Microsoft 365 has become the backbone of communication for many businesses. Email, file sharing, Teams conversations, invoices, contracts, and customer communications often flow through a single platform.
Unfortunately, cybercriminals know this.
Most Microsoft 365 hacks and breaches don't involve sophisticated hacking techniques. Instead, they start with common security gaps that allow attackers to gain access to user accounts, monitor communications, and exploit trust within an organization.
For businesses in St. Louis and beyond, understanding how these attacks happen is the first step toward preventing them.
6 Common Ways Businesses Get Hacked Through Microsoft 365
Most Microsoft 365 breaches can be traced back to a handful of common attack methods. In many cases, cybercriminals aren't exploiting advanced vulnerabilities—they're taking advantage of weak security practices, compromised credentials, and human error. Here are six of the most common ways businesses lose control of Microsoft 365 accounts and data.
1. Business Email Compromise (BEC): When Attackers Use Trusted Email Accounts Against You
Many business owners picture cyberattacks as someone breaking through a firewall or deploying sophisticated malware.
In reality, some of the most costly attacks begin with something much simpler: access to a trusted email account.
Business Email Compromise (BEC) occurs when cybercriminals gain access to a legitimate business email account and use that access to deceive employees, customers, vendors, or financial institutions. Because the messages come from a real account, they often appear completely legitimate.
Unlike traditional phishing scams, BEC attacks may go unnoticed for days, weeks, or even months. Attackers often spend time monitoring email conversations, learning how a company operates, identifying financial processes, and waiting for the right opportunity to act.
Once they understand the flow of communication, they may attempt to redirect payments, change banking information, request wire transfers, or obtain sensitive business data.
Business Email Compromise is also known by several other names, depending on the tactic being used, including:
Email Account Compromise (EAC)
CEO Fraud
Vendor Email Compromise (VEC)
Man-in-the-Email Attacks
Payroll Diversion Scams
Common Examples of Business Email Compromise
CEO Fraud
An attacker impersonates a company executive and sends an urgent request to accounting or finance personnel.
Example: A controller receives an email that appears to come from the company president requesting an immediate wire transfer for a confidential business matter.
Vendor Email Compromise (VEC)
Attackers gain access to a vendor's email account and alter payment instructions on legitimate invoices.
Example: A St. Louis business receives an invoice from a trusted supplier. The invoice looks legitimate, but the banking information has been changed to an account controlled by the attacker.
Man-in-the-Email Attacks
A Man-in-the-Email (or email eavesdropping) attack is a form of Man-in-the-Middle (MITM) attack where a cybercriminal secretly intercepts and monitors email communications between two or more parties.
Example: An attacker observes discussions between a company and a customer regarding a large payment. Just before payment is due, they send updated payment instructions from the compromised account.
Payroll Diversion
Human resources and payroll personnel are frequent targets because they have access to employee compensation information.
Example: An employee's email account is compromised and used to request a direct deposit change, causing future payroll payments to be sent to a fraudulent bank account.
The common thread in all of these attacks is trust. Rather than exploiting software vulnerabilities, attackers exploit legitimate business relationships and communications that employees already trust.
For many organizations, a compromised Microsoft 365 account can quickly become a financial problem long before anyone realizes there has been a security incident. Because BEC attacks frequently result in financial losses, many organizations are also reviewing their cyber insurance readiness and security controls to ensure they meet policy requirements and reduce overall risk.
2. Credential Theft Through Phishing Emails
Many Business Email Compromise attacks begin with a phishing email.
Phishing is a type of cyberattack designed to trick users into revealing sensitive information, including Microsoft 365 usernames, passwords, and multifactor authentication codes. Rather than targeting software vulnerabilities, phishing attacks target people.
A typical phishing email may appear to come from Microsoft, a coworker, a vendor, a customer, or another trusted source. The message often creates a sense of urgency by claiming an account has been locked, a password is expiring, or a document requires immediate review.
