Internet security has been and always will be an important matter and an interesting topic. With each year passing by, the cyber-attacks appear to be gradually getting worse in terms of their targets.

While some may have just purposes, a couple seems to have vile roots, ranging from an attack that compromises the private information of hundreds of thousands of children to a just cause to take down terrorism.

Here's a list of some of the renowned hacks and data breaches that rocked the online world this 2015:

OPM

The attack on the Office of Personnel Management is arguably the biggest case of 2015 because of the stolen information's sensitivity. According to reports, Chinese hackers were able to infiltrate and stay under the radar within the office's networks for over a year.

Initially, estimates indicated that 4 million were victims of the attack. However, the figure boomed to 21.5 million after further investigation, consisting of 19.7 million applicants for security clearances who had undergone background checks and 1.8 million spouses and other partners.

What's more, the hackers got ahold of the SF-86 forms of the individuals who applied for clearances, which is not only full of sensitive information of the workers but also of each one's relatives, friends and husbands and wives.

To make things worse, OPM soon disclosed that the hackers also got their hands on the fingerprint records of about 5.6 million employees who can access secured facilities and computers using said biometrics.

Ashley Madison

Ashley Madison has 37 million users whose sensitive information such as personal and financial details were exposed to the public.

The adultery website featured a paid "full delete" service that promises to get rid of every identifiable detail of its patrons, but the fact of the matter was that it did nothing of the sort. As anyone can imagine, the active users on the website were more than desperate to keep their identities incognito, where a lot of them opted for the said service.

The hacker group who exposed this lie spread the information on the Internet, resulting in problems of catastrophic proportions, where some users took their own life.

Anthem

When Anthem was attacked, it made history as the biggest data breach regarding healthcare information, not to mention that it's considered as the largest and earliest attack of 2015.

The hackers gained access to over 78.8 million patient files and 8.8 million to 18.8 million employee records, according to the estimation of the health insurance company. The latter is believed to include names along with dates of birth, addresses and social security numbers.

ISIS

Anonymous has been going against ISIS for some time now, waging war after the Paris attacks back in November.

The hacktivist group is vigilant for any online activity of the terrorists, particularly on Twitter, which is the preferred social media platform of the radicals to recruit new members, spread propaganda and communicate with one another.

Thousands of ISIS-affiliated Twitter accounts were reportedly taken down this year.

VTech

Cyber-attacks that target children are pretty rare, but it still happens. That's why a certain hacker carried out an attack to VTech, gaining access to 6.4 million children and 4.8 million parents' private information via Learning Lodge and Kid Connect servers and making the company rethink the current security measures it applies.

On a global scale, names, passwords, download history and IP addresses were exposed, including the children's dates of birth and genders.

This event reiterated just how careful everyone should be in giving their children's information online, regardless of the integrity of the recipient.

IRS

The IRS has been targeted before, and it has been attacked yet again, where initial reports indicate that 100,000 taxpayers had their records leaked. Much like what happened with OPM, the figure increased to 330,000 after a deeper investigation.

The hackers were able to get in using stolen credentials through the Get Transcript feature on the agency's website.

Juniper Networks

Juniper Networks discovered that there were two backdoors in specific ScreenOS versions, which is the OS used on the firm's NetScreen VPN/firewalls. Government agencies and various companies use this OS, making plenty of entities worried over the matter.

The first backdoor is made up of a hardcoded master password, essentially letting hackers gain complete control over vulnerable online NetScreen devices.

Meanwhile, the second backdoor seems to undercut Dual_EC, which the company uses to encrypt the traffic in the NetScreen VPN. Interestingly, this backdoor appears to be based on the one that NSA purportedly installed for its purposes.

"The backdoor in Dual EC is in two parts. Think of part 1 as adding a second keyhole that overrides the normal lock on a door [and] part 2 as a specific lock cylinder that goes into that keyhole," cryptographer Matthew Green says.

This case highlights how setting up backdoors could lead to severe vulnerability issues.

Bonus: Sony Pictures

Technically speaking, the hack occurred back in 2014, but the huge impact of the attack came to light in 2015.

The purportedly North Korean hacking group that goes by the name The Guardians of Peace carried out an attack against Sony, stealing massive information, data and emails and wiping data from the servers using malware. According to reports, approximately 10 terabytes of data were compromised.

In the slew of stolen files, an email conversation between Sony's Amy Pascal and producer Scott Rudin stirred up Hollywood news, where Angelina Jolie was called a "spoilt brat."

Pascal resigned after the entire hacking scandal.