AT&T informed 1,600 users that an employee gained unauthorized access to their personal data in August.
The company said the person may have obtained Social Security numbers, driver's license numbers and AT&T services customers subscribed to, according to a letter posted on the website of the Attorney General of Vermont on Oct. 1.
Authorities have been notified regarding the incident, and the employees have been fired, the carrier said. The company declined to give the employee's specific role at the company.
"We take our customers' privacy very seriously and value the trust they have in us," Mark Siegel, an AT&T spokesman, said in a statement. "Unfortunately, we recently learned that one of our employees did not follow our strict privacy rules and inappropriately obtained some customer information. This individual no longer works at AT&T and we are directly contacting the limited number of affected customers."
Unauthorized charges made to customer accounts will be reversed, and affected customers would be eligible for a free year of credit monitoring services, according to AT&T.
Click here to read the AT&T notice.
The news follows breaches that includes a theft involving Apple Inc's APPL.O iCloud and an alleged international computer hacking ring charged with stealing more than $100 million worth of software and data, some of it used to train military pilots and some related to Microsoft Corp's Xbox.
A massive cyberattack at JPMorgan Chase & Co was reported last week as well.
"To strengthen your account security, we recommend that if you currently have a passcode on your account, you change it. If you do not have a passcode on your account, we recommend you add one," Michael Chiaramonte, director of finance billing operations said in the company notice.
People with more questions regarding the AT&T breach can contact the company at 866-704-0179, between 8 a.m. and 6 p.m.
"On behalf of AT&T, please accept my sincere apology for this incident," Chiarmonte said in the letter.