Attackers Use Legacy IMAP Protocol to Bypass Multifactor Authentication in Cloud Accounts, Leading to Internal Phishing and BEC

Threats to cloud-based applications
have been growing, and passwords — the traditional method used to secure accounts — are often no longer enough to protect users from the dangers that they potentially face. The need for more comprehensive security in cloud-based applications has led to vendors offering multifactor authentication (MFA) as an integral feature of their products and services. By using MFA, users limit the risk that an attacker will gain control of their accounts by spreading authentication across multiple devices.

However, while MFA provides an additional layer of security for protecting account access, it’s not a fool-proof feature. For example, a recent study from Proofpoint examined brute-force attacks against user accounts in major cloud services. The attacks reportedly took advantage of legacy email protocols, phishing, and credential dumps to bypass MFA.

Notably, attackers were able to abuse legacy protocols — most commonly the IMAP authentication protocol — to bypass even multifactor authentication. The study noted that the IMAP protocol can be abused under certain situations, such as when users employ third-party email clients that do not have modern authentication support. IMAP abuse can also be performed in two other cases: when the targets do not implement applications passwords and when it is done against shared email accounts where IMAP is not blocked and/or MFA cannot be used. The report also said these attacks can often go undetected, instead looking like failed logins rather than external attempts. Threat actors use these accounts as entry points into the system, after which lateral movement is carried out via internal phishing and BEC to expand their reach within the organization.

The six-month study saw over 72 percent of cloud tenants being targeted at least once by attackers, while 40 percent had at least one compromised account within their system. Even more concerning, 15 out of every 10,000 active user accounts were successfully breached. Hijacked servers and routers were used as the main attack platforms, with the network devices gaining access to approximately one new tenant every 2.5 days during a 50-day period.

Roughly 60 percent of the tenants involved in the study that were using Microsoft Office 365 and G Suite were targeted with the password-spraying attacks via IMAP, and 25 percent fell victim to a successful breach.

As more companies across industries adopt cloud-based services, it’s expected that cybercriminals will go after accounts for cloud-based platforms. Once an account has been compromised, whether through hacking or brute force, the account could be used to communicate with executives and their staff. Internal BEC emails could trick the targets into transferring funds and personal or corporate data or downloading malicious files. Compromised email accounts, for example, had been found replying to email threads to deliver malware. These BEC attempts can be difficult to detect given that they come from legitimate (though compromised) email accounts.

A feature such as MFA is only one part of an effective multilayered security implementation. Organizations looking to boost their security can start with these best practices:

  • Passwords still have a role to play as a component of multifactor authentication. Ensure that users have passwords that are strong and regularly changed to stay protected from brute-force attacks. This could mean includes using at least 12 characters with a mix of upper and lowercase letters, numbers, and special characters. Ask users to avoid common or easily-guessable passwords or passwords that show obvious information such as names or birthdates.
  • Educate employees on how to identify phishing attacks. Common indicators that an email is a phishing attempt include suspicious-looking email addresses and the presence of misspellings and typographical errors.
  • Furthermore, attackers often try to make their phishing attempts as convincing as possible. Thus, users should avoid giving out personal and company information unless they are absolutely certain that the person or group they are communicating with is legitimate.

Given that cybercriminals use compromised accounts and internal BEC emails, organizations should also consider the use of security solutions designed to combat the growing threat. Trend Micro’s existing BEC protection uses AI, including expert rules and machine learning to analyze email behavior and intention. The new and innovative Writing Style DNA technology goes further by using machine learning to recognize the DNA of an executive’s writing style based on past written emails. Designed for high-profile users who are prone to being spoofed, Writing Style DNA technology can detect forged emails when the writing style of an email does not match that of the supposed sender. The technology is used by Trend Micro™ Cloud App Security™ and ScanMail™ Suite for Microsoft® Exchange™ solutions to cross-match the email content’s writing style to the sender’s by taking into account the following criteria: capital letters, short words, punctuation marks, function words, word repeats, distinct words, sentence length, and blank lines, among 7,000 other writing characteristics.


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