--- title: "Know Your Breach: SK Telecom" description: "The Take: USIM data is information stored on a Universal Subscriber Identity Module (USIM), which typically includes International Mobile Subscriber Identity (IMSI), Mobile Station ISDN Number (MSISDN), authentication keys, network usage data, and SMS or contacts if stored on the SIM. This data could be used for targeted surveillance, tracking, and SIM-swap attacks." --- [![CH Diligence (thicker line)](https://www.castlehalldiligence.com/hs-fs/hubfs/CH%20Diligence%20(thicker%20line).jpg?width=200&height=78&name=CH%20Diligence%20(thicker%20line).jpg "CH Diligence (thicker line)")](http://castlehalldiligence.com) [![Castle Hall Diligence](https://www.castlehalldiligence.com/hs-fs/hubfs/bobble.png?width=60&height=61&name=bobble.png "Castle Hall Diligence")](https://www.castlehalldiligence.com) [![Sign In](https://no-cache.hubspot.com/cta/default/4256284/c25d63cf-000a-4c8c-97b3-422965550826.png)](https://cta-redirect.hubspot.com/cta/redirect/4256284/c25d63cf-000a-4c8c-97b3-422965550826) ** ** ** Search this site on Google Search Google **Menu** ** ** [![Sign In](https://no-cache.hubspot.com/cta/default/4256284/c25d63cf-000a-4c8c-97b3-422965550826.png)](https://cta-redirect.hubspot.com/cta/redirect/4256284/c25d63cf-000a-4c8c-97b3-422965550826) ![shutterstock_490960141-1](https://www.castlehalldiligence.com/hs-fs/hubfs/shutterstock_490960141-1.jpg?width=7000&name=shutterstock_490960141-1.jpg "shutterstock_490960141-1") # Industry News: ESG5 [Content Title](https://www.castlehalldiligence.com/cyber-news/know-your-breach-sk-telecom#) Description < Previous page Next page > ## Know Your Breach: SK Telecom Apr 25, 2025 7:38:38 AM **The Target:** ​ ​ SK Telecom is the largest mobile network operator in South Korea, holding approximately 48.4% of the mobile phone service market in the country, corresponding to 34 million subscribers. **The Take:** USIM data is information stored on a Universal Subscriber Identity Module (USIM), which typically includes International Mobile Subscriber Identity (IMSI), Mobile Station ISDN Number (MSISDN), authentication keys, network usage data, and SMS or contacts if stored on the SIM. This data could be used for targeted surveillance, tracking, and SIM-swap attacks. **The Vector:** A malware infection allowed threat actors to access sensitive USIM-related information for customers. This breach highlights the extreme importance of timely software updates for known software vulnerabilities, not only in systems directly under a firm’s control, but in third-party systems the firm relies upon as well. The longer a firm, or its vendors, hold out on deploying the most up-to-date software for their systems, the greater the chance an attacker will exploit the issue. 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