The bank said in a statement that the data was from customers in Spain, Chile and Uruguay, as well as all current and some former employees. No data on transactions, nor any credentials that would allow to perform transactions were stored in the database, it said.
Customer data in all other markets and businesses were not affected, the bank said, adding that customers could continue to transact securely.
Santander, the euro zone's second-biggest bank by market value, said it had "immediately implemented measures to contain the incident," including blocking the compromised access to the database.
Without elaborating further on how the database was breached, Santander said it also established additional fraud prevention controls to protect the affected customers.
The bank declined to comment how many clients had been affected. It has notified regulators and law enforcement agents and will continue to work closely with them while proactively contacting those affected directly, it said.
Last month, the International Monetary Fund said in a blog that rising cyber threats posed serious concerns for financial stability, adding that incidents in the financial sector could erode confidence in the financial system, disrupt critical services, or cause spillovers to other institutions.
In March, the European Central Bank told lenders in the euro zone to better prepare for risks related to cyberattacks after having said earlier this year that it would conduct a cyber resilience stress test on 109 directly supervised banks in 2024.
The exercise would be aimed at assessing how banks respond to and recover from a cyberattack, rather than their ability to prevent it.
