Images and other secrets that your documents can reveal
If you work with sensitive data, these features of PDF, MS Office, and cloud documents are worth learning about.

Reports, articles or marketing materials are some examples of documents that most of us deal with at some point. We write and edit them on computers, email them to colleagues or friends, share them in the cloud, deliver them to clients, and much more.

However, if a file that you want to show to other people contains information that they shouldn’t see, you could be in trouble. Let’s find out how to prevent this situation.

whimsical images
Secrets often show up in images, like background passwords , and the problem is that not all editing tools get rid of them correctly. For example, even if you completely blur sensitive information with a semi-transparent brush, simply adjusting the brightness and contrast is enough to reveal the secret. To find out how an image can inadvertently reveal sensitive information and how to hide it, don’t miss this post .

In a nutshell, to really hide passwords, barcodes, names, and other secret data in images with a graphical editor, you need to remember two things. First, do any blurring with 100% opaque tools, and second, post the image in a “flat” format, such as JPG or PNG, to prevent others from splitting it into separate layers.

But what if you see secret information in an image that is embedded in a text document?

The naughty PDF
Let’s say you’re about to send a brochure to a client when you notice that one of the images contains a colleague’s personal information. You draw a black rectangle on it, using the paid version of Adobe Acrobat on your office computer. All set, right?

Hide part of an image in Adobe Acrobat Reader

Unfortunately, if you send that document, the client will still be able to collect too much information about your partner. Acrobat is not designed for image editing and does not have the ability to merge an image with what you draw on it, so anyone opening the file could remove or move rectangles and other surface graphics.

Any user could easily remove a drawn black rectangle in Adobe Acrobat Reader

Export from Word to PDF
In some cases, it can be useful to modify an image in the format in which you created the document (for example, DOCX) and then export it to PDF. For example, if you crop an image, the cropped part will not be transferred to the PDF. Many people use this very simple method to make simple edits to a document image.

However, it’s important to remember that not all image edits work to hide information in this way. For example, the black rectangle cheat will still fail.

original image

Hiding part of an image with a rectangle in Microsoft Word

If, after exporting the file from MS Word to PDF, you open the resulting file in Adobe Reader and copy and paste the image back into Word, you will see the original image without dark rectangles.

Copy an image from a PDF

Paste it into Word and magic!

When you export from Word to PDF, the original image and the object drawn on top are not merged. They are saved separately. The hidden bits also remain in the file.

Ultimately, like Acrobat Reader, Microsoft Word is not designed for image editing. If you see an image that needs to be modified in a text document, make the edit in a suitable graphics editor and then insert it back into the document.

Microsoft Office Document Inspector
Images are not the only elements of a document that can retain private information. Others include headers and footers; tracking reviews, comments and hidden text; linked files, such as Excel spreadsheets, that form the basis of charts in a report; sometimes even the name of the author of a document that should be anonymous. A single file can be full of these little things and it’s easy to forget all or some of them.

To help catch these potential information leaks early, Microsoft Office provides the Document Inspector tool , which analyzes everything mentioned above, including metadata (such as author name), headers and footers, hidden text, embedded objects , etc.

To inspect a file with the Document Inspector in Office 365:

Open the File tab .
Select Information .
Click Check for problems .
Select Inspect Document .
The names of the settings may differ depending on the version of Word.

If the Document Inspector finds sensitive data, it suggests deleting it or recommends a more secure alternative. For example, if you’ve added an Excel chart as an interactive object, the tool recommends inserting it as an image, so the recipient will see it, but won’t be able to browse the original table.

Document Inspector finds sensitive data

However, when it comes to image secrets, the Document Inspector is of no use, because it doesn’t even look at them. You will have to re-verify them manually, following the tips above.

Google Docs remembers everything
Sometimes a team of colleagues has to edit a single document, in which case PDF is usually not the best format (due to its relative paucity of collaboration tools). Using Word documents locally and sending them via email is also not a good option; version control is practically impossible and the process takes too long, especially since they will have to do it in turns.

Help is available in the form of cloud solutions, which allow co-editing of the same copy of a document. However, with privacy in mind, it is important to remember that cloud office suites record all actions and anyone who edits the file can access the change log.

If you accidentally added an object or text with sensitive information to a cloud document, even if you catch the mistake and delete the information immediately, it will remain in the change history so your colleagues can see it.

The change history contains the deleted images

Even if you’ve removed all sensitive information from the cloud file before it’s publicly available, anyone with access to the file can view the history of changes and revert it.

Google Docs allows users to revert changes

The problem has a simple solution. If you plan to invite someone to edit a document online with sensitive data in images or other elements that you want to hide, create a new file and copy into it only what you want your partner to see.

Another tip: To avoid accidentally pasting something into a shared document, first check what’s on the clipboard by pasting it into a local file to make sure it’s exactly what you want to share.

How not to leak information in a document
In a nutshell, here’s how to keep information private in shared documents, from peers and co-editors, not to mention the general public:

Double check the content of the document before sharing it.
Use graphic design programs dedicated to editing images and 100% opaque elements to block information and save images in formats that do not support layers: JPG or PNG.
Pay particular attention to cloud documents, which keep a full change history log for each file, potentially allowing others to restore deleted or modified information.
Don’t give co-authors access to cloud documents that once contained secret data; instead, create a new file and copy only the non-sensitive information.
Check Word documents with the Document Inspector. Download the cloud documents in DOCX format and review them too.
Pay close attention and don’t be in a hurry.