There has been a marked increase in email campaigns using malicious PDF files to distribute malware, rather than the typical uses of PDF files for obtaining sensitive information such as login credentials.

Increased security measures implemented by Microsoft have made it harder for cybercriminals to use macros in Office documents in their email campaigns, with PDF files a good alternative. Malicious links can be embedded in PDF files that drive victims to web pages where credentials are harvested. By using PDF files to house the links, they are less likely to be blocked by email security solutions.

Over the past few months, PDF files have been increasingly used to distribute malware. One of the currently active campaigns uses malicious emailed PDF files to infect users with DarkGate malware. DarkGate malware is offered under the malware-as-a-service model and provides cybercriminals with backdoor access to infected devices. In this campaign, emails are sent to targets that contain a PDF attachment that displays a fake image from Microsoft OneDrive that suggests there was a problem connecting which has prevented the content from being displayed. The user is given the option to download the PDF file; however, the downloaded files will install DarkGate malware.

In this campaign, clicking the link does not directly lead to the malware download, instead, the click routes through an ad network, so the final destination cannot be identified by checking the link of the download button. Further, since the ad network uses CAPTCHAs, the threat actors can make sure that the destination URL is not revealed to email security solutions. If the CAPTCHA is passed, the user will be redirected to the malicious URL where they can download the file.  This is often a compressed file that contains a text file and a URL file, with the latter downloading and running JavaScript code which executes a PowerShell command that downloads and executes the malicious payload.

PDF files have been used in many other malware campaigns, including those that distribute the Ursnif banking Trojan and WikiLoader malware. Recent campaigns distributing these malware variants have used parcel delivery lures with PDF file attachments that contain a link that prompts the user to download a fake invoice. Instead of the invoice, a zip file is downloaded that contains a JavaScript file. If executed, the JavaScript file downloads an archive, extracts the contents, and executes the malware payload. Another campaign uses PDF files to install the Agent Tesla remote access trojan using lures.

Not only do PDF files have a greater chance of evading email security solutions, they are also more trusted by end users than Office file attachments. Security awareness campaigns are often focused on training employees about the risks of phishing, such as clicking links in unsolicited emails and the risks of opening unsolicited office files. Malicious email campaigns using PDF files arouse less suspicion and end users are more likely to be tricked by these campaigns.

It is important for businesses to incorporate PDF files into their security awareness training and phishing simulation campaigns to better prepare employees for this growing threat. With SafeTitan, adding new content in response to the changing tactics, techniques, and procedures of threat actors is a quick and easy process. Get in touch with the TitanHQ team today to find out more about the SafeTitan security awareness training and phishing simulation platform and discover the difference the solution can make to your organization’s security posture.