How to hide sensitive information in a screenshot on a Mac

A screenshot pasted into a chat, an issue or a document carries whatever was on screen at that moment. That often includes an email address, a phone number, an API key. Here are three ways to hide those details, from a manual shape to one that finds them for you.

The details usually hide in plain sight: a message header, a support reply, a settings window, a terminal where you just pasted a key. Read the shot before you send it.

Option 1: draw a solid shape in Preview

Open the screenshot in Preview.
Open Markup and choose Tools > Annotate > Rectangle.
Set the fill to a solid color and turn the line off.
Drag the rectangle over the detail.

This works with what your Mac already has. It is manual: one shape per detail, placed by eye, and it is easy to miss one.

Two warnings. A semi-transparent highlighter or a light blur does not really hide text, because the letters are still under it. Use a solid shape with nothing showing through. And export a flattened copy, a PNG, before you send it. In a PDF the shapes stay as separate objects, so they can be moved or deleted and the text underneath can be read.

Option 2: crop it out

If the detail sits at the edge of the shot, take it again with ⇧⌘4 and a tighter drag, or open the screenshot in Preview, drag a selection and press ⌘K to crop.

Cropping removes the pixels instead of covering them, so there is nothing left to reveal. It is the cleanest result when it works. It only works when the secret is at the edge, which is rarely true in the middle of a message thread.

Option 3: the mosaic tool in Lightcap

Take a screenshot, then open it from the card that appears in the corner to annotate it.

Click the lock button in the title bar.
Review the mosaics it places over the details it found.
Move, resize or delete any of them like a normal shape. One undo removes them all.

It looks for email addresses, phone numbers, card numbers, IP addresses, API keys, and passwords written after a label such as password: or token=.

It only finds things with a recognisable pattern. Names, street addresses and account numbers are not found, so look over the result and cover the rest with the mosaic tool, M. All of this runs on the Mac; nothing is uploaded.

The app is 3.3 MB. Free for 7 days with every feature, no account or card. Then $10 for 1 Mac or $25 for 3 Macs at the launch price through December 31, 2026, and $15 or $35 after.

Which one to use

One detail, and you want to use nothing new: a solid rectangle in Preview, saved as a PNG.
The detail is at the edge of the shot: crop it out. Removed pixels cannot come back.
A support reply, a log or a settings window with several details: the lock button in Lightcap finds the common patterns, then you check and cover the rest with M.
Anything going into a public issue: cover it, then read the whole image once before you post.

Questions

Does a blur hide text in a screenshot?

A light blur or a semi-transparent highlighter does not. The letters are still in the image. Use a solid shape, a mosaic, or crop the area out.

Can the shapes I put over a screenshot be removed?

Yes, if you save the file as a PDF, the shapes are separate objects. Export a flattened PNG instead.

What does the lock button in Lightcap find?

Email addresses, phone numbers, card numbers, IP addresses, API keys and passwords written after labels. Names, street addresses and account numbers are not found, so check the result and cover the rest by hand.

Download Lightcap for Mac and try every feature free for 7 days. No account or card needed. It runs on macOS 15 or later.