It hasn’t been a good week for telecommunications companies: security researchers have uncovered security flaws with systems at AT&T, Sprint, and T-Mobile that could have left customer data accessible to bad actors.

Yesterday, BuzzFeed News reported two flaws that left customer information information vulnerable at AT&T and T-Mobile. In T-Mobile’s case, an “engineering mistake” between Apple’s online storefront and T-Mobile’s account validation API allowed for an unlimited number of attempts on an online form, which would allow a hacker to use commonly-available tools to guess an account PIN or the last four digits in a customer’s social security number, in what’s called a brute-force attack.

A similar problem occurred with phone insurance company Asurion and its AT&T customers. An online claims form would allow anyone with a customer’s phone number to access a form that allowed them unlimited guesses to guess a customer’s passcode, leaving it vulnerable to another brute-force attack.

Once reported, AT&T and T-Mobile fixed the problems

In each case, both companies fixed the vulnerabilities when contacted by BuzzFeed News.

In another instance this weekend, TechCrunch reported that security researchers were able to access an internal staff portal at Sprint because of “weak, easy-to-use usernames and passwords,” compounded with the lack of two-factor authentication. Once in, the researcher was reportedly able to access customer account information for Sprint, Boost Mobile, and Virgin Mobile. The researcher also reported that anyone who gained access could make changes to customer accounts, and that customer PINs could be brute-forced. A Sprint spokesperson confirmed the vulnerability to TechCrunch, and noted that it didn’t believe that any customers were affected by the vulnerability, and noted that they’re working to fix the issue.

It’s worth noting that vulnerabilities aren’t necessarily breaches, but it’s vulnerabilities such as these that allow bad actors to gain access to a system and exploit the customer data that they access. These systems are by necessity complicated: companies like AT&T, Sprint, and T-Mobile have to balance providing access to employees to do their jobs and to customers to gain access to their information. But given the harm that a malicious actor can play with the vast amounts of data these companies have, it’s clear that they need to be more proactive in protecting their customers.