While free smartphone apps may not cost you any money, they're paid for in other ways.
There are more than 2 million apps globally available to be downloaded.
Many of them are free and paid for through advertisements.
Australian security expert, Troy Hunt says advertiser funded apps aren't necessarily 'free'.
"Primarily that when you don't pay for the product, often you become the product! This means that monetisation is done through the audience which means showing ads. The more information ads can extract from the user, the more valuable they become and personal information is very valuable to advertisers."
Whenever a user installs an advertiser backed app, a window or advertising banner is displayed, often at the bottom of the screen. The content of that ad is chosen by an advertising network.
But your privacy may be at risk, with the right combination of phone, app and advertising network.
Troy Hunt adds, "Certainly advert-funded apps are extremely prevalent in Australia although the impact on privacy can range from very minor, like tracking of IP address and location. through to outright malicious and subvertly retrieving personal data."
Hackers can access photos and contacts, and can even make calls on your phone.
He does say however, that devices like Apple's have very solid security models around them and that security issues usually occus because users are opting into excessive sharing of personal data.
Watch below as Troy Hunt offers these two very important tips to keep in mind when using advertiser funded apps.