(CNN) Asen Genov is pretty furious. His personal data was made public this week after records of more than 5 million Bulgarians got stolen by hackers from the country's tax revenue office.

In a country of just 7 million people, the scale of the hack means that just about every working adult has been affected.

"We should all be angry. ... The information is now freely available to anyone. Many, many people in Bulgaria already have this file, and I believe that it's not only in Bulgaria," said Genov, a blogger and political analyst. He knows his data was compromised because, though he's not an IT expert, he managed to find the stolen files online.

The attack is extraordinary, but it is not unique.

Government databases are gold mines for hackers. They contain a huge wealth of information that can be "useful" for years to come, experts say.

"You can make (your password) longer and more sophisticated, but the information the government holds are things that are not going to change," said Guy Bunker, an information security expert and the chief technology officer at Clearswift, a cybersecurity company.

"Your date of birth is not going to change, you're not going to move house tomorrow," he said. "A lot of the information that was taken was valid yesterday, is valid today, and will probably be valid for a large number of people in five, 10, 20 years' time."

Hackers' paradise

Data breaches used to be spearheaded by highly skilled hackers. But it increasingly doesn't take a sophisticated and carefully planned operation to break into IT systems. Hacking tools and malware that are available on the dark web make it possible for amateur hackers to cause enormous damage.

A strict data protection law that came into effect last year across the European Union has placed new burdens on anyone who collects and stores personal data. It also introduced hefty fines for anyone who mismanages data, potentially opening the door for the Bulgarian government to fine itself for the breach.

Still, attacks against government systems are on the rise, said Adam Levin, the founder of CyberScout, another cybersecurity firm. "It's a war right now -- one we will win if we make cybersecurity a front-burner issue," he said.

The notion that governments urgently need to step up their cybersecurity game is not new. Experts have been ringing alarm bells for years.

The US Department of Veterans Affairs suffered one of the first major data breaches in 2006, when personal data of more than 26 million veterans and military personnel were compromised.

"And it was all, 'Oh, this is dreadful. We must do things to stop it.' ... And here we are, 13 years later, and an entire country's data has been compromised, and in between, there's been incidents of large swathes of citizen data being compromised in different countries," Bunker said.

Out-of-date systems are often the problem. Some governments may have used private companies to manage the data they collected before the array of hacks and breeches brought their attention to cybersecurity.

"In many cases, our data was sent to third-party contractors years ago," Levin said. "The way we looked at data management 10 years ago seems antiquated today, yet that old data is still out there being managed by third parties, using legacy systems."

If the "old data" hasn't changed, it's still valuable to hackers.

The Bulgaria incident is concerning, said Desislava Krusteva, a Bulgarian privacy and data protection lawyer who advises some of the world's biggest tech companies on how to keep their clients' information safe.

"These kinds of incidents should not happen in a state institution. It seems like it didn't require huge efforts, and it's probably the personal data of almost all Bulgarian citizens," said Krusteva, a partner at Dimitrov, Petrov & Co., a law firm in Sofia.

The Bulgarian Commission for Personal Data Protection has said it would launch an investigation into the hack.

A National Revenue Agency spokesman would not comment on whether the data was properly protected.

"As there is undergoing investigation, we couldn't provide more details about reasons behind the hack," Communications Director Rossen Bachvarov said.

'Very embarrassing for the government'

A 20-year-old cybersecurity worker has been arrested by the Bulgarian police in connection with the hack. The computer and software used in the attack led police to the suspect, according to the Sofia prosecutor's office.

The man has been detained, and the police seized his equipment, including mobile phones, computers and drives, the prosecutor's office said in a statement. If convicted, he could spend as long as eight years in prison.

"It's still too early to say what exactly happened, but from political perspective, it is, of course, very embarrassing for the government," Krusteva said.

The embarrassment is made worse by the fact that this was not the first time the Bulgarian government was targeted. The country's Commercial Registry was brought down less than a year ago by an attack.

"So, at least for a year, the Bulgarian society, politicians, those who are in charge of the country, they knew quite well about the serious cybersecurity problems in the government infrastructures," Genov said, "and they didn't do anything about it."