SK Telecom, South Korea’s largest telecom company, disclosed a data leak involving a malware infection.

SK Telecom is South Korea’s largest wireless carrier — it has tens of millions of subscribers and holds roughly half of the local market.

The company revealed on Tuesday in a Korean-language statement posted on its website that it detected an intrusion on April 19. An investigation showed that the attackers deployed malware and managed to obtain personal information belonging to customers.

Following the incident, SK Telecom is offering customers a free SIM protection service designed to prevent SIM swapping, which suggests that the leaked data could be leveraged for such activities.