Shares of SK Telecom (NYSE:SKM), South Korea’s largest mobile carrier, dropped by as much as 8.5% on Monday, reaching their lowest point since August of the previous year.

This sharp decline followed the company’s disclosure of a customer data leak caused by a cyberattack earlier this month.

In a public statement, SK Telecom announced that it had detected the breach on April 18.

The company described the incident as a large-scale data leak triggered by malware but did not provide further details.

The company pledged to take complete responsibility for any damage resulting from this breach.