Multi-factor authentication (MFA) is one of the most important steps a business can take to protect its Microsoft 365 accounts. Even if an attacker steals an employee’s password, MFA adds another layer of protection that makes unauthorized access more difficult.
But there is an important detail many businesses overlook: MFA doesn’t mean your Microsoft 365 account can’t be compromised.
Cybercriminals are increasingly targeting the digital sessions that users create after signing in. Through techniques such as session-token theft and malicious cookie theft, attackers may gain access to an account without needing to repeat the normal login process.
Understanding how these attacks work can help your business build a stronger security strategy.
What Is Session-Token Theft?
When you sign in to Microsoft 365, your browser or application needs a way to remember that you have successfully authenticated. Otherwise, you would need to enter your credentials and complete MFA every time you opened an email, accessed a document, or switched between applications.
To make this possible, authentication systems use digital tokens and browser cookies to maintain your signed-in session.
Think of a session token as a temporary digital pass. After you prove your identity, the system issues this pass so you can continue working without repeatedly signing in.
The problem arises when a cybercriminal manages to steal that pass.
Instead of trying to guess your password or trick you into approving another MFA request, an attacker may attempt to steal an active session token. If the stolen token remains valid and the service accepts it under the circumstances, the attacker could potentially use it to access the account as the authenticated user.
In simple terms, MFA helps prove who you are when you sign in, but a stolen session token can sometimes allow an attacker to misuse an already authenticated session.
These attacks may involve convincing employees to visit fraudulent websites, exploiting compromised devices, or using other techniques to obtain session information. The exact method and likelihood of success depend on the attack and the security controls in place.
Why Businesses Need More Than MFA
MFA remains essential. The answer is not to abandon it, but to strengthen it with additional security measures.
For small and medium-sized businesses, Microsoft 365 often contains sensitive information, including customer records, financial documents, internal communications, and confidential business files. A compromised account can expose that information or give an attacker an opportunity to impersonate an employee.
Here are four practical ways to improve protection.
1. Strengthen Security with Microsoft 365 Business Premium
Microsoft 365 Business Premium brings together productivity tools and security capabilities designed for many small and medium-sized organizations.
Depending on licensing, configuration, and the services deployed, businesses can use capabilities such as Microsoft Entra ID Conditional Access, Microsoft Intune device management, and Microsoft Defender for Business to help strengthen identity, device, and endpoint security.
However, purchasing a license alone does not secure your environment. These tools need to be configured appropriately, monitored, and maintained.
2. Use Conditional Access Policies
Conditional Access allows organizations to apply access requirements based on signals such as user identity, device compliance, application, and sign-in risk where supported.
For example, a company may require employees to access sensitive Microsoft 365 resources from compliant, managed devices. It may also apply additional authentication requirements or restrict access when specified risk conditions are detected.
These controls can reduce the opportunities available to attackers, although no single policy can prevent every session-based attack. Policies should be tested carefully to avoid disrupting legitimate business operations.
3. Manage and Protect Employee Devices
A compromised laptop can undermine even a well-designed identity security strategy.
Managed endpoints allow IT teams to apply security policies, enforce encryption requirements, manage updates, and assess device compliance. Endpoint protection can also help detect suspicious activity and malicious software.
Businesses should establish clear rules for accessing company email and files from personal devices, particularly when those devices are not centrally managed.
4. Consider Phishing-Resistant Authentication
Traditional MFA methods offer meaningful protection, but some approaches are more resistant to phishing than others.
Where supported, passkeys and security keys using FIDO2 or other appropriately configured phishing-resistant authentication methods can help protect users against credential-harvesting attacks.
Organizations should evaluate their applications, devices, licensing, and user requirements before selecting an authentication method.
Turn Security Awareness into Action
Security is strongest when identity protection, device management, employee awareness, and ongoing monitoring work together.
Start by reviewing which accounts have access to sensitive information, whether MFA is consistently enforced, which devices can access company resources, and how suspicious sign-ins are investigated. Review session policies and token protections where available, and make sure your incident response process accounts for the possibility of compromised sessions.
It is also important to train employees to recognize suspicious links, unexpected sign-in requests, and unusual authentication prompts.
The bottom line: MFA is a foundation, not a complete security strategy. By combining strong authentication with Conditional Access, managed endpoints, endpoint protection, and regular security reviews, your business can make Microsoft 365 significantly harder to compromise.
If your organization relies on Microsoft 365 every day, a security review can help identify gaps in your current setup and prioritize improvements based on your actual risks.