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How A PDF Can Contain Malware

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A PDF is one of the most frequently used file formats for sending documents, commonly used everywhere, a successful replacement for print-outs, and favourite means of attack among malicious actors. PDF files are not easily edited in comparison to rich text files and cannot just be opened and altered.

A PDF is typically used for contracts, bank statements, and other important documents that need to be signed and sent. Unfortunately, a PDF file can contain a virus, but it’s not the text or illustrations that harm your device.

Instead, it is everything else within these files, combined with software, that triggers the threats. The “fill & sign” abilities of PDFs are where the danger is located, working from inside with the help of special scripts. This article will discuss how malicious code can be lying dormant within a PDF file and how PDFs need proactive, multi-tiered protection against viruses and malware to keep them from infecting your device.  

How PDFs Can Contain A Virus

Security threats come in a number of different ways. PDFs may sometimes be embedded with a code allowing documents to be signed and edited and may also contain viruses. A virus is a program that may change or delete data, while trojans typically gather information on a user or their device.

This is comparable to Microsoft Word files that are infected with viruses as the malware is hidden inside macros scripts, and the infected PDF file contains malicious JavaScript code. Viruses, trojans, and malware can often be found in email downloads or attachments like eBooks and other documents, and sent from unknown or unfamiliar senders.

How a PDF Can Contain Malware

Because PDF files have the ability to execute code on your device, dynamic and static elements can be manipulated to inject malicious scripts, such as: 

  • Javascript: Javascripts are used in coding to control browser appearance and functionality and have previously been used to exploit vulnerabilities in Adobe and other PDF readers.
  • Hidden Objects: PDFs can have embedded and encrypted objects that are executed when a file is opened by the user, which prevents antivirus scanners from analyzing them.
  • Multimedia Control: Embedded objects in a PDF can also be a quicktime media or flash file, which have vulnerabilities attackers can exploit.

How An Infected PDF Can Contaminate Your Device

As PDFs often contain scripts for extended capabilities, such as the fill and sign functionality, they can also display the date, add print buttons, and format data. Unfortunately, hackers will likely use more sophisticated methods to add malicious code to PDF files.

PDF readers are an unwilling accomplice of hackers due to the apps themselves or their plugins being able to run the injected code. Third-party plugins in PDF-reading software can also be a gateway for malicious scripts.

Another method attackers will use is PDF phishing, an approach where emails are sometimes executed more efficiently than generic phishing attempts and target specific recipients. The content of the email won’t have suspicious links, instead will contain files with hidden viruses in the scripts, download links to malware files, or have a trojan virus disguised as a PDF in the attachment.

How To Protect From A PDF Virus

Should you receive a PDF from a suspicious sender, scanning the document for viruses is vital. It is possible to extract a safe copy of the file, but sometimes it’s best not to open the document. Some other tips to prevent an infection include:

  • Disable JavaScript on your PDF reader.
  • Do not allow PDF readers to execute Non-PDF files using external applications.
  • Disable PDF reader from Startup programs of Windows.
  • Keep Macros disabled. Malicious files might persuade you to enable them, but you should not unless very much necessary.
  • Do not download or open file attachments sent by unknown email senders.
  • Ensure Windows OS, PDF reader program, and Antivirus is up to date.
  • Backup regularly and keep it encrypted.
  • Be cautious when clicking links in PDFs from unknown senders.

The Bottom Line

Regardless of whether or not a PDF is infected, it does not solely depend on the file extension, but also depends on the vulnerabilities in the software. Because a PDF reader may potentially contain a buffer overflow vulnerability, an attacker can construct a special PDF file for exploiting that vulnerability.

A PDF can contain a virus hidden in multimedia files, JavaScript code, hyperlinks, or system commands. The virus can be triggered in your system by opening the file or executing specific tasks.

To prevent malicious actors from infecting your device, you can test the PDF for malware and protect your systems from infection using best cybersecurity practices.


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Marie Beaujolie
Marie Beaujolie
Marie B. is a computer network engineer by day, and a versatile writer by night. She spends more time in Paris than every other city.
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