SK Telecom reveals cyberattack, customer USIM data stolen could be used in attacks

• SK Telecom confirmed suffering a malware infection on a Saturday night
• The attackers tried to steal USIM data
• Law enforcement and government has been notified

SK Telecom, South Korea’s largest telecommunications provider, suffered a malware infection recently, through which threat actors tried to grab customer USIM information.

The attack happened in mid-April 2025, at night between Saturday and Sunday, when most employees were away for the weekend.

The company’s IT team spotted the intrusion and moved quickly to isolate the affected endpoints and prevent more data from being stolen. In the morning, SK Telecom notified relevant authorities and started an investigation to determine exactly what happened and what was stolen.

GitHub not affected

So far, no data appeared on the dark web and no one took responsibility for the attack.

A USIM (Universal Subscriber Identity Module) is the successor of the old SIM card. It’s used in 3G, 4G, and 5G mobile networks and is found in most devices with cellular connectivity.

At its core, a USIM authenticates the device’s identity on a network, stores subscriber data (International Mobile Subscriber Identity, authentication keys, contacts, and sometimes text messages), encrypts the information passing through, and enables secure access to mobile services such as calls, data, or texts.

A cybercriminal could use the data to close the SIM, track people’s location, bypass app security, or eavesdrop on communications.

In other words, this was a rather dangerous attack. That is why the company strengthened blocks of USIM swaps and abnormal authentication attempts. Furthermore, it said it would immediately suspend service for accounts linked to suspicious activity.

SK Telecom is the biggest telecommunications provider in South Korea, servicing some 34 million subscribers (roughly 49% of the market). The rest is mostly divided between KT and LG U+.

It offers mobile services, internet and IPTV, AI and cloud services, IoT solutions, smart factory and connected car technology, quantum cryptography communication, and more. It was one of the world’s first companies to launch commercial 5G, back in 2019.

Via BleepingComputer

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