How to check if a website uploads your files

What you are looking for

When a web page sends your file to a server, the browser makes a network request that carries the file's data. That request is almost always a POST or a PUT, and its payload is about as large as the file. The browser's developer tools list every request the page makes, so you can spot such a request, or confirm that none exists, while you use the tool.

Two things make this easier. Use a test file you do not mind exposing, for example a dummy PDF or a harmless photo of a few hundred kilobytes to a few megabytes, so that an upload would be easy to see. And clear the request list right before you pick the file, so earlier page-load traffic does not bury what you are looking for.

Chrome and other Chromium browsers (Edge, Brave)

1. Open the tool page and let it finish loading.
2. Press F12, or Ctrl+Shift+I on Windows and Linux, or Cmd+Option+I on a Mac. Click the Network tab.
3. Tick Preserve log if you want requests to survive a page change, and click the clear button (a circle with a line through it) to empty the list.
4. Choose your test file in the tool and run the task, such as merge, compress or convert.
5. Watch the list. Click the Method column header or the Fetch/XHR filter to narrow it. Look for POST or PUT requests, and check the Size column. A request with a size close to your file's size is an upload.
6. Click a suspicious request and open Payload to see what was sent. For uploads you may see a form data entry with your file's name.

Firefox

1. Open the tool page, then press F12 or Ctrl+Shift+E (Cmd+Option+E on a Mac) to open the Network panel.
2. Click the trash-can icon to clear existing requests.
3. Select your file and run the task.
4. Use the filter bar to show XHR requests, or sort by Method. Check the Transferred and Size columns for something close to your file size.
5. Click a request and open the Request tab to see the data that was sent.

Safari

On iPhone and iPad, the easiest route is to connect to a Mac and use its Web Inspector, or to use the airplane-mode test below, which needs no tools.

1. Safari hides developer tools by default. Open Safari settings, go to Advanced, and tick the option to show features for web developers (called Show Develop menu in the menu bar in older versions).
2. Open the tool page, then choose Develop, then Show Web Inspector, or press Option+Cmd+I.
3. Click the Network tab and clear the list using the clear icon.
4. Pick your file and run the task. Watch for entries whose Method is POST or PUT and whose size is close to your file.
5. Select an entry to see its request headers and body.

The airplane-mode test

This is the most convincing check and it works on any device. Load the tool page while online. Once the page is fully loaded, turn on airplane mode or switch off Wi-Fi and mobile data. Now select a file and run the task.

If the tool still produces your result, the processing is happening on your device, because there is no connection for a file to travel over. If the tool hangs, shows a network error or never finishes, it was waiting for a server. A few caveats: some tools legitimately use a network connection for something other than your file, such as live currency rates, and some pages will not work offline if you refresh the page after disconnecting, because the page itself needs to be reloaded from the internet.

A tool that still works after you disconnect from the internet cannot be uploading your file at that moment.

Normal requests versus file uploads

A busy page can show dozens of requests even when nothing is uploaded, which can look alarming. Most of them are routine. Here is how to tell them apart.

Caveats and edge cases

• A small request is not necessarily harmless. A site could send a few details about your file, such as its name, without sending the file. Click a request and read its payload if you want certainty.
• Some sites upload in chunks, so you may see several medium-sized requests instead of one large one. Add up the sizes.
• Browser extensions can add their own traffic to the list. Test in a private window with extensions disabled for a cleaner picture.
• Compressed uploads can be smaller than the original file, so do not rely on exact size matching.
• Service workers and WebSocket connections can appear under separate filters. Check the WS filter as well as Fetch/XHR.
• Network inspection shows what happens now. It cannot prove what the site will do after a future update, so repeat the check if you are handling sensitive material on a site you have not used for a while.

Try it on our tools

You can run exactly this test on Robince Studio. Open a tool such as Compress PDF or Image Compressor, clear the Network list, process a test file, and look for a request carrying your file. You should find page, font and advertising requests, but no upload of your document. Then try the airplane-mode test: the tool should still finish. If you ever find something that contradicts this, we would want to know.

Frequently asked questions

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