Facebook wishes to inform 1 million users that their login credentials might be in the wrong hands.
The information was reportedly collected using fake Android and Apple apps pretending to be legitimate mobile applications.
According to David Agranovich, Facebook's director of threat disruption, the company's researchers unveiled over 400 malicious applications this year. These apps were designed to steal login information from Facebook users in order to compromise their accounts.
Some of these apps were pretending to be photo-editing apps, while others were posing as music players or free games. In general, the apps were pretending to be unrelated to Facebook.
Android and Apple's iOS are the two most popular mobile operating systems globally. They are currently running on billions of smartphones and tablets.
Facebook has reported that the malicious apps they discovered had already been taken off the iOS and Android app stores. This was done even before the company informed users about them.
Only about 10 percent of the programs were on Apple App Store.
Mr. Agranovich also assured users that they were helping people whose information had been compromised were getting help securing their accounts.
These apps took advantage of features such as "Sign in with Facebook." However, instead of linking to Facebook servers, the apps forwarded the information to the creators of these apps.
Cybercriminals use stolen information because it gives them access to additional personal information about the users. This information can be used in crimes such as identity theft and other forms of fraud.
The usernames and passwords are matched by criminals to stolen accounts with information such as home addresses or credit card details obtained through other means.
Facebook has requested its users to be more careful about installing mobile apps that demand them to log in to Facebook to work.
"Be suspicious of a photo-editing app that needs your Facebook login and password before allowing you to use it."
Apple and Google have a tough job trying to vet new apps being published in their app stores. Apple has earlier said that it hosts more than 2 million apps, while Google's Play Store has about 2.65 million.