T-Mobile today disclosed a data breach affecting tens of millions of customer accounts, its second major data exposure in as many years. In a filing with federal regulators, T-Mobile said an investigation determined that someone abused its systems to harvest subscriber data tied to approximately 37 million current customer accounts.

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In a filing today with the U.S. Securities and Exchange Commission, T-Mobile said a “bad actor” abused an application programming interface (API) to hoover up data on roughly 37 million current postpaid and prepaid customer accounts. The data stolen included customer name, billing address, email, phone number, date of birth, T-Mobile account number, as well as information on the number of customer lines and plan features.

APIs are essentially instructions that allow applications to access data and interact with web databases. But left improperly secured, these APIs can be leveraged by malicious actors to mass-harvest information stored in those databases. In October, mobile provider Optus disclosed that hackers abused a poorly secured API to steal data on 10 million customers in Australia.

The company said it first learned of the incident on Jan. 5, 2023, and that an investigation determined the bad actor started abusing the API beginning around Nov. 25, 2022.

T-Mobile says it is in the process of notifying affected customers, and that no customer payment card data, passwords, Social Security numbers, driver’s license or other government ID numbers were exposed.

In August 2021, T-Mobile acknowledged that hackers made off with the names, dates of birth, Social Security numbers and driver’s license/ID information on more than 40 million current, former or prospective customers who applied for credit with the company. That breach came to light after a hacker began selling the records on a cybercrime forum.

Last year, T-Mobile agreed to pay $500 million to settle all class action lawsuits stemming from the 2021 breach. The company pledged to spend $150 million of that money toward beefing up its own cybersecurity.

In its filing with the SEC, T-Mobile suggested it was going to take years to fully realize the benefits of those cybersecurity improvements, even as it claimed that protecting customer data remains a top priority.

“As we have previously disclosed, in 2021, we commenced a substantial multi-year investment working with leading external cybersecurity experts to enhance our cybersecurity capabilities and transform our approach to cybersecurity,” the filing reads. “We have made substantial progress to date, and protecting our customers’ data remains a top priority.”

Despite this being the second major customer data spill in as many years, T-Mobile told the SEC the company does not expect this latest breach to have a material impact on its operations.

While that may seem like a daring thing to say in a data breach disclosure affecting a significant portion of your active customer base, consider that T-Mobile reported revenues of nearly $20 billion in the third quarter of 2022 alone. In that context, a few hundred million dollars every couple of years to make the class action lawyers go away is a drop in the bucket.

The settlement related to the 2021 breach says T-Mobile will make $350 million available to customers who file a claim. But here’s the catch: If you were affected by that 2021 breach and you haven’t filed a claim yet, please know that you have only three more days to do that.

If you were a T-Mobile customer affected by the 2021 incident, it is likely that T-Mobile has already made several efforts to notify you of your eligibility to file a claim, which includes a payout of at least $25, with the possibility of more for those who can document direct costs associated with the breach. OpenClassActions.com says the filing deadline is Jan. 23, 2023.

“If you opt for a cash payment you will receive an estimated $25.00,” the site explains. “If you reside in California, you will receive an estimated $100.00. Out of pocket losses can be reimbursed for up to $25,000.00. The amount that you claim from T-Mobile will be determined by the class action administrator based on how many people file a legitimate and timely claim form.”

There are currently no signs that hackers are selling this latest data haul from T-Mobile, but if the past is any teacher much of it will wind up posted online soon. It is a safe bet that scammers will use some of this information to target T-Mobile users with phishing messages, account takeovers and harassment.

T-Mobile customers should fully expect to see phishers taking advantage of public concern over the breach to impersonate the company — and possibly even send messages that include the recipient’s compromised account details to make the communications look more legitimate.

Data stolen and exposed in this breach may also be used for identity theft. Credit monitoring and ID theft protection services can help you recover from having your identity stolen, but most will do nothing to stop the ID theft from happening. If you want the maximum control over who should be able to view your credit or grant new lines of credit in your name, then a security freeze is your best option.

Regardless of which mobile provider you patronize, please consider removing your phone number from as many online accounts as you can. Many online services require you to provide a phone number upon registering an account, but in many cases that number can be removed from your profile afterwards.

Why do I suggest this? Many online services allow users to reset their passwords just by clicking a link sent via SMS, and this unfortunately widespread practice has turned mobile phone numbers into de facto identity documents. Which means losing control over your phone number thanks to an unauthorized SIM swap or mobile number port-out, divorce, job termination or financial crisis can be devastating.