Full Breakdown
Mozilla Study Ranks Period-Tracking Apps by Privacy Performance
7/19/2026, 3:55:08 AM
Core Findings of the Privacy Investigation
Mozilla’s “Privacy Not Included” team evaluated six popular period-tracking apps—Euki, Clue, Flo, Period Calendar, Planned Parenthood’s Spot On, and Stardust—through hands-on Android testing and network-traffic analysis. The study assigned a 10-point privacy score to each app. Euki received a perfect 10/10, while Stardust earned the lowest 2/10. Scores for the remaining apps fell between 5 and 8 points.
Background & Context
The research was conducted amid heightened concern over reproductive-health data after the 2022 Supreme Court decision overturning *Roe v. Wade*. In the United States, many consumer health apps fall outside HIPAA’s protections, leaving users vulnerable to data collection practices that could be subpoenaed in legal proceedings. Mozilla partnered with Harvard University’s Berkman Klein Center Transparency Hub and incorporated expertise from the University of Illinois’ Siebel School of Computing and Data Science.
Key Organizations and Individuals
- Mozilla – privacy-focused nonprofit that performed the testing and released the report.
- Euki – open-source period tracker managed by the Euki nonprofit; co-founder Ana Ramirez commented on a minor in-app-browser issue.
- Flo – commercial app with 81 million monthly active users; previously settled FTC allegations in 2021 and an $8 million settlement in 2025.
- Stardust – astrology-themed tracker that markets privacy-first features but was found to share extensive health data.
Data & Statistics
| App | Privacy Score (out of 10) | Notable Data Practices |
|---|---|---|
| Euki | 10 | Stores all health data locally; no account required; PIN protection, automatic deletion, decoy screen. |
| Clue | 8 | Granular consent controls; builds long-term health profiles but does not broadcast records to third parties. |
| Flo | 7 | Optional advertising and analytics can be disabled; “Anonymous Mode” routes traffic through Cloudflare. |
| Period Calendar | 6 | Sends device model, screen size, time zone, and app name to Google AdMob, DoubleClick, and InMobi. |
| Spot On | 5 | No external sharing during core tracking; web-browser searches leak city, care type, age, and last period. |
| Stardust | 2 | Transmits birth dates, contraceptive methods, reproductive goals, and symptoms to RudderStack, AppsFlyer, and Facebook. |
Why It Matters
The study highlights that even when health records remain on a device, ancillary data—such as device identifiers or web-browser inputs—can create persistent advertising profiles. In a post-Dobbs legal environment, such profiles may be subpoenaed, exposing users to privacy and legal risks. The findings also underscore regulatory gaps: most U.S. consumer health apps are not covered by HIPAA, and pending legislation like the American Data Privacy and Protection Act (stalled since 2023) has yet to provide comprehensive safeguards for sensitive health information.
Official Statements & Responses
- Mozilla noted that “simply using a reproductive-health app can become part of a persistent advertising profile.”
- Ana Ramirez, co-founder of Euki, told Mozilla the in-app-browser analytics issue “would be reviewed.”
- Flo asserted that it “has significantly strengthened its privacy and security practices since 2019.”
Criticism & Opposition
Privacy advocates warn that anonymized datasets can be re-identified when combined with location data. “A 2019 study found that menstrual cycle data combined with location information could identify individual users with high accuracy,” emphasizing the fragility of “anonymous” claims. Critics also point to Stardust’s prior marketing of end-to-end encryption that, upon investigation, proved to be only standard transport encryption.
Verbatim Quotes
- “Euki earned the study’s only perfect score because it stores health information locally and requires no account, while the astrology-themed tracker Stardust ranked last due to its extensive third-party data sharing.” — Mozilla report
- “Stardust shared sensitive period tracking data with third-party analytics firms, according to new privacy research from Mozilla.” — TechCrunch summary
- “Stardust told Mozilla that RudderStack routes data only to the company's analytics systems and does not receive information that can identify individual users.” — Stardust response
- “Flo said it has significantly strengthened its privacy and security practices since 2019.” — Flo statement
What’s Next
Mozilla plans to update its “Privacy Not Included” database annually, incorporating any policy changes announced by the evaluated apps. Stakeholders, including U.S. lawmakers debating the American Data Privacy and Protection Act, are expected to reference the study when shaping future consumer-health privacy regulations.
