What is ransomware? How it works and how to remove it

What is ransomware? How it works and how to remove it

Ransomware is a form of malicious software (or malware) that, once it's taken over your computer, threatens you with harm, usually by denying you access to your data. The attacker demands a ransom from the victim, promising — not always truthfully — to restore access to the data upon payment. 

Users are shown instructions for how to pay a fee to get the decryption key. The costs can range from a few hundred dollars to thousands, payable to cybercriminals in Bitcoin.

How ransomware works

There are a number of vectors ransomware can take to access a computer. One of the most common delivery systems is phishing spam — attachments that come to the victim in an email, masquerading as a file they should trust. Once they're downloaded and opened, they can take over the victim's computer, especially if they have built-in social engineering tools that trick users into allowing administrative access. Some other, more aggressive forms of ransomware, like NotPetya, exploit security holes to infect computers without needing to trick users.

There are several things the malware might do once it’s taken over the victim's computer, but by far the most common action is to encrypt some or all of the user's files. If you want the technical details, the Infosec Institute has a great in-depth look at how several flavors of ransomware encrypt files. But the most important thing to know is that at the end of the process, the files cannot be decrypted without a mathematical key known only by the attacker. The user is presented with a message explaining that their files are now are now inaccessible and will only be decrypted if the victim sends an untraceable Bitcoin payment to the attacker.

In some forms of malware, the attacker might claim to be a law enforcement agency shutting down the victim's computer due to the presence of pornography or pirated software on it, and demanding the payment of a "fine," perhaps to make victims less likely to report the attack to authorities. But most attacks don't bother with this pretense. There is also a variation, called leakware or doxware, in which the attacker threatens to publicize sensitive data on the victim's hard drive unless a ransom is paid. But because finding and extracting such information is a very tricky proposition for attackers, encryption ransomware is by far the most common type.

Who is a target for ransomware?

There are several different ways attackers choose the organizations they target with ransomware. Sometimes it's a matter of opportunity: for instance, attackers might target universities because they tend to have smaller security teams and a disparate user base that does a lot of file sharing, making it easier to penetrate their defenses.

On the other hand, some organizations are tempting targets because they seem more likely to pay a ransom quickly. For instance, government agencies or medical facilities often need immediate access to their files. Law firms and other organizations with sensitive data may be willing to pay to keep news of a compromise quiet — and these organizations may be uniquely sensitive to leakware attacks.

But don't feel like you're safe if you don't fit these categories: as we noted, some ransomware spreads automatically and indiscriminately across the internet.

How to prevent ransomware

There are a number of defensive steps you can take to prevent ransomware infection. These steps are a of course good security practices in general, so following them improves your defenses from all sorts of attacks:

  • Keep your operating system patched and up-to-date, to ensure you have fewer vulnerabilities to exploit.
  • Don't install software or give it administrative privileges unless you know exactly what it is and what it does.
  • Install antivirus software, which detects malicious programs like ransomware as they arrive, and whitelisting software, which prevents unauthorized applications from executing in the first place.
  • And, of course, back up your files, frequently and automatically! That won't stop a malware attack, but it can make the damage caused by one much less significant.

Ransomware removal

If your computer has been infected with ransomware, you'll need to regain control of your machine. CSO's Steve Ragan has a great video demonstrating how to do this on a Windows 10 machine:

The video has all the details, but the important steps are to:

  • Reboot Windows 10 to safe mode
  • Install antimalware software
  • Scan the system to find the ransomware program
  • Restore the computer to a previous state

But here's the important thing to keep in mind: while walking through these steps can remove the malware from your computer and restore it to your control, it won't decrypt your files. Their transformation into unreadability has already happened, and if the malware is at all sophisticated, it will be mathematically impossible for anyone to decrypt them without access to the key that the attacker holds. In fact, by removing the malware, you've precluded the possibility of restoring your files by paying the attackers the ransom they've asked for.

That brings up an obvious question: if your system has been infected with malware, and you've lost vital data that you can't restore from backup, should you pay the ransom? 

Should you pay the ransom?

When speaking theoretically, most law enforcement agencies urge you not to pay ransomware attackers, on the logic that doing so only encourages hackers to create more ransomware. That said, many organizations that find themselves afflicted by malware quickly stop thinking in terms of the "greater good" and start doing a cost-benefit analysis, weighing the price of the ransom against the value of the encrypted data. According to research from Trend Micro, while 66 percent of companies say they would never pay a ransom as a point of principle, in practice 65 percent actually do pay the ransom when they get hit.

Ransomware attackers keep prices relatively low — usually between $700 and $1,300, an amount companies can usually afford to pay on short notice. Some particularly sophisticated malware will detect the country where the infected computer is running and adjust the ransom to match that nation's economy, demanding more from companies in rich countries and less from those in poor regions.

There are often discounts offered for acting fast, so as to encourage victims to pay quickly before thinking too much about it. In general, the price point is set so that it's high enough to be worth the criminal's while, but low enough that it's often cheaper than what the victim would have to pay to restore their computer or reconstruct the lost data. With that in mind, some companies are beginning to build the potential need to pay ransom into their security plans: for instance, some large UK companies who are otherwise uninvolved with cryptocurrency are holding some Bitcoin in reserve specifically for ransom payments.

There are a couple of tricky things to remember here, keeping in mind that the people you're dealing with are, of course, criminals. First, what looks like ransomware may not have actually encrypted your data at all; make sure you aren't dealing with so-called "scareware" before you send any money to anybody. And second, paying the attackers doesn't guarantee that you'll get your files back. Sometimes the criminals just take the money and run, and may not have even built decryption functionality into the malware. But any such malware will quickly get a reputation and won't generate revenue, so in most cases — Gary Sockrider, principal security technologist at Arbor Networks, estimates around 65 to 70 percent of the time — the crooks come through and your data is restored.

Ransomware examples

While ransomware has technically been around since the '90s, it's only in the past five years or so that it's really taken off, largely because of the availability of untraceable payment methods like Bitcoin. Some of the worst offenders have been:

  • CryptoLocker, a 2013 attack that launched the modern ransomware age and infected up to 500,000 machines at its height
  • TeslaCrypt, which targeted gaming files and saw constant improvement during its reign of terror
  • SimpleLocker, the first widespread ransomware attack that focused on mobile devices
  • WannaCry, which spread autonomously from computer to computer using EternalBlue, an exploit developed by the NSA and then stolen by hackers
  • NotPetya, which also used EternalBlue and may have been part of a Russian-directed cyberattack against Ukraine
  • Locky, which started spreading in 2016, was "similar in its mode of attack to the notorious banking software Dridex."

And this list is just going to get longer. Even as this article was being put together, a new wave of ransomware, dubbed BadRabbit, spread across media companies in Eastern Europe and Asia. It's important to follow the tips listed here to protect yourself.

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