Dangerous-Files

Note: For a better browsing experience we give the "answer" here at the beginning. See below for mora more detailed explanation onof thethis problem here.threat.

What file types can be dangerous

Although none of the listed file types are dangerousmalicious per se, some of themthey are especiallyoften vulnerableused by attackers to attacks"hide" ofmalware. thisMost kind. For examplecommonly:

• LibreOffice:
  • .odt: Text documents (Writer)
  • .ods: Spreadsheets (Calc)
  • .odp: Presentations (Impress)
  • .odg: Drawings (Draw)
  • .odb: Databases (Base)
• Microsoft Office:
  • .docx: Word documents
  • .xlsx: Excel spreadsheets
  • .pptx: PowerPoint presentations
• .pdf: PDF's
• even image formats like .gif have reportedly been exploited, also on mobile devices.

[!tip]

It is recommended to avoid unnecessarythe complexabove "complex" file types if they are not necessary. If you still have to open such a file from an untrusted source, we recommend useing Dangerzone.

Use text files and markup languages like markdown if possible instead.

Why this matters

[!note] It is a matter of security culture to think,reconsider whetherif itit's isreally necessary to putsend youran invitation text insideas a PDF or a draft of a press release inside a LibreOffice or Microsoftas Word document. If it is enough to use the possibilities, that safe markup languages like markdown give you, them use just them.

Markdown is even compatible to collaborative tools like e.g. Nextcloud.

In many contexts we see,see that people are kind of ashamed of sending e.g. plain text invitations.invitations for example. They feel that they ownowe their friends some more effort than just text. While this shows a pretty nice property of friendship, we also have to talk about the problems that this brings along and that it might be worth it to break this behavior down towardtowards a more conscious approach.

What is a file type

Different programs expect their files to have a specific format. They expect the files to follow a pattern that the program know and recognizerecognizes to function correctly.

Each file type is typically identified by a specific extension (such as .odf, .pdf, .jpg), which signals to the operating system what program should open it and how it should behave. For example, if you click on a file that ends with .pdf, the operating system knows that is has to open the file with a PDF reader and not with you music player.

How can files be dangerous

Consider a simple text file (not a word document, but a simple plain text file!). A normal text file contains, no surprise, text, which is nothing else than characters, like "A", "a", ";", "/" and so on. Those text file can be read and displayed from simple programs like UbuntusGnome's "gedit", Windows notepad, aso.and Simpleso ason. they are, theyThey are not capable of advanced featuresfeatures, likesuch e.g.as calculating tables, like Excel, or LibreCalc.

More advanced programs like e.g. mentioned Excel, PowerPoint, or modern PDF viewers are capable of much more advanced features. PDF viewers for example can display interactive forms, that you can fill out right inside the PDF viewer. TheThey can have drop-down menus and more.

[!caution] This means, that thisyour PDF viewerviewer, isPowerPoint, Excel etc. are able to execute additional code, that is delivered inside the PDFfile itthey isare processing.

While this is necessary to use tothe full feature set of the program, thisthe capability to execute additional code can expose severe security risks.

You probably all heard about viruses being distributed through PDFs. This is exactly what is exploited here:

[!note] An attacker smugglescan smuggle some malicious code inside the PDF. You open the PDF with your PDF viewer. The PDF viewer detects some code and thinks: "Ah, I have to execute this, so that the user has the full functionality of this file" and executes the code.code, Thenwhich can then perform malicious actions such as stealing your data and sending it to the code itself could infect the machine.attacker.