Introduction

Cybersecurity has entered a high-stakes phase as Microsoft 365, a critical platform for businesses worldwide, faces aggressive cyberattacks. Two dominant threats—email bombing and vishing attacks—have surged recently, exploiting communication tools within Microsoft 365 to disrupt operations and extract sensitive data. These attacks are not mere nuisances; they are sophisticated intrusions that threaten enterprise security at a foundational level.

Background and Technical Overview

What is Email Bombing?

Email bombing is a form of Denial of Service (DoS) attack targeting email systems. Attackers flood an inbox with a deluge of unwanted emails, rendering the email service unusable. The goal extends beyond inconvenience: it masks more insidious activities such as ransomware deployment or credential phishing by hiding them under a flood of junk emails.

Attackers use bots or scripts to send thousands of junk emails either directly or through compromised Microsoft 365 accounts. This overwhelms the inbox and security monitoring systems, making it difficult for users to identify legitimate communications, including critical security warnings.

What is Vishing?

Vishing, or voice phishing, represents the social engineering arm of these threats. Cybercriminals impersonate Microsoft support personnel, typically leveraging Microsoft Teams and Office 365 communications to initiate seemingly legitimate interactions. The attackers often use:

  • Calls via Teams or other platforms
  • Screen-sharing or remote control sessions to deceive victims
  • Fake “tech support” narratives to coax users into granting system access

Once access is gained, attackers deploy ransomware variants like Black Basta or Python ransomware, encrypting files and threatening data leaks unless a ransom is paid.

Exploiting Microsoft 365 Infrastructure

The attackers demonstrate deep technical knowledge by abusing default configurations and permitted mail flow rules in Microsoft 365:

  • They set up their own Microsoft tenancy, embedding payloads into the organization names.
  • Mail flow rules auto-forward malicious or deceptive invoices to thousands of recipients, bypassing many standard filters.
  • Emails maintain full authentication integrity (passing SPF, DKIM, and DMARC checks), increasing their legitimacy in the eyes of users and security gateways.

Furthermore, vulnerabilities arise from default Microsoft Teams settings allowing external users to initiate chats or meetings, which attackers exploit to impersonate support staff convincingly.

Implications and Impact

The combined use of email bombing and vishing attacks creates a perfect storm:

  • Operational disruption: Overloaded inboxes cause critical emails and alerts to be missed.
  • Credential theft and ransomware: Once inside the system, attackers can exfiltrate data and deploy ransomware.
  • Widespread impact: Over 15 incidents within three months and spikes in attacks highlight the growing scale.

Threat groups identified as STAC5143 and STAC5777 have been linked to these attacks. They reuse and refine tactics from older cybercriminal campaigns (like Storm-1811), making defense more challenging.

What Organizations and Users Should Do

  1. Harden Microsoft 365 Access: Enable Multi-Factor Authentication (MFA) on all accounts, restrict Teams calls and meetings to known users.
  2. Educate and Train: Update anti-phishing training to cover vishing and email bombing tactics explicitly.
  3. Monitor and Respond: Set up email rules to segregate suspicious emails and monitor for unusual activity.
  4. Patch and Maintain: Keep all Microsoft 365 software and platforms updated to reduce vulnerability.
  5. Use Indicators of Compromise (IoCs): Apply IoCs shared by security providers like Sophos to proactively detect ongoing threats.

Conclusion

Microsoft 365, as a backbone of modern business communication, is under siege by increasingly sophisticated cyberattacks combining technical prowess with psychological manipulation. As cybercriminals refine their playbooks, organizations must also evolve their defenses to protect against these multi-faceted threats.

Vigilance, education, and robust security practices are crucial to mitigating the risks posed by email bombing and vishing in the Microsoft 365 environment. Failure to do so not only jeopardizes data integrity but also threatens operational continuity in an era where communication platforms are indispensable.