T-Mobile, one of the four major network carriers in the country, announced that personal details of 15 million of its customers have been compromised due to a cyber attack.

According company CEO John Legere, the attack targeted Experian, a credit bureau and T-Mobile's data vendor.

As reported by the Washington Post, the data breach began in September 2013 but was not discovered by the company until last month. This means the attack affected the information details of both customers and non-subscribers of T-Mobile. In other words, even those who just applied for T-Mobile's credit services but did not go through were still victimized in the security breach.

Based on its own investigation, Experian noted that the credit card information and other banking details of the victims were not stolen by hackers. However, this does not guarantee that their other personal details such as addresses, full names and Social Security numbers are secure since these were accessed by those behind the attack.

"Records containing name, address, Social Security number, date of birth, identification number (typically a driver's license, military ID or passport number) and additional information used in T-Mobile's own credit assessment were accessed," Experian said in a statement. "No payment or banking information was obtained."

The data vendor also mentioned that the details accessed by the hackers could be used for identity theft. As a precaution, Experian is offering T-Mobile subscribers and applicants free credit monitoring and identity resolution for two years.

The company also advised the public to closely monitor their accounts with various companies and institutions to avoid being victimized by identity thieves.

T-Mobile's CEO also issued a statement regarding the incident and said that the data breach has forced him to review the company's working relationship with Experian. But, Legere noted that the safety of the customers is still the company's top priority.

"Obviously I am incredibly angry about this data breach and we will institute a thorough review of our relationship with Experian, but right now my top concern and first focus is assisting any and all consumers affected," Legere said in a statement.

"I take our customers and prospective customer privacy very seriously," he added. "This is no small issue for us."

T-Mobile is the latest major corporation that was hit by a massive cyber attack. Over the past few months and years, other companies and organizations such as Home Depot, eBay, Target, Sony and Ashley Madison have also been targeted by hackers, according to BBC.