With millions of apps available on smartphones, it's no surprise that not all of them are beneficial. Indeed, many mobile apps exist solely to attack your device or steal your personal information.

We've taken a look at specific apps you should avoid, but there are entire categories of untrustworthy apps as well. Let's discuss some broad app categories you should avoid or be cautious about.

These especially apply to Android users since Google Play has less oversight than the App Store, but the topic is still relevant for iPhone users.

1. Flashlight Apps

There's no reason to use these apps when your phone already has a light.

People have used their smartphones as emergency flashlights for a long time. For a while, you needed an app to turn on the camera flash or screen at full power. But that was years ago, and now Android and iOS both have flashlights built into the OS. Despite this, people continue to use ad-filled flashlight apps that don't offer anything useful over the built-in solutions.

Search for "flashlight" on the Google Play Store and you'll see dozens of flashlight apps with millions of downloads. The most popular ones all feature tons of ads and require invasive permissions like your locations and contacts list. Of course, the developers then use these to sell your data to advertisers so they can make more money.

Ignore flashlight apps and just use the built-in functionality on your phone. On Android, you'll find this by dragging down twice from the top of the screen to open the Quick Access panel. iPhone users can swipe up from the bottom of the screen to access Control Center and start the flashlight there.

2. Keyboard Apps

Consider carefully which keyboard app you use.

Replacing your mobile keyboard has long been a draw for using Android, and Apple even allowed this starting with iOS 8. While a third-party keyboard can offer better predictions and functionality not seen in the stock offering, they also represent a big privacy concern.

Remember that your keyboard app can see everything you type, including passwords, personal messages to loved ones, and your financial information. But it's not just this---keyboards always want to improve. To do this, they upload data about your specific typing style to their companies' servers.

And when keyboard developers suffer a data breach, anything you've typed could be up for grabs. Users of Android keyboard ai.type had their personal data exposed when the company failed to protect the database server with a password, as ZDNet reported. And SwiftKey, which is owned by Microsoft, once suggested personal email addresses and other predictions for the wrong users as reported by a Reddit user.

Thankfully, iOS doesn't let third-party keyboards access the internet unless you enable the Full Access option. But if you do this (or if you use Android), you should be extremely careful of which keyboard app you use. If a giant like Microsoft can have a privacy problem with its keyboard, there's no telling what a no-name developer could do with your information.

3. Free Games

Be aware that free games often have hidden costs.

The rise of mobile gaming has given way to thousands of "freemium" games that don't cost anything to start, but they earn a payday in other ways. Many apps cram in loads of in-app purchases that require you to keep paying to play, and nearly all free games include ads. Unsurprisingly, most of them are also full of invasive permissions.

Popular free games often ask for access to your contacts list, location, camera, and more sensitive permissions when you install them. While there are "legitimate" reasons for these, such as sending invites to your friends, a lot of games use them for something more.

The New York Times reported in late 2017 that hundreds of games on Google Play and the App Store included software known as Alphonso. This is a tool used by advertisers that uses your phone's microphone to pick up the sounds of what TV shows you're watching. In fact, the software can match this with the locations you visit to track information like the ad that finally prompted you to go buy a new car.

Is the privilege of playing the latest freemium time-waster worth sharing this detailed information with advertisers?

4. Antivirus Apps

You'd be surprised what security-focused apps can do with what they know about you.

We've discussed before whether you need antivirus apps for your smartphone. iPhone antivirus apps are basically useless because of Apple's built-in protections and their inability to hook into the OS. On Android, you really don't need an antivirus app unless the device is rooted, or you regularly download apps from outside Google Play.

But it's another question entirely what mobile antivirus apps do with your data. It's no secret that despite their purpose as security apps, antivirus programs collect a lot of data about your computer and browsing. Why would they do any differently on your phone?

If you have an antivirus app on your device, even if it isn't doing anything for your security, it's still likely collecting your information. Why not uninstall them to save system resources, take back storage space, and stop letting antivirus companies collect your data?

5. Store Loyalty Apps

You're paying for discounts and freebies by providing your purchasing habits, and storing payment details isn't wise.

It seems that every restaurant, department store, and other business provides an app they want you to download today. While these can reward you with personalized offers and convenient ways to pay, they also carry new security risks. We've talked before about how loyalty cards compromise your privacy, but there are further considerations.

Many chain restaurant apps let you add your credit card so you can easily reload your balance once it drops. This becomes a problem when, in the case of Starbucks, user information on the app is vulnerable, as CNN reported in 2014. A year later, hackers broke into people's Starbucks apps and used their linked cards to steal money, again covered by CNN.

The more places you add your payment info to, the greater surface for attack. And companies are happy to learn even more about you when you have their apps installed.

Many Apps Betray Your Privacy Every Day

We've covered five big categories of apps that have no problem invading your privacy, but there are many more. Apps offering basic customization like wallpapers are often full of invasive permissions and ads. Even weather apps can log your IP address and other unnecessary information. And any app could transmit or store password insecurely.

And that's not even considering health and fitness apps like Fitbit. These track how well you sleep at night, how much exercise you get, your location, and more. That's a lot of information that you can't necessarily trust companies to store safely.

There's no way to avoid dangerous apps with 100 percent certainty. But you should take care with the above categories for sure. And for other apps, remember to always review the permissions and privacy policies before using them. It's clear that you can't assume any company is looking out for your best interest, and most "free" apps make you pay in other ways.

For more on this topic, check out how location tracking can violate your privacy.