The attack, known as an “ATM cash-out” or “unlimited operation,” is expected in the coming days, and will likely take place on a weekend or after banks have closed, according to information obtained by the FBI. Hackers will steal data by gaining access to bank systems or payment card processors. They’ll then alter bank data relating to fraud controls, maximum withdrawal limits, and even the amount of money in each account. Then, using the stolen information, they could withdraw large sums of cash from ATMs worldwide in the space of only a few hours.

The warning was issued only to banks, not the general public, but was publicized by Brian Krebs on his cybersecurity blog, Krebs on Security.

This kind of attack usually targets data acquired from small- and medium-sized banks because they tend to have less security than large institutions. That was the case in a series of attacks on Virginia’s National Bank of Blacksburg over an eight month period between 2016 and 2017. Hackers stole $2.4 million from ATMs around the country in two separate attacks on the same bank. Despite bulked-up security, the hackers managed to access the bank’s network twice through phishing emails.

The expected attack follows a spate of “jackpotting” attacks in the U.S. earlier this year, in which hackers programmed machines to spit out cash at specified times.