Pierluigi Paganini
May 21, 2025
SK Telecom is South Korea’s largest wireless telecom company, a major player in the country’s mobile and tech landscape.
It holds about 48% of the market share for mobile services, meaning around 34 million subscribers use its network. The company offers cellular service, along with 5G development, AI services, IoT solutions, cloud computing, and smart city infrastructure.
The carrier is part of the larger SK Group, which is one of South Korea’s biggest conglomerates (also active in energy, semiconductors, chemicals, and more).
In April, SK Telecom reported that threat actors gained access to USIM-related information for customers following a malware attack. The Universal Subscriber Identity Module (USIM) is a smart card used in mobile devices, it securely stores subscriber information, including the International Mobile Subscriber Identity (IMSI) and cryptographic keys.
The telecom giant detected an infection of its systems at 11 PM on Saturday, April 19, 2025. Upon discovering the infection, the company promptly reported it to the Korea Internet & Security Agency (KISA) on Sunday, April 20, sanitized the impacted systems, and isolated the suspected hacking device. No cases of misuse of the information have been confirmed to date.
SK Telecom announced it had enhanced defensive measures and blocked illegal SIM card changes and abnormal authentication attempts. The company is also offering impacted customers a subscription to the ‘SIM protection service’ for free.
SK Telecom revealed that the security breach began in 2022, exposing USIM data of 27 million users. To counter SIM-swapping risks, it is replacing all SIM cards and enhancing security to block unauthorized number porting.
On May 8, 2025, the Personal Information Protection Committee found that malware compromised 25 types of data in the SK Telecom breach. SKT was instructed to notify all 25.64 million users, including budget phone users, by May 9. Leaked data includes phone numbers, IMSIs, SIM keys, and HSS-stored info.
“The personal information that has been confirmed to have been leaked so far is a total of 25 types, including users’ mobile phone numbers, IMSI (subscriber identification number), SIM authentication keys, and other SIM-related information that were stored in HSS*.” states the Personal Information Protection Committee. “This measure was taken based on the committee’s judgment that the leaked information is important personal information that is used as a key means of identifying and connecting individuals in the mobile era, and therefore requires special attention.”
SK Telecom announced it will notify 26.95 million users affected by a malware breach that exposed sensitive data. The firm found 25 malware types across 23 servers, revealing a far more extensive attack than expected.
“We have isolated 25 types of malware discovered and 23 infected servers, and there has been no confirmation of leakage of 290,000 IMEIs, terminal cloning is virtually impossible, and the network is also blocked through FDS.” reads an update published by the company.
A joint probe found the initial infection occurred on June 15, 2022, staying undetected for nearly three years.
“The second investigation results additionally revealed that SKT did not notice that it had been infected with malicious code for nearly 3 years. The investigation team announced that after inspecting all 30,000 of SKT’s Linux servers four times, a total of 23 servers were infected with malicious code.” reported the website KBS. “The first malware infiltration occurred on June 15, 2022, and was confirmed to be a ‘web shell’ series code that took over the server. The investigation team said that the web shell code subsequently played a role in calling in difficult-to-detect backdoor (BPF door) series malware, and a total of 25 types of malware were found in SKT’s internal system.”
SK Telecom ensures illegal USIM and device changes are fully blocked and pledges full responsibility if any damage still occurs.
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(SecurityAffairs – hacking, data breach)
