Microsoft Warns of Emails Bearing Crafty PDF Phishing Scams

Instead of trying to cram malware into inboxes, attackers are increasingly using PDF-based social engineering schemes to trick victims into handing over sensitive data or email login passwords.

PDF Email Phishing

On the heels of a disturbingly convincing Gmail phishing scam, Microsoft is warning email users of other crafty schemes, this time involving PDF attachments.

PDF, short for the Portable Document Format pioneered by Adobe, is a popular method of distributing content online. Cyber-attackers are banking on its ubiquity, particularly in the workplace, to ensnare office workers.

The latest phishing attempts may slip through an antivirus software's defenses. "Unlike in other spam campaigns, the PDF attachments we are seeing in these phishing attacks do not contain malware or exploit code," blogged Alden Pornasdoro, Microsoft Malware Protection Center team member.

"Instead, they rely on social engineering to lead you on to phishing pages, where you are then asked to divulge sensitive information," continued Pornasdoro. Sometimes spoofing real employees at legitimate companies, one attack involves sending a product or service quote as a file attachment (Quote.pdf). Once opened, the PDF file, crafted to mimic an error message, leads users to an online login page that offers access to the ostensibly confidential information contained in the PDF file.

It's a ruse, of course. The site exists to collect email account credentials. Microsoft has issued a Windows Defender threat signature (Trojan:Win32/Pdfphish.BU), which the company classifies as "Severe," to help protect users of the built-in antivirus software in Windows. Similarly, the SmartScreen technology used by the Edge browser in Windows 10 will alert users attempting to reach fraudulent sites.

Another phishing scam attempts to exploit the growing use of cloud file storage and collaboration services like Dropbox.

Similar to the example above, the PDF attachment (ScannedbyXerox.pdf) falsely notifies users that they are attempting to access a secure document stored on Dropbox. If they click on the embedded link, they are taken to a fake Dropbox login page that collects Google, Office 365,, AOL and Yahoo account credentials.

Again, Windows Defender can detect this threat (PWS:HTML/Misfhing.B) and SmartScreen will splash a warning message in the Edge browser.

To combat these threats, Microsoft is advocating awareness. The company is advising users to cast a suspicious eye on email attachments and links, even if they seem to come from known colleagues.

"In these times, when we're seeing heightened phishing attacks with improved social engineering techniques, a little bit of paranoia doesn't hurt," said Pornasdoro. "For instance, question why Adobe Reader is trying to open an Excel file. Ask why Dropbox is requiring you to enter your email credentials, not your Dropbox account credentials."

Security vendor Sophos is taking a more direct approach to phishing awareness and education.

This week, the company launched its Phish Threat attack simulator. Backed by Sophos Central, the company security management and advanced threat detection platform, the service mimics phishing attempts using the latest tactics employed by attackers and gauges how employees respond to them.

Acquired in November 2016 from Silent Break Security, the technology is used to educate users, making them less prone to falling for phishing scams over time.

Pedro Hernandez

Pedro Hernandez

Pedro Hernandez is a contributor to eWEEK and the IT Business Edge Network, the network for technology professionals. Previously, he served as a managing editor for the network of...