Period trackers leak intimate health data to ad firms, Mozilla finds

Mozilla researcher Shoshana Wodinsky conducted a detailed audit of six popular period-tracking apps in partnership with Harvard's Berkman Klein Center, examining how each app handles sensitive reproductive health data. The astrology-themed app Stardust emerged as the worst performer, scoring 2 out of 10.

Stardust's privacy failures begin immediately upon opening: the app pings third-party trackers before any user enters data. Once a user logs a symptom—whether tender breasts, stomach cramps, or any other detail—that information travels instantly to analytics firm RudderStack alongside a persistent user ID. Critically, RudderStack is built to route data onward to unknown destinations that Mozilla's auditors could not even observe. In addition, Stardust hands Facebook an ad identifier that ties in-app behavior directly to the platform's existing user profiles, enabling ad targeting based on reproductive health details. The app also provides no in-app method for users to shut off data sharing. When asked by TechCrunch about data security practices, Stardust's company stated it has never received a legal demand for user data—a response that sidesteps the audit's core finding.

In sharp contrast, Euki, a nonprofit-run period tracker, earned a perfect score of 10. Euki requires no user account; all health data remains on the device and never leaves the phone. Users can set a PIN to protect access, schedule automatic deletion of entries, or pull up a decoy screen if someone physically forces the phone open. The one minor weakness is an in-app browser for educational pages that loads standard web trackers, but even there, Euki resets identifiers between visits to minimize tracking persistence.

The audit demonstrates that the commercial period-tracking ecosystem treats intimate reproductive health data as an asset for ad-targeting and analytics, whereas a nonprofit model can prioritize user privacy as the default. For millions of users relying on period trackers for health management, the findings raise urgent questions about data exposure and the lack of transparency in how their reproductive data flows to advertisers and data brokers.

The Mozilla Foundation audit, conducted in partnership with Harvard's Berkman Klein Center, reveals a stark privacy gap between commercial period-tracking apps and privacy-focused alternatives. Stardust's practice of sending detailed reproductive health data—including birth control type, pregnancy status, and specific symptoms—to third-party analytics and ad platforms without transparent user control reflects a broader pattern in the app ecosystem: health apps often treat intimate data as a commodity for monetization through ad targeting. The audit found that Stardust shares data with RudderStack, which is designed to route information to unknown downstream destinations, creating an opaque data flow that users cannot observe or control within the app.

The contrast with Euki's approach underscores that privacy-first design is technically achievable. Euki demonstrates that a period tracker can operate without requiring an account, keeping all sensitive data local to the user's device, and offering users concrete controls—a PIN, automatic deletion schedules, and decoy screens. This design eliminates the intermediaries (ad networks, analytics firms) that characterize commercial trackers. The audit's public grading system (0–10) provides users with a concrete metric for comparing apps, though the finding that only one of six apps earned a passing score suggests most mainstream options prioritize data extraction over user privacy.