Macy’s confirmed on Tuesday that its website suffered a security breach for a week in October exposing the personal information of customers including their payment details.

Macy’s stock lost 10.9% in Tuesday trading, its worst one-day performance in three months. The shares are down just about 0.5% in Wednesday trading.

“We are aware of a highly sophisticated and targeted data security incident related to that affected a small number of customers during a one-week period in October,” the company stated in an email.

“Affected customers have been notified and will receive additional information, including instructions on how to enroll in consumer protection services at no cost.”

The data breach occurred on and is known as a Magecart attack. Hackers managed to insert malicious code into Checkout and My Wallet pages of the department store’s website, which then proceeded to collect the personal information of customers who used the site.