We are still in testing and will be updated soonĬontact us if you want us to help you test it Meanwhile, Proofpoint was affected by the issue at the time of its discovery. Gmail users and users protecting their Office 365 with Mimecast were not vulnerable to the attack. If you are protecting Office 365 with Mimecast, you are secure. Proofpoint is also vulnerable – if you are using Proofpoint you also have this problem,” continues the post. If you are using Gmail, you don’t have this issue. “We have tested the vulnerability on several configurations and found that anyone using Office 365 in any configuration is vulnerable.
Tests of the baseStriker attack technique demonstrated that Office 365 users were vulnerable. The following image shows a traditional phishing link that is blocked by the filter because the URL is classified as malicious and a link that is split using the BaseStriker attack technique.įigure 2 – Link split with the BaseStriker attack technique “The attack sends a malicious link, that would ordinarily be blocked by Microsoft, past their security filters by splitting the URL into two snippets of HTML: a base tag and a regular href tag.” “The name baseStriker refers to the method hackers use to take advantage of this vulnerability: splitting and disguising a malicious link using a tag called the URL tag,” reads the analysis published by Avanan. Otherwise, the user is redirected to the original link.īaseStriker attack technique leverages the URL tag in the header of an HTML email to split and disguise a malicious link. If the scan detects suspicious activity, it then warns users. When the user clicks on a link included in an incoming email, it first redirects the user to a domain owned by Microsoft and used to checks the original URL for anything suspicious. The security feature works by replacing all URLs in an incoming email with Microsoft-owned secure URLs.
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The Safe Links feature is designed by Microsoft to protect Office users from malicious codes and phishing attacks it is part of Microsoft’s Advanced Threat Protection (ATP).īeginning in late October 2017, ATP Safe Links protection is being extended to apply to web addresses (URLs) in email as well as URLs in Office 365 ProPlus documents, such as Word, Excel, PowerPoint on Windows, iOS, and Android devices, and Visio files on Windows. Researchers at cloud security company Avanan discovered in May a technique, dubbed baseStriker that was used by attackers to bypass the Safe Links security feature implemented in Microsoft Office 365. baseStriker attack technique allows bypassing Microsoft Office 365 anti-phishing filter
Natural language processing is essential to prevent phishing attacks, but a technique like ZeroFont demonstrated that attackers could bypass filters with a trick. 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. “Microsoft can not identify this as a spoofing email because it cannot see the word ‘Microsoft’ in the un-emulated version. This text, of course, doesn’t appear as a malicious content if analyzed with natural language processing: Microsoft’s filter will see the overall text including words written with “FONT-SIZE: 0px” attribute. Summarizing, while the user sees a classic phishing content like this:
The content of the email is composed to be a phishing message, but Microsoft’s filters are not able to detect it because the attackers have introduced a font size text that alters the text making it harmless to the security mechanisms. 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. “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 researchers from Avanan have recently discovered phishing campaigns using emails in which some of the content is set to be displayed with zero-size font using, for this reason, they dubbed the technique ZeroFont.
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One of the detection mechanisms implemented by Microsoft in Office 365 leverages the natural language processing for the identification of the content of the email messages associated with malicious campaigns.įor 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.