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

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.