Common phishing email examples include:
Fake Microsoft 365 sign-in requests
Password expiration notices
Shared document notifications
Invoice or payment requests
Messages claiming suspicious account activity
Fake voicemail or Teams notifications
The goal is to convince the user to click a link and enter their credentials into a fraudulent website that closely resembles a legitimate Microsoft login page.
Once credentials are stolen, attackers can gain access to email, OneDrive, Teams, SharePoint, and other connected Microsoft 365 services. In many cases, users are unaware that their account has been compromised until suspicious emails are sent from their account or fraudulent activity is discovered.
Modern phishing campaigns have become increasingly sophisticated.
Attackers often research organizations in advance, personalize messages, and use branding that closely matches legitimate companies. Some attacks are so convincing that even experienced employees can be fooled.
This is why businesses should view phishing prevention as more than a user training issue. Strong email filtering, multifactor authentication, account monitoring, and proactive Microsoft 365 security management all play an important role in reducing risk.
Organizations should also ensure that laptops, desktops, and mobile devices accessing Microsoft 365 are protected through effective endpoint management and device security practices. A compromised device can make it easier for attackers to steal credentials, capture sessions, or gain persistent access to business systems.
For businesses without a dedicated security team, ongoing managed IT services can help identify suspicious activity, strengthen security controls, and reduce exposure to common phishing threats.
3. MFA Fatigue and Session Hijacking Attacks
Many businesses enable multifactor authentication (MFA) and assume their Microsoft 365 accounts are fully protected.
While MFA remains one of the most effective security controls available, attackers have developed techniques designed to work around it.
One common tactic is known as MFA fatigue.
In this type of attack, cybercriminals repeatedly attempt to sign in using stolen credentials, triggering a flood of authentication requests to the user's phone or authenticator app. After receiving multiple prompts, some users eventually approve a request simply to stop the notifications.
Another growing threat involves session hijacking.
When a user successfully signs in to Microsoft 365, the platform creates a trusted session that allows them to remain logged in without repeatedly entering their credentials. If an attacker can steal that session, they may be able to access the account without needing the user's password or MFA approval again.
This can occur through malware, compromised devices, malicious browser extensions, or advanced phishing attacks that capture authentication tokens during the login process.
The result is the same: an attacker gains access to a legitimate Microsoft 365 account while appearing to be an authorized user.
These attacks are particularly challenging to detect because they often originate from valid accounts and may not trigger the same alerts associated with traditional login attempts. In some cases, organizations don't discover the compromise until suspicious emails are sent, files are accessed, or financial transactions are questioned.
Protecting against these threats requires more than simply turning on MFA.
Businesses should regularly review sign-in activity, monitor for unusual behavior, enforce conditional access policies, and maintain strong Microsoft 365 security management practices.
It's also important to remember that many session hijacking attacks originate on infected or unmanaged devices. Implementing effective endpoint management and device security controls can help reduce the risk of attackers gaining access to authentication tokens and user sessions.
For organizations with limited internal IT resources, co-managed IT services can provide additional oversight and monitoring to help identify suspicious account activity before it develops into a larger security incident.
4. Weak Passwords and Password Reuse
Despite years of security warnings, weak passwords remain one of the most common ways attackers gain access to Microsoft 365 accounts.
Many users continue to choose passwords that are easy to remember, reuse the same password across multiple websites, or make only minor variations when updating credentials. While these habits may seem harmless, they can create significant security risks for businesses.
How Credential Stuffing Attacks Work
When a website or online service experiences a data breach, stolen usernames and passwords often become available to cybercriminals. Attackers then use automated tools to test those credentials against other services, including Microsoft 365. This technique is commonly known as credential stuffing.
Example: An employee may use the same password for a personal shopping account and their Microsoft 365 account. If that shopping website suffers a breach, attackers may be able to use the exposed credentials to access company email, files, and business applications.
Even organizations with password policies in place can be vulnerable if employees reuse passwords across personal and business accounts or choose passwords that are predictable and easy to guess.
