ZeroFont phishing attack – Crooks are using a new technique that involves manipulating font sizes to bypass Office 365 protections.
According to cloud security firm Avanan, one of the detection mechanisms in Office 365 involves natural language processing to identify the content of the messages typically used in malicious emails.
For example, an email including the words “Apple” or “Microsoft” that are not sent from legitimate domains, or messages referencing user accounts, password resets or financial requests are flagged as malicious.
Experts from Avanan discovered phishing campaigns using emails in which some of the content is set to be displayed with zero-size font using <span style=”FONT-SIZE: 0px”>, for this reason, they dubbed the technique ZeroFont.
“Recently, we have been seeing a number of phishing attacks using a simple strategy to get their blatant email spoofs past Microsoft’s phishing scans. The tactic, which we are calling ZeroFont, involves inserting hidden words with a font size of zero that are invisible to the recipient in order to fool Microsoft’s natural language processing.” reads the analysis published by Avanan.
The email appears to the recipient as normal, but Microsoft’s filters are able to analyze also the text having a font size of “0”.
Summarizing, while the user sees a classic phishing content like this:
Microsoft’s filter will see the overall text including words written with “FONT-SIZE: 0px” attribute. This text, of course, doesn’t appear as a malicious content:
“Microsoft can not identify this as a spoofing email because it cannot see the word ‘Microsoft’ in the un-emulated version. Essentially, the ZeroFont attack makes it possible to display one message to the anti-phishing filters and another to the end user,” Avanan’s Yoav Nathaniel said in a blog post.
Natural language processing is essential to prevent phishing attacks, but a technique like ZeroFont demonstrated that attackers can bypass filters with a trick.
Pierluigi Paganini is member of the ENISA (European Union Agency for Network and Information Security) Threat Landscape Stakeholder Group and Cyber G7 Group, he is also a Security Evangelist, Security Analyst and Freelance Writer.
Editor-in-Chief at "Cyber Defense Magazine", Pierluigi is a cyber security expert with over 20 years experience in the field, he is Certified Ethical Hacker at EC Council in London. The passion for writing and a strong belief that security is founded on sharing and awareness led Pierluigi to find the security blog "Security Affairs" recently named a Top National Security Resource for US.
Pierluigi is a member of the "The Hacker News" team and he is a writer for some major publications in the field such as Cyber War Zone, ICTTF, Infosec Island, Infosec Institute, The Hacker News Magazine and for many other Security magazines.
Author of the Books "The Deep Dark Web" and “Digital Virtual Currency and Bitcoin”.