25% of Phishing Emails Sneak into Office 365: Report

One in every 99 emails is a phishing attack, and a new study shows 25% of those phishing attacks bypass default security measures built into Office 365, researchers reported today.

The data comes from Avanan's Global Phish Report, which analyzed 55.5 million emails sent to Microsoft Office 365 and Google G Suite accounts. They found roughly 1% of all messages are phishing threats that use malicious attachments or links as the attack vector. Of those, 25% were marked safe by Exchange Online Protection (EOP) built into Office 365 and delivered to users.

Cloud-based email has rung in a new era of phishing, explains Yoav Nathaniel, Avanan lead security analyst and report author. "The connected nature of cloud email allows an attacker to get access to a bigger bounty from a single successful phishing attack since the credentials give them access to other connected accounts such as cloud file sharing or cloud HR," he says.

Of 55.5 million total emails analyzed, 561,947 were phishing attacks. Researchers broke the malicious messages into four categories: over half (50.7%) had malware, 40.9% were harvesting credentials, 8% were extortion emails, and 0.4% were spearphishing attempts.

Researchers scanned about 52.4 million emails directed to Office 365, of which 546,427 (1.04%) were phishing emails. They only analyzed 3.12 million emails for G Suite, of which 15,700 (0.5%) were phishing emails. In the report, researchers note how the messages were scanned after they had gone through default security but before they were delivered to users' inboxes.

They then took a closer look at how phishing emails were classified by Office 365 EOP, Microsoft's cloud-based filtering service. In EOP, emails are first sent through connection filtering, which verifies the sender's reputation and scans for malware. Most spam is deleted here, Microsoft says. Messages continue on through policy filtering, where they're evaluated against custom rules admins can create and enforce. They're also passed through content filtering, where they're checked for words and properties associated with spam. Based on settings, spam can be redirected to the Junk folder or quarantined.

After going through these layers, messages deemed benign are delivered to the inbox.

Avanan reports of the phishing emails that made it through EOP, 20.7% were marked as phishing emails and about half (49%) were marked spam. About 5% were whitelisted by admin configurations, and 25% were marked clean and successfully sent to the target user.

Bypassing Filters

How do some emails sneak through? Nathaniel says part of the reason is obfuscation, which rely on emails being displayed to end-users differently than how they appear to the machine-based security layer. Obfuscation comes in different ways: rare but legitimate email formats, which aren't properly analyzed by security but are delivered to inboxes; malformed emails and attachments that parse HTML to confuse the security layer but appear safe to the email client; and hidden characters in the email body, which are intended to trick the security filer.

Obfuscation makes up "quite a small number of attacks," says Nathaniel. "We see them targeting extremely high-profile individuals … they save it for special occasions." This may include targeting a CEO or C-suite executives of Fortune 500 companies, using attacks they don't want to land in a Junk folder.

Analysts also pulled data on different characteristics of phishing emails, which yielded some interesting data. For example, 35% of messages containing links to WordPress websites are phishing attacks. "Just the fact that it sent you a link to a WordPress site already makes the email suspicious," Nathaniel points out. And Bitcoin wallet links are almost a sure red flag: 98% of messages with cryptocurrency wallet links turn out to be malicious, researchers learned.

"It's important to note there's rarely a legitimate reason to send a cryptowallet address via email," he continues. This is typically done via text, or money is sent using an app.

Finally, the report notes out of every 25 branded emails, it's likely at least one is a phishing attempt. Microsoft is the most frequently impersonated brand (43%) for most of the year, followed by Amazon (38%), which takes the lead during the holiday shopping season.

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