Mozilla Foundation research has found that period-tracking app Stardust shares users' sensitive health data with third-party analytics firm RudderStack, despite the app's website stating "Your data is private. Period." The finding is significant because Stardust markets itself as privacy-focused and previously claimed to use end-to-end encryption, a claim TechCrunch disproved in 2022 by analysing the app's network traffic; sharing health data with third parties raises risks around data breaches and law enforcement requests, particularly sensitive given the app's popularity following the overturning of the US constitutional right to abortion.

The shared data included users' birthdates, birth control type, reproductive goals and specific symptoms, linked to a unique identifier rather than a name, which the FTC has warned does not amount to true anonymisation. Mozilla researcher Shoshana Wodinsky tested six period trackers and found Stardust was the only one sharing such data with another company; a Stardust spokesperson said RudderStack is contractually barred from selling or using the data itself, and later stated the firm has never received any data requests from authorities. Mozilla named rival app Euki as "squeaky clean", as it shares no data and keeps health information on the user's device.