Why Administrator Accounts Are High-Value Targets
The risk becomes even greater when privileged accounts are involved. Administrative accounts often have access to user management, security settings, email systems, SharePoint environments, and other critical Microsoft 365 services. A compromised administrator account can quickly lead to a much larger security incident.
A compromised administrator account can turn a single stolen password into a company-wide security incident.
Strong password policies, password managers, and multifactor authentication all help reduce the likelihood of unauthorized access. Organizations should also regularly review privileged accounts and security settings as part of their overall Microsoft 365 security management strategy.
Businesses that support remote workers and hybrid environments should pay particular attention to account security. Combining strong password policies with endpoint management and device security helps reduce the risk of compromised credentials being used to access corporate systems.
5. Excessive Permissions and Misconfigured Microsoft 365 Access
Not every Microsoft 365 security incident begins with a sophisticated attack. In many cases, attackers take advantage of permissions and access settings that were never properly reviewed.
As organizations grow, Microsoft 365 environments often become more complex. Employees change roles, new applications are connected, shared mailboxes are created, and temporary access is granted to support projects or business needs. Over time, these permissions can accumulate and create unnecessary security risks.
If an attacker gains access to a single account, excessive permissions can make it much easier to access additional data, sensitive documents, or administrative functions.
Common examples include:
Former employees who still have active accounts
Users with administrative privileges they no longer need
Shared mailboxes with broad access permissions
Third-party applications connected to Microsoft 365 without proper oversight
Teams, SharePoint, or OneDrive resources shared more broadly than intended
Example: A compromised user account may initially provide access only to email. However, if that user has unnecessary permissions across SharePoint sites, Teams channels, or shared file repositories, attackers may be able to access significantly more information than expected.
Misconfigured access controls can also create compliance concerns for organizations that handle financial records, client information, healthcare data, or other sensitive business information.
The problem isn't always who gets in. It's what they're allowed to access once they do.
Access Reviews Help Reduce Risk
This is why businesses should periodically review user permissions, administrative roles, group memberships, and connected applications. Regular Microsoft 365 administration and security reviews can help identify accounts, permissions, and configurations that no longer align with business needs.
Organizations that rely heavily on collaboration tools should also evaluate how files, folders, and resources are being shared across Microsoft Teams, SharePoint, and OneDrive. Improper sharing settings can expose sensitive information even when user accounts have not been directly compromised.
For businesses that lack the time or expertise to perform these reviews internally, managed IT services and ongoing Microsoft 365 support can help ensure access controls remain aligned with security best practices.
6. Compromised Devices and Endpoint Security Gaps
Microsoft 365 security doesn't stop at the login screen.
Even with strong passwords, multifactor authentication, and properly configured security settings, a compromised laptop, desktop, or mobile device can provide attackers with another path into business systems.
Cybercriminals often target endpoints because they are used every day to access email, files, Teams conversations, and other Microsoft 365 services. If a device becomes infected with malware, attackers may be able to capture credentials, steal authentication tokens, install remote access tools, or monitor user activity.
In some cases, employees may not realize a device has been compromised. The attacker can continue operating in the background while accessing Microsoft 365 resources through what appears to be a legitimate user session.
Common risks include:
Malware infections
Unpatched operating systems and software
Unauthorized applications or browser extensions
Lost or stolen devices
Personal devices accessing business data without proper security controls
These risks have become more common as organizations adopt Microsoft 365 security remote for and hybrid work environments. Employees frequently access Microsoft 365 from multiple locations and devices, making it more important than ever to maintain visibility and control over endpoint security.
Microsoft 365 Security Requires Endpoint Protection
A strong security strategy should include more than account protection alone. Businesses should combine Microsoft 365 security controls with effective endpoint management and device security to help protect the devices that connect to business systems every day.
A secure Microsoft 365 account can still be vulnerable if the device accessing it is compromised.
For many businesses, the most effective approach is a layered strategy that combines Microsoft 365 security, endpoint protection, proactive cybersecurity monitoring, and ongoing managed IT services to reduce risk across the entire technology environment.
