OAKLAND — A ransomware attack against Oakland that has affected city services and exposed reams of sensitive personnel data is creating a nightmare for city officials who aren’t sure what it will take to resolve the crisis.

While there is much still unknown about the full extent of the attack that has unfolded over the last month, experts in cybercrime say the resolution is not likely to be a happy one for those affected.

“This is a really devastating cyberattack for sure,” said Sarah Powazek, the director of a cybersecurity academic program at UC Berkeley. “It’s a big deal, and it’s really unfortunate how poorly prepared folks are for dealing with this. And I’m not blaming the city at all — it’s sad that cities are supposed to be prepared and know what to do with what is an international cyber attack.”

The attack was carried out by hackers associated with the ransomware group Play, also known as PlayCrypt, that has targeted municipalities around the globe, including the small city Cordoba in Argentina, as well as hotels in Brazil and other private businesses.

The city has released few details about the attack, and has not revealed how its data was compromised or the dollar amount sought by the hackers.

As the frequency of ransomware attacks has increased, public entities like Oakland have found themselves more vulnerable, and with fewer resources to defend themselves. Other victims of such attacks in the Bay Area include Bay Area Rapid Transit and Contra Costa County.

Over the past weekend, the Play hackers released about 11 gigabytes of data from the Oakland attack to the dark web, including home addresses and social security numbers of numerous city employees — including the current mayor, and her predecessor — as well as police files and other city data, according to multiple city sources who reviewed the data.

The city has offered one year of free credit protection to employees whose data may have been compromised.

Ransomware attacks in recent years have become more frequent, reaching what some experts call epidemic levels since 2019. Earlier this month, President Joe Biden declared ransomware attacks a national security threat, and a report on…