(KPL/Yonhap) A joint government-private investigation team looking into SK Telecom Co.'s recent large-scale data breach has extended its probe to the servers of two other major mobile carriers, KT Corp. and LG Uplus Corp., but found no signs they have been compromised, industry sources said Monday.

Initially, the team had asked local telecommunications and platform companies to conduct their own cybersecurity inspections. However, the approach was revised last week amid growing concerns that hackers using BPFDoor malware variants may have also targeted other South Korean mobile carriers, according to the sources.

Following the expanded investigation, no traces of hacking activity have yet been found on the servers of KT or LG Uplus, they added.

In a media briefing last week, the investigation team revealed interim findings indicating that 25 malware variants had been discovered on 23 servers belonging to SK Telecom. These included 24 variants of the BPFDoor malware and one variant of WebCell.

Two of the affected servers had been used as temporary storage for personal data, such as names, birthdates, phone numbers and email addresses, as well as international mobile equipment identity (IMEI) data.

The IMEI is a unique identifier for each device on a network and could potentially be exploited in financial transactions.

SK Telecom discovered the breach April 18.

KPL