How to blur sensitive information in a screenshot

What actually leaks

The thing you are demonstrating is rarely the problem. The problem is everything else in frame:

• Real customer names and email addresses in a list you were only showing the layout of.
• API keys, session tokens and bearer headers in a DevTools network panel.
• The URL bar — internal hostnames, tenant IDs, and query strings carrying tokens.
• Browser tab titles across the top: the ticket you have open, the client you are working for.
• Notification toasts that appear mid-capture.
• Account numbers, invoice totals, and salary figures on the row next to the one you meant.

The habit worth building is to look at the four edges of the image before you send it, not the middle. The middle is the part you were paying attention to. The edges are where the leaks live.

Why a light blur is not redaction

A Gaussian blur does not delete information — it averages it. If the blur radius is small relative to the text, the original characters are still recoverable, and there is a decade of published work on reversing exactly this. Pixelation is worse: a mosaic of a known font, at a known size, on a known background is a solvable puzzle, and there are tools that solve it automatically.

This matters most for the things people most often blur badly: short strings from a small alphabet. A six-digit code, a card number, a phone number — the search space is small enough that a weak blur is barely an obstacle.

Doing it properly

1. Crop first

The cheapest redaction is not capturing the thing at all. Use a region capture rather than a full window and most of the risk disappears before you have drawn anything. The URL bar, the tab strip and the neighbouring rows are gone because they were never in the file.

2. Cover, do not blur, for secrets

A filled rectangle in a solid colour removes the pixels. There is nothing left to recover, because there is nothing left. Use this for every credential, token and account number without exception.

3. Blur only for context

Blur is still the right tool when you want the reader to see that something is there — that this is a list of eight customers, that the avatar column exists — without being able to read it. Used that way, the fact that a determined attacker could partly reverse it does not matter, because you were never trying to hide the shape.

If you do blur real data, use a heavy radius. A blur you can still almost read is the worst of both worlds.

4. Flatten before you send

This is the one that bites hardest. If your tool saves the annotation as a separate editable layer — as some design tools and some PDF workflows do — the black box is an object sitting on top of the original, and anyone can move it. Redactions have been undone this way in published legal filings and government documents more than once.

Export to a flat PNG or JPG. In a flat raster file the covered pixels are genuinely gone.

5. Check the metadata

An image can carry EXIF data, and a PDF can carry the original text layer underneath a drawn box. A screenshot taken by a browser is usually clean, but a photo of a screen is not, and a PDF exported from a document editor very often is not.

How CaptureFlow handles it

The editor has both tools deliberately, because they are for different jobs:

• Blur — drag over a region to obscure it while leaving the layout readable.
• Filled shape — a solid rectangle for anything that must be unrecoverable.

Everything you draw is composited into the image when you export, so the downloaded PNG, JPG or PDF is flat — there is no layer to peel back. And because capture, annotation and export all happen on your own machine, the unredacted original never leaves it. Cloud upload is a separate button you press on a specific capture, after you have finished redacting. The tool reference covers both tools in full.

A thirty-second checklist

1. Capture a region, not the whole window, wherever you can.
2. Scan the four edges — URL bar, tab titles, sidebar, notifications.
3. Solid box over every credential, token, card and ID number.
4. Heavy blur over names, emails and anything else that is personal but not secret.
5. Export flat. PNG or JPG, not a layered or editable format.
6. Open the exported file and look at it once before you attach it.

That last step catches more mistakes than the other five combined.

Related: taking a full page screenshot in Chrome, and recording your screen without leaking your desktop.