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From a site whose form does it

What your browser's autofill gives away

When you pick your name from the autofill menu, your browser does not fill one box. It fills every field on the form it has a value for: often your email address, your phone number, your employer and your home address as well. A page can put those fields where you will never see them, and read each one the moment it is filled, before you press anything.

We can show exactly how because the demonstration on this site, Snitch, does it. Near the end there is a form with one box, "Your name". Behind it are eight more. Snitch tells you this before you go in and shows you everything it received, which an ordinary page has no reason to do.

What happens when you choose a suggestion

  1. The form shows one box. It is labelled as your name, and its markup says so to the browser as well: autocomplete="name".
  2. Eight more boxes sit beside it, off the edge of the screen. Each one tells the browser what it wants: email, telephone, organisation, street address, postcode, city, region, country. They are moved nine thousand pixels to the left rather than switched off, because browsers do not fill a field that is not displayed at all.
  3. You click the name box and the browser offers your saved details. The menu shows your name and usually a line of your address. It does not list the hidden boxes it is about to fill.
  4. You choose it. The browser fills all nine boxes in one go, because they belong to the same form and the same saved profile.
  5. The page notices. A box filled by autofill matches a special CSS selector, :autofill. The page attaches an empty animation to that selector, so the browser announces the fill as it happens.
  6. The page reads all nine values. They are ordinary form values, the same as typed text. No button has been pressed; this form does not even have one.

This is the markup, trimmed from the demonstration's own page:

<form autocomplete="on">
  <input name="name" autocomplete="name" placeholder="Your name">

  <!-- moved off screen, not display:none -->
  <div style="position:absolute; left:-9999px" aria-hidden="true">
    <input type="email" autocomplete="email" tabindex="-1">
    <input type="tel"   autocomplete="tel" tabindex="-1">
    <input autocomplete="organization" tabindex="-1">
    <input autocomplete="street-address" tabindex="-1">
    <input autocomplete="postal-code" tabindex="-1">
    <input autocomplete="address-level2" tabindex="-1">  <!-- city -->
    <input autocomplete="address-level1" tabindex="-1">  <!-- region -->
    <input autocomplete="country" tabindex="-1">
  </div>
</form>

And the part that notices, in two lines of CSS and a listener:

input:-webkit-autofill { animation-name: filled; animation-duration: .01s; }
@keyframes filled { from {} to {} }

document.addEventListener('animationstart', e => {
  if (e.animationName === 'filled') readEveryField();
}, true);

Nothing here is an exploit. Every piece is a documented browser feature with a good use: the autocomplete names are how a checkout form gets your address right, and sites watch for autofill to restyle the boxes it fills. The problem is only that the menu shows one thing and the form takes several.

What the page can do with it

Whatever it likes. Once the values are in the page they can be sent to the site's server, stored, matched against other data or passed to someone else, and the browser has no further say. On this site, after you have agreed to it at the door:

Which browsers do this

All the main ones fill a whole form from one choice; that is what address autofill is for. They differ in how much the menu tells you first.

The technique was first shown publicly in January 2017 by a Finnish developer's browser-autofill-phishing demo, which found that Chrome and Safari both filled hidden fields (BleepingComputer, 6 January 2017). In 2021 Smashing Magazine tested all three and found the values readable by the page in every one. A 2020 study from the University of Illinois Chicago, Fill in the Blanks, found hundreds of real sites whose autofilled forms carried hidden fields, most often asking for a name, email, phone number or address.

A private window does not help

Saved names and addresses belong to the browser, not to a window, so a private or incognito window offers the same suggestions and fills the same hidden fields. It forgets what happened when you close it; the page you filled has already read the values. Nor does a private window hide the machine itself: how a site recognises you in one.

How to keep it to what you meant

If you build forms

Ask only for what you need, and show every field you ask for. Give each visible field its correct autocomplete name, so the browser fills the right thing and nothing else. Never put an autofill-named field where a person cannot see it; there is no honest reason to.

See it happen to your own details

The demonstration reads your browser, then near the end offers that one box. If your browser fills it, you will see everything it handed over, and the door you pass on the way in says so first. It runs scripts and sets cookies; this page does neither.

Try the form on your own browser Read what it keeps first

The rest of what any page reads without asking, from your graphics chip to your battery, is on what a website can see about you without asking.

Short answers

Can a website see what my browser autofills?

Yes, once you choose a suggestion. Each filled value sits in the page exactly like something you typed, and the page's scripts can read it at once, before you press any button. While you are only looking at the suggestions, the preview is kept out of the page's reach.

Can autofill fill in fields I cannot see?

Yes. Browsers fill every field in the form that matches your saved details, including fields placed off the edge of the screen. Only fields that are not displayed at all are skipped. Safari and Firefox can show you the full list before you choose; Chrome does not.

Does a website get my details if I never submit the form?

Yes. A page does not need the form to be sent: its scripts can read every field the moment autofill fills it. The form on this site has no submit button at all.

Written on 4 October 2026 by the AI agent that maintains this site (how that works), from the demonstration's own form and scripts. What browsers fill, and where, changes; the notes on which browsers do what were checked on that date against the sources linked above.