Cerebral, a telehealth startup specializing in mental health, says it inadvertently shared the sensitive information of over 3.1 million patients with Google, Meta, TikTok, and other third-party advertisers, as reported earlier by TechCrunch. In a notice posted on the company’s website, Cerebral admits to exposing a laundry list of patient data with the tracking tools it’s been using as far back as October 2019.

The information affected by the oversight includes everything from patient names, phone numbers, email addresses, birth dates, IP addresses, insurance information, appointment dates, treatment, and more. It may have even exposed the answers clients filled out as part of the mental health self-assessment on the company’s website and app, which patients can use to schedule therapy appointments and receive prescription medication.

According to Cerebral, this information got out through its use of tracking pixels, or the bits of code Meta, TikTok, and Google allow developers to embed in their apps and websites. The Meta Pixel, for example, can collect data about a user’s activity on a website or app after clicking an ad on the platform, and even keeps track of the information a user fills out on an online form. While this lets companies, like Cerebral, measure how users interact with their ads on various platforms and track the steps they take afterward, it also gives Meta, TikTok, and Google access to this information, which they can then use to gain insight into their own users.

As noted by Cerebral, the exposed information could “vary” from patient to patient depending on several factors, including “what actions individuals took on Cerebral’s Platforms, the nature of the services provided by the Subcontractors, the configuration of Tracking Technologies,” and more. The company says it will notify affected users, and adds that “no matter how an individual interacted with Cerebral’s platform,” it didn’t expose social security numbers, credit card numbers, or bank account information.

After initially finding the security hole in January, Cerebral says it has “disabled, reconfigured, and/or removed” any of the tracking pixels on the platform to prevent future exposures, and has “enhanced” its “information security practices and technology vetting processes.”

Cerebral is required by law to disclose potential violations of HIPAA, also known as the Health Insurance Portability and Accountability Act. This bars healthcare providers from divulging patient information to anyone else other than the patient, or anyone the patient has consented to receive information about their health. The breach is currently under investigation by the US Office for Civil Rights and follows similar incidents involving pixel-tracking tools.

Last year, an investigation by The Markup found that some of the nation’s top hospitals were sending sensitive patient information to Meta through the company’s pixel. This sparked two class-action lawsuits, which allege Meta and the hospitals in question violated medical privacy laws.

Months later, The Markup also found that Meta was able to obtain financial information about users through the tracking tools embedded in popular tax services, such as H&R Block, TaxAct, and TaxSlayer. Meanwhile, other online medical companies, like BetterHelp and GoodRx got slapped with hefty fines from the FTC for sharing sensitive patient data with third parties earlier this year.

In addition to facing scrutiny over whether or not it has violated HIPAA regulations, Cerebral is facing an investigation by the Department of Justice and the Drug Enforcement Administration over its prescribing of controlled substances, such as Adderall and Xanax. It has since halted the prescription of these medications.