How to blur sensitive information in a screenshot
What has to be covered, what almost everyone forgets, and how to cover it so it cannot be read back. Includes the checks for status bars, tab titles, QR codes and file names — the parts that leak after the obvious thing has been hidden.
You are about to send a screenshot and something on it should not travel with it. A phone number, an address, an account balance, a card number, a colleague’s name in a chat log, a face. The mechanical part takes ten seconds in any markup tool. The part that goes wrong is deciding what to cover, and it goes wrong in a very predictable direction: people cover the thing they were thinking about and miss the four things around it.
So this page is a checklist first and a technique second. The technique is short — for anything that would genuinely hurt, put a solid block over it and flatten the image. The checklist is where the actual work is.
The obvious layer
- Full names, in the message, in the contact header, in the To field, and in the group member list.
- Phone numbers, email addresses, and any national ID or tax number.
- Bank card numbers — all of them, not the middle. The first six digits identify the issuer and the last four are what everyone uses to identify a card, which makes them the useful half to an attacker rather than the harmless half.
- Home and work addresses, including the delivery address quietly sitting in an order confirmation.
- Account balances and salary figures, which are rarely the point of the screenshot and always the part that gets quoted.
- Faces, including in avatars, which are small enough to feel invisible and large enough to be recognised by someone who knows the person.
- Anything that looks like a token: an order number, a booking reference, a password reset link, a verification code, a session URL with a long random string in it.
The layer almost everyone forgets
These are the leaks that survive a careful redaction, because they are outside the rectangle the person was concentrating on.
- The status bar. Carrier name, exact time, battery percentage, and on a Mac the Wi-Fi network name and the menu-bar icons of every app you run.
- A notification banner that arrived during the capture. It is a screenshot of an unrelated private conversation, pasted on top of yours.
- Browser tab titles. You covered the page content and left seven tabs across the top spelling out what else you were doing.
- The address bar. A URL frequently contains an account id, a workspace name, or a share token that grants access on its own.
- Autocomplete and suggestion rows. The three suggestions under a search box are your search history.
- The file name. Exporting as "invoice_2026_acme_final.png" undoes the redaction inside the image.
- A photograph of a screen rather than a screenshot: reflections in the glass, and whatever is on the desk behind it.
- Metadata. A screenshot carries less than a camera photo, but exported files can carry a creation timestamp, a device model, and in some pipelines a colour profile that identifies the machine. Re-exporting through a markup app that writes a fresh file is the simplest way to shed most of it.
- The scrollbar, which tells a reader how much you cropped away.
QR codes need more covering than you think
A QR code is designed to survive damage. The four error-correction levels recover roughly 7, 15, 25 and 30 percent of a symbol, which means a code with a corner blacked out very often still scans. How much you can actually damage before it fails depends on where the damage falls, but the practical rule is simple: do not cover part of a QR code. Cover essentially all of it, or crop it out of the image entirely.
The same reasoning applies to barcodes on tickets and to the machine-readable strip at the bottom of an ID document, which is intentionally redundant and intentionally easy to read under bad conditions.
How to cover it so it cannot be read back
Three techniques, in increasing order of how much information they destroy. A blur redistributes the pixels but keeps them all, which is why light blur over short predictable text is the weakest option available. A mosaic replaces each block with one average colour, discarding what was inside the block — a coarse grid destroys a great deal, a fine one destroys very little. A solid block replaces the area with one colour and keeps nothing at all.
There is published research on reversing both blur and pixelation, and it is worth being precise about what it shows rather than repeating the slogan. The attacks work by making guesses and testing them: render a candidate string in the likely font, push it through the same blur or the same pixelation grid, and compare. That is devastating for a six-digit code, a licence plate, a date of birth or a name from a short list, and it is why those should never be mosaicked. It is much weaker against a coarse mosaic over content the attacker cannot enumerate. What it does not license is the claim that a mosaic is always reversible; a coarse mosaic flattened into an export is not, in general, recoverable. The honest rule is a decision rule rather than a physics claim: if you would be harmed by being wrong, use a solid block, because you do not need to reason about the attacker at all.
The failure that has nothing to do with the filter
The classic redaction disaster is not a weak mosaic. It is a strong cover that was never flattened: a black rectangle sitting as an object on top of a PDF, in a layered file, or in an editor’s non-destructive edit history, where selecting it and pressing delete restores the original. This has embarrassed governments repeatedly and it will keep doing so.
The defence is one habit. Export a new flat image file, open that file, and look at it. Not the editor’s canvas — the file you are about to attach. If your editor keeps an original for a revert function, know that it does, and either delete the original or accept that the cover protects the recipient rather than protecting the device.
Crop beats cover
Whenever the sensitive thing is near an edge, the best redaction is a crop. Cropped pixels are not in the file at all, so there is nothing to reverse, nothing to un-layer and nothing to argue about. Ask before every cover: could I just cut this off instead? The answer is yes more often than people expect, particularly for status bars, sidebars and the account row at the top of a settings screen.
Doing it in Marka
Marka handles this as a brush rather than a box: you drag over the thing and it goes. There are three modes — mosaic, blur, solid colour — with a strength slider and three brush widths, and the mosaic mode is free. On Mac there is additionally an on-device scan that uses Apple’s Vision framework to propose candidates: phone numbers, email addresses, ID numbers, card-like sequences, faces and URLs, each with a checkbox, and the numbers it finds are themselves masked in that list. It covers only what you tick. It proposes and never acts, and it will miss things — it is a second pair of eyes on the checklist above, not a replacement for it, and it does not exist on iPhone or iPad. Everything happens on the device: no server, no account, no analytics, nothing uploaded, which for a page about sensitive screenshots is not a marketing line but the point. Note before installing that the interface is in Simplified Chinese in this version.
Frequently asked questions
Is blurring enough to hide a phone number?
No, and a phone number is the worst case for it. A number has a fixed length, a known format and a small alphabet, so an attacker can render candidates, blur them the same way and compare. Use a solid block for anything with a fixed format — codes, card numbers, plates, dates of birth. Blur is a stylistic choice for things nobody is trying to recover.
Should I cover only the middle digits of a bank card?
Cover all of it. The first six digits identify the issuing bank and the last four are the digits every service on earth uses to identify a card, so the two ends you were planning to leave are the useful ones. If the card image is near an edge of the screenshot, crop rather than cover.
Does covering part of a QR code stop it working?
Usually not. QR codes carry error correction that recovers roughly 7 to 30 percent of the symbol depending on the level, so a code with a corner obscured very often still scans. Cover the whole code or crop it out. The same is true of barcodes and of the machine-readable strip on an ID document.
Do screenshots contain metadata that identifies me?
Less than a camera photo — there is no GPS in a screenshot — but exported files can still carry a creation time, a device or software identifier and a colour profile. Re-exporting through an editor that writes a fresh file sheds most of it. The bigger leak is almost always visible in the image itself: the status bar, the tab titles and the file name you chose.
What is the single check that catches the most mistakes?
Open the exported file — not the editor canvas — and read it as a stranger, starting at the top-left corner and moving in a Z. Nearly everything that leaks is at the edges: status bar, tab strip, address bar, sidebar, a notification banner, the account row. The middle of the image is the part you already thought about.
Do it in Marka
Marka is a screenshot markup app for iPhone, iPad and Mac. Brush a mosaic over what should not be seen, point an arrow at what should, number the steps, crop it to the shape you are posting in, and send. Everything is processed on the device — no upload, no account, no analytics, and a 2.4 MB download. Opening, cropping, the mosaic brush and export are free; Pro is a one-time purchase that covers all three platforms.