More than 5 billion Android apps that have already been downloaded are vulnerable to being hacked, according to cybersecurity researchers, CNBC reports.

Attackers have been trying to find and exploit flaws in Google's operating system. About 96 percent of malware used by hackers target Android, according to U.S. firm FireEye. FireEye analyzed more than 7 billion Android and iOS apps between January and October 2014.

Apps that are designed to steal financial data are very popular, researchers found. Since Android is an open-sourced operating system, hackers are able to find the code behind a popular app, like a national bank's app, and then recreate the app. They then add malicious code to the app and infect users.

App users have no clue their app's code has been changed since it still looks and acts like it did before.

"You can get all the code and then you can insert additional instructions and make it look and feel like the original app and no way for a consumer to tell the difference when they download it," said Jason Steer, director of technology strategy at FireEye. 

Malware specifically targeted at Android has surged in the last couple of years. In 2013, 240,000 unique samples were found. In the first three quarters of 2014, more than 390,000 unique samples were found according to research.

FireEye says Android is vulnerable due to the way its apps communicate information back to servers. The company found most apps send unencrypted information. This leaves information open to hackers to intercept and install malicious code that can affect app users.

Even third party ads are causing Android apps to become vulnerable to hackers. These ads use data collection about the app user, and sometimes the collection process is done in an unsecured way. That means hackers could get their hands on the data.

Google did not respond to a request for comment on the issue.

Recently, IBM found 60 percent of dating apps it tested had medium or high security vulnerabilities. 

What do you think about Android apps? Are they secure? Leave us a comment below and let us know your thoughts.