What the research found
A 2024 study tested 30 popular password managers against web forms containing hidden fields (invisible to a normal user through techniques like CSS positioning or zero-height containers), the same techniques legitimate sites sometimes use for layout, but which can also be used to harvest data without anyone noticing. Every password manager tested filled data into at least one hidden field across the scenarios tried, with an overall fill rate of 58.7% across more than 1,000 test cases. Login forms were the most exposed, with hidden fields getting filled 65.7% of the time. Password managers built into a browser turned out to be about four times more likely to fill hidden fields than separately installed ones, and several browser-built-in managers filled passwords into hidden fields no matter which concealment technique was used.
A separate 2023 study found something related from a different angle: passwords sitting in plain, unencrypted text within a webpage’s own source code, readable by anything with basic page access, on a meaningful share of sites tested, including high-traffic ones.
Why convenience and safety pull against each other here
Autofill exists to reduce friction: one click and a form is done. That same design goal is what creates the exposure. A password manager has to guess which fields deserve which saved data, and a hidden field designed to capture information exploits exactly that guesswork. In more than a third of the scenarios tested, the fill happened with what researchers judged too little user interaction, meaning a person had reasonable grounds to be unaware it happened at all.
The practical takeaway
Turn off automatic, one-click autofill where your password manager allows it, and switch to a mode that requires you to select which field gets filled rather than filling every recognized field on a page on its own. This adds a small amount of friction back in exchange for a meaningful reduction in this specific exposure. It’s also worth being more cautious on unfamiliar or unofficial-looking sites, since a hidden field built to harvest data is far more likely to show up on a page built to exploit autofill than on a site you already trust and use often.
This research tested password managers under controlled conditions built to surface the vulnerability, not real-world attack rates. It shows the capability exists across most major password managers, not that any specific site is exploiting it against you right now.
References
Fu, Y., Wang, D. “Leaky Autofill: An Empirical Study on the Privacy Threat of Password Managers’ Autofill Functionality.” Proceedings of the 40th Annual Computer Security Applications Conference, 2024. https://www.researchgate.net/publication/385420522
Nayak, A., et al. “Exposing and Addressing Security Vulnerabilities in Browser Text Input Fields.” ArXiv, 2023. https://arxiv.org/abs/2308.16321v1
