Over the past several months, threat actors have been increasingly targeting Office 365 and G Suite cloud accounts that are using the legacy IMAP protocol, in an attempt to bypass multi-factor authentication (MFA), Proofpoint reports.
Targeted brute-force attacks have increased in sophistication over the past months, attempting to compromise accounts using variations of the usernames and passwords exposed in large credential dumps, and phishing campaigns continued to provide additional avenues into corporate accounts.
An analysis of over one hundred thousand unauthorized logins across millions of monitored cloud accounts revealed that more than 2% of the user accounts were targeted, and that 15 in 10,000 were successfully breached.
Nearly three quarters (72%) of the cloud service tenants were targeted at least once and 40% of them had at least one compromised account in their environment, Proofpoint says.
Provided that the initial target does not have the access needed to move money or data, the attackers mainly aim to leverage compromised accounts for internal phishing or internal BEC attacks, which are more difficult to detect compared to external phishing attempts.
The security firm also noticed that 40% of all successful attacker logins originate from Nigerian IP addresses (their number increased between November 2018 and January 2019 by 65%), followed by Chinese IP addresses, at 26%. The United States, Brazil, and South Africa were also major attack sources.
IMAP emerged as the most commonly abused legacy protocol in these attacks, as it bypasses multifactor authentication and allows attackers to avoid account lock-out. Service accounts and shared mailboxes are particularly vulnerable, Proofpoint says.
The security firm says over half (60%) of Microsoft Office 365 and G Suite tenants were targeted with IMAP-based password-spraying attacks, which resulted in around 25% of Office 365 and G Suite tenants experiencing a successful breach. Overall, the attack success rate was at 44%.
Proofpoint said it observed a large number of IMAP-based password-spraying campaigns between September 2018 and February 2019. Ten percent of active user accounts in targeted tenants were hit and 1% of targeted user accounts were successfully breached.
“Attackers utilized thousands of hijacked network devices around the world — primarily vulnerable routers and servers — as operational attack platforms. These hijacked devices gained access to a new tenant every 2.5 days on average during a 50-day period,” Proofpoint reports.
China was the source for most IMAP-based attacks (53%), followed by Brazil (39%) and the United States (31%). The attacks, however, often originated from multiple geographies.
The affected organizations are from various industries and countries, with K-12 and higher education sectors being most vulnerable. Over 13% of successful attacks were aimed at educational institutions, and 70% of all educational institutions’ tenants experienced breaches from these IMAP-based brute force attacks.
Following email phishing campaigns, threat actors use the stolen credentials to infiltrate users’ cloud application accounts and Proofpoint says 31% of all cloud tenants were subject to breaches originating from successful phishing campaigns. Retail, finance, and technology were also targeted.
Most of the attacks (63%) originated from Nigerian IP addresses, followed by South African infrastructure (21%), and the United States via VPNs (11%).
“Attackers parlay successful compromises into internal phishing attacks, lateral movement in organizations, and additional compromises at trusted external organizations. Organizations need to implement layered, intelligent security measures – including user education – to combat these evolving threats that are increasingly successful in compromising user cloud accounts,” Proofpoint concludes.
Related: G Suite Admins Can Now Disable Phone 2-SV
Related: Attackers Are Landing Email Inboxes Without the Need to Phish
Ionut Arghire is an international correspondent for SecurityWeek.
Tags: 
