Cyber security is a major concern in today's world, as almost every person is connected to the Internet in some way or another. The year 2015 proved to be one of the worst years as a series of hacking attempts were deployed successfully. Here's a look at the five biggest and scariest hacks of 2015 that shocked us.

FREAK attack

Earlier this year, a major cyber attack on Windows PCs left millions at risk. It was so intense that Microsoft issued an advisory note regarding the dangerous FREAK vulnerability, which stands for Factoring Attack on RSA-EXPORT Keys. FREAK also threatened mobile devices and Mac computers.

[Read: Windows PCs Vulnerable to FREAK Attacks; Important Tips on How to Protect Your Computer]

Yahoo hack

A group of hackers called D33Ds Co. broke into Yahoo's database and stole login credentials of more than 400,000 users. Yahoo said the list of hacked accounts had less than 5% of valid passwords. The hacker group had posted a total of 453,492 email addresses and passwords of Yahoo users on a public website.

[Read: Yahoo Hacked: Over 400,000 Passwords Leaked; Orders Probe Into Security Breach]

Stagefright bug

Widely referred to as the "Mother of all Android vulnerabilities", the Stagefright bug put millions of Android smartphone users at great risk. Almost 95% of Android phones were at risk of Stagefright's deadly attack, affecting nearly 950 million devices in total. Affected devices were at high risk of granting uninterrupted access to hackers. Over the course of time, Google rolled out a fix for the malicious bug and other smartphone manufacturers followed suit.

[Read: 'Mother Of All Android Vulnerability' Stagefright Bug Slips Google's Security Patch]

Ashley Madison hack

A group of hackers known as the Impact Team released personal details of nearly 37 million users of Ashley Madison, an infidelity site. The leaked data, which had 10GB worth of content, also linked email IDs to the United Nations and even the Vatican. It was one of the most high-profile hacks of 2015.

[Read: Who Cheated on Ashley Madison? Leaked Email IDs Linked to US, UK Govts, Nasa and Vatican]

225,000 Jailbroken iPhone accounts hacked

A malicious malware infected thousands of iPhones and stole iTunes login information to make illicit purchases and even demand ransom from device owners in September. The hack, which was discovered by the researchers at Palo Alto Networks and WeipTech, largely affected jailbroken iPhones.

[Read: 225,000 Plus Jailbroken iPhone Accounts Hacked: How to Identify Infection and Remove It?]

These are just five of many hacking attacks that took place this year. As 2015 comes to an end, it remains unclear if the hacking attempts will continue to take place in 2016 or companies will be able to defeat hackers by improving security.