Kaspersky How to convert files safely

The Online file conversion websites are an attractive way to change the format of your files, but they carry risks. Read below how you can to convertψyou files avoiding the risk of infection with trojan.

We've all been in the situation where we can't open a file type that a friend or colleague sent us. For example, we expect some photos in JPEG or PNG format, but instead receive them in HEIC files. In this case, most people look for a free online file converter.

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Converting a file isn't just a matter of changing its extension — otherwise, you could simply rename the file from, say, EPUB to MP3. Instead, the conversion program has to read the file, understand what it contains, convert the data, and re-save it in a different format — and each of these phases carries its own risks.

Personal data leakage, malware and other risks

The first risk that comes to mind is the leakage of personal data . Even if you are one of those who says “but who could possibly need my data?”, you should be careful: Photos from your vacation may not interest anyone, but confidential business documents are a completely different matter. When you upload a file to an online converter, you can never be sure that the website will not save a copy of the file for its own purposes. The data you upload can easily end up in the hands of crooks and even be used to carry out an attack on your company. Being the entry point for attackers into the corporate network will certainly not make you popular with your company’s information security team.

If you think this threat only affects text or spreadsheet files, and that a photo of a financial statement can be safely uploaded and converted to a PDF, think again. Optical character recognition (OCR) was invented in the last century, and now, with artificial intelligence, even mobile trojans have learned to extract data of interest to attackers from photos in your smartphone gallery.

Another common risk is malware infection . Some questionable conversion websites can modify your files or add malicious code to them—something you won't be able to detect in time without reliable protection. After conversion, the files may contain malicious scripts, trojans, macros, and other dangerous files that we have analyzed many times.

Conversion websites can also be phishing sites , so you should be suspicious of services that ask you to register, enter personal information, and pay a fee to convert a file, for example from PDF to DOC. If you still plan to use an online converter, look for one that doesn't require registration and never enter your payment information.

How to convert files locally

The safest way is to convert files locally, i.e. on your own computer without using external websites. This way guarantees the confidentiality of the data — at least until you connect to the Internet. You can change the format of a file using either the system tools or well-known programs.

For text and spreadsheet files, as well as presentations , use the Microsoft Office features. Office can read many file formats with the File → Open or File → Import commands (depending on the version and operating system) and save them in different formats with the File → Save As → Save As (or File Format ) or File → Export commands . The list of available formats is long, from PDF and HTML to the standard OpenDocument.

If you don't have access to Microsoft products, you can use the free alternatives LibreOffice and OpenOffice , which also support various text and spreadsheet file formats. On Windows, you can also convert text documents with the built-in WordPad editor, although it reads far fewer file types.

For macOS users, Apple's office apps (Pages, Numbers, Keynote) recognize and save documents in many different formats.

When it comes to graphic files, things are even simpler. The operating system’s built-in tools can help convert images from PNG to JPEG. On Windows, just use the command in Paint: File → Save As . macOS users don’t even need to open any programs — just right-click the image in Finder and choose Quick Actions → Convert Image . The window that opens gives you options for the format (PNG, JPEG, HEIF) and size of the image you want to convert.

If the above options aren't enough — for example, if you're dealing with audio/video files or specific file formats — look for well-known offline tools with good reviews, that are free and use open source software.

For video files (and many audio files), it's worth checking out Handbrake (Windows, macOS, Linux) and Shutter Encoder (Windows, macOS, Linux). For audio files, try Audacity , and for images, ImageMagick (Windows, macOS, Linux).

With a simple graphical addition, most online media converters work like FFmpeg , which is probably the leading tool for converting file formats. Its only drawback (which for some is an advantage) is that it only works via the command line.

If you’re comfortable with the command line, FFmpeg is clearly the best option (and you probably already have it installed). Another great option for command line enthusiasts is Pandoc — a versatile text and markup format converter. Note that in the Extras section of the Pandoc website, you can find many external applications to add a graphical interface to the converter or integrate it with other editors, services, or even operating systems.

All of the above converters are FOSS (free and open source software) and support at least the most popular operating systems: Windows, macOS, Linux.

When choosing other offline converters, make sure that the conversion is actually done locally , as many tools simply provide the interface for online converters, but still send the source files to a server. This is very easy to check by disconnecting your computer from the internet before starting the conversion. If the tool does not work, then it is not an offline converter.

How to convert files online with the greatest possible security

Sometimes you can't avoid online converters — for example, when you receive files in a very strange or outdated format. The next section looks at how you can minimize the risks when converting files online. When using an online converter, the confidentiality of the files is unfortunately never guaranteed. Its creators can write whatever they want in the website's policies, but you will never know what really happens to your data after you upload it. So the golden rule is: Never convert sensitive information online.

If you have a Google account (and who doesn’t?), you can upload the file you want to convert to Google Drive (most Office file formats are supported), right-click and open it in Google Docs/Sheets/Slides, and then download it in a different format. Among other advantages, this method also works on mobile devices — although in this case it’s more convenient to open the file in Google’s corresponding editing tool.

Another fairly secure way to convert text or graphic files is Adobe's online converter . You can use it for free even on a smartphone — but beware: All the data you enter is stored on Adobe's servers, making this option unsuitable for sensitive files.

Follow these rules for maximum security when converting files online:

• Use reliable online converters.
• Open the converter website in a new browser window in Incognito mode: This will reduce the amount of information collected about you — but it will not nullify them.
• Use one reliable VPN to hide your real IP address from the converter website.
• Check the online converter's privacy policy to understand how it will handle your data. Make sure the service doesn't collect, store, or transfer information without your consent — or at least claims not to.
• Check that the files you are converting do not contain confidential information.
• Scan your converted files with antivirus software. Be very careful if the converter website asks you to download the final file — especially if it is password-protected, as this is the most common method of hiding a virus from security software. If you don't have antivirus software on your device (alas!), you can scan the downloaded file using online file scanner us.
• Avoid unverified websites that require registration and payment information.

Decompress Files, Instead of Converting

Finally, a little tip that few people know about: Sometimes you don’t even need to convert a file to another format, but simply extract the information from it. For example — extracting images from a text document or presentation in their original format. This usually takes a lot of time and is more difficult — since you have to extract the images one by one, and the editors may resize or compress them, resulting in worse image quality.

But there is a way around this problem. In reality, files in various formats are nothing more than compressed folders with subfolders that store the “puzzle pieces,” i.e. text, images, embedded videos, and more. All of this is in zip format. This means that almost all Office Suite files are ZIPs with the extension changed to DOCX, PPTX, PAGES, etc.

To extract all the contents from this "archive", you will simply have to rename it, changing its extension to ZIP, and then unzip it. The result will be a folder with subfolders in which all the original "components" of the document will be available and organized.

So if you see an unknown file format, first scan it for viruses with a reputable security solution, then make a copy of it, change the extension to ZIP (on macOS, if the file extension is hidden, you may need to press ⌘+I to change it), and try unzipping the file — in many cases this will work. Then, look inside the folder that will be created and you’ll discover a bunch of “goodies”!

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