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The “20 million” figure in the headline is not a verified count of people. South Korea’s Ministry of Science and ICT (MSIT) confirmed that about 26.96 million IMSI records were leaked from SK Telecom (SKT); records are not the same as unique subscribers. The breach involved a telecom carrier’s systems, not a South Korean government database.
What happened in the SK Telecom breach?
An attacker gained access to SKT systems and, on April 18, 2025, accessed Home Subscriber Server (HSS) nodes, compressed USIM data and sent it out through a server with an internet connection. MSIT traced the intrusion back to August 6, 2021, when CrossC2 was installed on an internet-facing server in a management subnet.
Key dates
- August 6, 2021: MSIT traced the attacker’s access to the installation of CrossC2 on a management-subnet server.
- April 18, 2025, 11:20 p.m.: SKT detected unusually large outbound traffic.
- April 20, 2025, 4:46 p.m.: SKT notified the Korea Internet & Security Agency (KISA). MSIT said the notification came later than the statutory 24-hour window.
- April 23, 2025: MSIT established a public-private joint investigation team.
- April 23–June 27, 2025: The team conducted its forensic investigation of SKT’s servers.
What investigators found
MSIT’s final investigation reported 28 infected servers and 33 malware variants among the 42,605 SKT servers scanned. The 33 variants included 27 instances of BPFDoor, a stealthy malware family. Investigators confirmed that 25 categories of USIM data, totaling 9.82 GB, had been exfiltrated.
“Because the incident involved the nation’s largest mobile carrier, featured highly stealthy BPFDoor malware, and exposed subscribers to SIM-cloning risk, the Team conducted a full forensic inspection of all 42,605 servers operated by SKT between April 23 and June 27.” — MSIT, final investigation release
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How many people were affected?
Reported totals use different measures and should not be treated as interchangeable. In particular, an IMSI-record count does not establish how many distinct people were affected.
| Reported figure | What it refers to | Source and status |
|---|---|---|
| “20 million” | The number in the headline; it is not the verified unique-person total established by MSIT’s final investigation. | Headline framing. |
| About 26.96 million | Leaked IMSI records, not a verified count of unique people. | MSIT’s 2025 final investigation. |
| About 23 million | Users described as affected. | A secondary report published September 2, 2026; the underlying primary decision’s detailed findings and exact final count are not established here. |
The Personal Information Protection Commission (PIPC) decision index records a deliberation on corrective action for SKT on August 27, 2025, but the index entry does not provide the detailed findings. The exact final number of unique affected people and the full details of the later sanctions therefore remain unsettled by the available primary decision information.
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What information was exposed?
MSIT confirmed the exfiltration of USIM-related data. A USIM is the subscriber identity module used by a mobile network to identify and authenticate a subscriber. IMSI records are identifiers associated with mobile subscriptions; their total cannot be read as a count of distinct people.
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What security failures did MSIT identify?
MSIT identified several organizational and technical weaknesses that contributed to the breach:
- Poor credential management, including administrator credentials stored in plain text and reused across systems.
- An inadequate response to an anomaly detected in February 2022.
- Failure to encrypt critical data.
- Gaps in security governance, supply-chain oversight, malware detection and endpoint protection.
- Firewall-log retention shorter than SKT’s stated rules, limiting investigators’ ability to determine whether some earlier data had been taken.
The ministry recommended stronger password controls, encryption of critical data, wider endpoint detection and antivirus coverage, quarterly vulnerability scans, tighter supply-chain controls and security governance across the company.
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What did SKT and regulators tell subscribers?
SKT’s reported response
In an April 2025 filing, SKT said it had not found actual or attempted misuse at that stage. The company described free USIM-protection services and free USIM-card replacement as mitigation measures. That was SKT’s statement at the time; it does not establish that misuse never occurred or that those services remain available on the same terms today.
PIPC’s notification order
On May 2, 2025, the PIPC ordered SKT to notify people individually if their data was confirmed leaked or might have been exposed. The order also covered subscribers using SKT’s network through mobile virtual network operators (MVNOs). The commission called for protections for older and disabled users and more practical support, citing USIM supply problems, service delays and difficulties accessing support.
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Was your SK Telecom data leaked, and should you replace your USIM?
The confirmed IMSI-record total does not identify which individual subscribers’ records were involved. The PIPC’s notice order was intended to reach people whose data was confirmed leaked or might have been exposed, including eligible MVNO users. Check current communications from SKT or your MVNO to see whether you received an individual notice and what measures are available to you.
The materials describing free protection and replacement measures date from SKT’s April 2025 filing. They do not establish present-day eligibility or availability, so do not assume you need to buy a generic replacement SIM or a third-party security product. Follow current official guidance from your carrier about whether a USIM replacement or protection measure is appropriate.
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