A popular file-clean up program called CCleaner has become the latest known target of hackers after security researchers discovered that it had been compromised with a “backdoor.” That backdoor would have allowed hackers to target unsuspecting users who thought that they had downloaded and were using the app from a legitimate source. Instead, CCleaner for a period of weeks was compromised and according to security researchers at Cisco Talos, hackers could have targeted users “for any number of malicious purposes.”

Fortune has compiled the following quick look at everything you need to know to stay safe:

What Is CCleaner?

CCleaner is a program designed for both Windows and the Mac. The app features a host of tools that remove old files, clean up fragmented hard drives, and remove junk. Ultimately, CCleaner aims at making computers faster and more reliable. It’s designed by a company named Piriform, which was acquired earlier this year by security app maker Avast.

How Popular Is CCleaner?

CCleaner is an exceedingly popular app with download estimates of more than 2 billion, according to Cisco Talos. The security researcher estimates that CCleaner attracts more than 5 million new downloads a week.

Get Data Sheet, Fortune’s technology newsletter

How Did the Hack Work?

According to Piriform, it’s believed that hackers “illegally modified” a version of its software released in August that allowed them to remotely execute malicious activities on infected machines, like collecting a person’s keystrokes or sending ransomware to make a quick profit off stolen data.

The malicious software would allow hackers to access information about a victim’s computer, including the type of software it was running, processes it was running, and more. That information could all be used to dupe users and ultimately steal sensitive information.

Worst of all, Piriform wasn’t immediately aware that its software had been hacked, leaving users at risk for weeks.

How Many People Were Affected?

Unfortunately, that’s a bit of a moving target right now, since Piriform hasn’t completed its investigation. However, Cisco Talos says anyone who was running CCleaner version 5.33 was susceptible to the hack. And given the billions of downloads the app has secured, and the millions that it attracts each week, it’s likely that millions of users were infected.

What Is Piriform Saying About the Hack?

In a statement on Monday, Piriform apologized to its users and said that it contacted law enforcement. The software company also said that it has shut down the hackers’ access to its users and updated its software.

“In other words, to the best of our knowledge, we were able to disarm the threat before it was able to do any harm,” said Paul Yung, Piriform’s vice president of products, in a statement.

Am I Safe Now?

It depends on who you ask. Piriform seems clear that it’s handled the threat and no users were affected by the problem. However, some security researchers say that this might only be the beginning and there’s a possibility of this getting worse before it gets better.

In a statement to Forbes, Martijn Grooten, editor of the Virus Bulletin security publication, said that Piriform is “downplaying” the hack.

What Do I Do Next?

Unfortunately, there isn’t much you can do. If you’re running CCleaner on your computer, update to the latest version, 5.34. After that, keep an eye on the latest developments to see if there’s anything else you should do to stay safe.