South Korea's Ministry of Science and ICT said Tuesday that SK Telecom (KRX:017670)'s cyberattack, which took place on April 18, did not compromise device identifiers such as International Mobile Equipment Identity (IMEI) numbers.

However, data potentially usable for Universal Subscriber Identity Module (USIM) cloning, including phone numbers and International Mobile Subscriber Identity (IMSI) data, was leaked.

Authorities are expanding the probe as BPFDoor malware was detected on compromised servers. BPFDoor is a stealthy backdoor that uses Linux's BPF tool, making hacker activity hard to detect, the release said.

Customers are being urged to replace their USIMs or enroll in the USIM Protection Service, which is now fully guaranteed by SK Telecom, upon reservation, according to the Science Ministry release.

Shares of SK Telecom fell nearly 2% in recent trade on Friday.