SK Telecom CEO Ryu Young-sang has admitted that a recent cyberattack targeting user SIM card data was the worst hacking incident in South Korea’s telecom history. 

Speaking at a parliamentary hearing on Wednesday, Ryu agreed with lawmakers that the breach compromised systems previously considered impenetrable. 

He confirmed that three Home Subscriber Servers were affected but said further investigation is needed to determine if other servers were also compromised.
 
Lawmakers raised concerns that the hack, which occurred within a closed network, may have exploited VPN vulnerabilities and remained undetected for up to a year. 

Ryu said SK Telecom plans to secure 11 million SIM cards by the end of June as part of a pledge to replace the universal SIM cards, or USIM cards, for all 23 million users free of charge.