, and others may just spread to other computers.

What is a worm?
A worm is a type of virus that can spread without human interaction. Worms often spread from computer to computer and take up valuable memory and network bandwidth, which can cause a computer to stop responding. Worms can also allow attackers to gain access to your computer remotely.

What is a Trojan horse?
A Trojan horse is a computer program that is hiding a virus or other potentially damaging program. A Trojan horse can be a program that purports to do one action when, in fact, it is performing a malicious action on your computer. Trojan horses can be included in software that you download for free or as attachments in email messages.

Can I get a virus by reading my email messages?
Most viruses, Trojan horses, and worms are activated when you open an attachment or click a link contained in an email message. If your email client allows scripting, then it is possible to get a virus by simply opening a message. It's best to limit what HTML is available in your email messages. The safest way to view email messages is in plain text.

How can I avoid a virus infection from email?
Most users get viruses from opening and running unknown email attachments. Never open anything that is attached to an email message unless you know the contents of the file. If you receive an attachment from a familiar email address, but were not expecting anything, you should contact the sender before opening the attachment. If you receive a message with an attachment and you do not recognize the sender, you should delete the message.

Selecting the option to view your email messages in plain text, not HTML, will also help you to avoid a virus.

What are some tips to avoid viruses and lessen their impact?

• Install anti-virus software from a reputable vendor. Update it and use it regularly.
• In addition to scanning for viruses on a regular basis, install an "on access" scanner [included in most anti-virus software packages] and configure it to start each time you start up your computer. This will protect your system by checking for viruses each time you run an executable file.
• Use a virus scan before you open any new programs or files that may contain executable code. This includes packaged software that you buy from the store as well as any program you might download from the Internet.
• If you are a member of an online community or chat room, be very careful about accepting files or clicking links that you find or that people send you within the community.
• Make sure you back up your data [documents, bookmark files, important email messages, etc.] on disc so that in the event of a virus infection, you do not lose valuable work.

How to Stay Protected from Malware Attacks

There are a few things you can do to help protect yourself from malware attacks:

• Install antivirus software and keep it up to date.
• Be careful about which websites you visit and what files you download.
• Use strong passwords and keep your computer software up to date.
• Don't open attachments or click on links in emails from people you don't know
• Back up your data regularly.

How do I get malware?

Malware is usually distributed through malicious websites, emails, and software. Malware can also be hidden in other files, such as image or document files, or even in seemingly innocuous files, such as .exe files.

Users can unintentionally install malware when they click on a link in a phishing email, or when they download and install software from a website that is not reputable. Malware can also be installed on a computer when the user plugs in an infected USB drive, or when the user visits a website that is infected with malware.

How malware can infect your PC

There are many different ways that malware can infect your PC. One common way is through infected files that you download from the Internet. Malicious code can be hidden in all kinds of files, including videos, pictures, and software. When you open these files on your PC, the malware can infect your system and cause damage.

Another common way that malware can infect your PC is through malicious websites. If you visit a website that is infected with malware, the malware can automatically download and install itself on your PC without your knowledge.

In addition, malware can also be spread through email attachments. If you open an email attachment that is infected with malware, the malware can install itself on your PC and cause damage.

Is malware a virus?

No, malware is not a virus. Malware is a type of software that is designed to cause harm to a computer or its users. Viruses are a specific type of malware that can spread from one computer to another.

How to detect malware?

Malware is software that is installed on a computer without the user's consent and that performs malicious actions, such as stealing passwords or money. There are many ways to detect malware, but the most common is to scan the computer for malicious files or programs.

Malware can be installed in a variety of ways, including through email attachments, drive-by downloads, or by clicking on links in malicious websites. It can also be installed through vulnerabilities in software that the user has installed on their computer.

How to remove malware

" There is no one-size-fits-all answer to this question, as the best way to remove malware may vary depending on the specific malware that is installed on your computer. However, some common methods for removing malware include using an antivirus program to scan your computer for malware and then delete any malware that is found, using a malware removal program to scan your computer for malware and then delete any malware that is found, or manually deleting any malware that is found on your computer.

Do mobile devices get malware?

Yes, mobile devices can get malware. In fact, there’s been a surge in malware targeting mobile devices in recent years. Some of the primary ways that mobile devices can get malware include downloading infected apps, opening infected email attachments, and visiting infected websites.

There are a variety of measures that you can take to help protect your mobile device from malware, including installing a quality antivirus app, being careful about which apps you download, and avoiding suspicious-looking websites.

Types of malware?

Unfortunately, there is a lot of malware out there, but understanding the different types of malware is one way to help protect your data and devices:

Viruses

A virus usually comes as an attachment in an email that holds a virus payload, or the part of the malware that performs the malicious action. Once the victim opens the file, the device is infected.