How Businesses Can Reduce Microsoft 365 Security Risks
While the attack methods discussed above may sound concerning, the good news is that most Microsoft 365 compromises can be prevented through a combination of security controls, user awareness, and ongoing management.
Businesses can reduce risk by focusing on a few key areas:
Enforce multifactor authentication for all users
Use strong, unique passwords and password managers
Regularly review user permissions and administrative access
Monitor sign-in activity and investigate suspicious behavior
Keep laptops, desktops, and mobile devices updated and protected
Remove unused accounts and unnecessary third-party integrations
Train employees to recognize phishing attempts and social engineering tactics
Conduct regular security reviews of Microsoft 365 settings and configurations
It's also important to remember that Microsoft 365 security is not a one-time project. New threats emerge, employees change roles, devices are added, and business requirements evolve over time. Security settings that were appropriate a year ago may no longer provide adequate protection today.
This is why many organizations invest in ongoing Microsoft 365 security management rather than relying solely on default configurations. Regular reviews, monitoring, and proactive maintenance can help identify potential risks before they become costly security incidents.
Businesses should also view Microsoft 365 security as part of a broader technology strategy. Combining Microsoft 365 protections with endpoint management and device security, cloud IT management, and ongoing managed IT services can help create multiple layers of defense against today's most common cyber threats.
The goal isn't to eliminate every possible risk. It's to make your organization a much harder target and improve your ability to detect and respond to suspicious activity before significant damage occurs.
Frequently Asked Questions About Microsoft 365 Security
What Is Another Name for Business Email Compromise?
Business Email Compromise (BEC) may also be referred to as Email Account Compromise (EAC), CEO fraud, Vendor Email Compromise (VEC), Man-in-the-Email attacks, or payroll diversion scams.
While the tactics vary, they all involve attackers using email to impersonate trusted individuals or organizations in an attempt to steal money, sensitive information, or account access.
Can Microsoft 365 Accounts Be Hacked?
Yes. Like any online platform, Microsoft 365 accounts can be compromised if attackers obtain user credentials, bypass security controls, exploit misconfigured settings, or gain access through compromised devices.
The good news is that many Microsoft 365 security incidents can be prevented through strong security practices, multifactor authentication, and ongoing account monitoring.
Does Multifactor Authentication Prevent All Microsoft 365 Attacks?
No. Multifactor authentication significantly reduces risk and should be enabled for all users, but it does not stop every attack.
Cybercriminals may attempt MFA fatigue attacks, session hijacking, social engineering, or other techniques designed to bypass authentication protections. This is why MFA should be part of a broader Microsoft 365 security management strategy.
What Are the Signs of a Compromised Microsoft 365 Account?
Common warning signs include:
Unexpected password reset notifications
MFA prompts you didn't request
Suspicious emails sent from your account
Login alerts from unfamiliar locations
New inbox rules you didn't create
Missing, moved, or deleted emails
Unauthorized changes to account settings
If any of these signs are present, the account should be investigated immediately.
How Often Should Microsoft 365 Security Settings Be Reviewed?
Most organizations should review Microsoft 365 security settings, user permissions, administrative access, and connected applications on a regular basis.
Businesses with compliance requirements, remote workforces, or sensitive data often benefit from ongoing Microsoft 365 administration and security monitoring to help identify potential risks before they become larger problems.
Need Help Securing Microsoft 365?
Certified NETS helps businesses throughout the St. Louis area strengthen their Microsoft 365 security posture through proactive Microsoft 365 security management, ongoing administration, and practical cybersecurity best practices.
Robyn Howes is the President and visionary leader of Certified NETS, where she combines decades of experience in IT strategy, cybersecurity, and operations with a passion for building lasting client relationships. Named to CRN’s Women of the Channel Power Solution Provider list multiple times, Robyn leads with both innovation and integrity—bringing strategic focus and real-world expertise to every engagement. Read Robyn’s full bio »