Ransomware

One of the most profitable, and therefore one of the most popular, types of malware amongst cybercriminals is ransomware. This malware installs itself onto a victim’s machine, encrypts their files, and then turns around and demands a ransom [usually in Bitcoin] to return that data to the user.

Scareware

Cybercriminals scare us into thinking that our computers or smartphones have become infected to convince victims to purchase a fake application. In a typical scareware scam, you might see an alarming message while browsing the Web that says “Warning: Your computer is infected!” or “You have a virus!” Cybercriminals use these programs and unethical advertising practices to frighten users into purchasing rogue applications.

Worms

Worms have the ability to copy themselves from machine to machine, usually by exploiting some sort of security weakness in a software or operating system and don’t require user interaction to function.

Spyware

Spyware is a program installed on your computer, usually without your explicit knowledge, that captures and transmits personal information or Internet browsing habits and details to its user. Spyware enables its users to monitor all forms of communications on the targeted device. Spyware is often used by law enforcement, government agencies and information security organizations to test and monitor communications in a sensitive environment or in an investigation. But spyware is also available to consumers, allowing purchasers to spy on their spouse, children and employees.

Trojans

Trojans masquerade as harmless applications, tricking users into downloading and using them. Once up and running, they then can steal personal data, crash a device, spy on activities or even launch an attack.

Adware

Adware programs push unwanted advertisements at users and typically display blinking advertisements or pop-up windows when you perform a certain action. Adware programs are often installed in exchange for another service, such as the right to use a program without paying for it.

Fileless malware

Fileless malware is a type of malicious software that uses legitimate programs to infect a computer. Fileless malware registry attacks leave no malware files to scan and no malicious processes to detect. It does not rely on files and leaves no footprint, making it challenging to detect and remove.

How do I know I’ve been infected with malware?

The most common signs that your computer has been compromised by malware are:

• Slow computer performance
• Browser redirects, or when your web browser takes you to sites you did not intend to visit
• Infection warnings, frequently accompanied by solicitations to buy something to fix them
• Problems shutting down or starting up your computer
• Frequent pop-up ads

The more of these common symptoms you see, the higher the likelihood your computer has a malware infection. Browser redirects and large numbers of pop-up warnings claiming you have a virus are the strongest indicators that your computer has been compromised.

How can I protect myself from malware?

Even though there are a lot of types of malware out there, the good news is, there are just as many ways to protect yourself from malware. Check out these top tips:

Protect your devices

• Keep your operating system and applications updated. Cybercriminals look for vulnerabilities in old or outdated software, so make sure you install updates as soon as they become available.
• Never click on a link in a popup. Simply close the message by clicking on “X” in the upper corner and navigate away from the site that generated it.
• Limit the number of apps on your devices. Only install apps you think you need and will use regularly. And if you no longer use an app, uninstall it.
• Use a mobile security solution like McAfee® Security, available for Android and iOS. As malware and adware campaigns continue to infect mobile applications, make sure your mobile devices are prepared for any threat coming their way.
• Don’t lend out your phone or leave your devices unattended for any reason, and be sure to check their settings and apps. If your default settings have changed, or a new app has mysteriously appeared, it might be a sign that spyware has been installed.
• If you don’t already have comprehensive security protection on all your devices, then try out McAfee® Total Protection, which protects all your PCs, Macs, tablets and smartphones from online threats while safeguarding your data and identity.

Be careful online

• Avoid clicking on unknown links. Whether it comes via email, a social networking site or a text message, if a link seems unfamiliar, keep away from it.
• Be selective about which sites you visit. Do your best to only use known and trusted sites, as well as using a safe search plug-in like McAfee® WebAdvisor, to avoid any sites that may be malicious without your knowing.
• Beware of emails requesting personal information. If an email appears to come from your bank and instructs you to click a link and reset your password or access your account, don't click it. Go directly to your online banking site and log in there.
• Avoid risky websites, such as those offering free screensavers.

Pay attention to downloads and other software purchases

• Only purchase security software from a reputable company via their official website or in a retail store.
• Stick to official app stores. While spyware can be found on official app stores, they thrive on obscure third-party stores promoting unofficial apps. By downloading apps for jailbroken or rooted devices, you bypass built-in security and essentially place your device’s data into the hands of a stranger.
• When looking for your next favorite app, make sure you only download something that checks out. Read app reviews, utilize only official app stores, and if something comes off as remotely fishy, steer clear.
• Do not open an email attachment unless you know what it is, even if it came from a friend or someone you know.

Perform regular checks

• If you are concerned that your device may be infected, run a scan using the security software you have installed on your device.
• Check your bank accounts and credit reports regularly.
• With these tips and some reliable security software, you’ll be well on your way to protecting your data and devices from all kinds of malware.