Browser Extension Session Hijacking Risks for Microsoft 365 and SaaS Apps

ProArch SOC has observed an increase in threats leveraging malicious or unauthorized browser extensions to hijack authenticated sessions and bypass traditional security controls including multi-factor authentication (MFA). These attacks primarily target Microsoft 365 and other SaaS platforms where access is maintained through persistent session tokens.

By abusing browser extensions, attackers can silently capture session data, enabling account takeover without requiring credentials or MFA interaction.

ProArch SOC Observations on Session Hijacking Attacks

• The SOC identified suspicious browser-based activity consistent with session token abuse in multiple environments.
• In several cases, access occurred without corresponding successful authentication events.
• Indicators included:
  • Access from new geolocations without MFA prompts
  • Session reuse across multiple IP addresses
  • Activity inconsistent with normal user behavior patterns
  • Some users had recently installed browser extensions with elevated permissions to access browsing data.
  • Early detection through identity telemetry, Defender for Endpoint visibility, and session analysis enabled rapid containment.

Who Is at Risk: Microsoft 365, Entra ID & SaaS Users

Organizations at risk of Microsoft 365 session hijacking attacks include:

• Organizations using Microsoft 365, Entra ID, and cloud-based SaaS platforms
• Environments relying on browser-based access and persistent authentication sessions
• Users with privileged or high-value access to business-critical systems
• Organizations without strict controls on browser extension usage

How Browser Extension Attacks Work: Session Hijacking Explained

Malicious Browser Extension Abuse:

• Attackers are leveraging browser extensions as a stealthy access vector:
• Users install extensions that appear legitimate (productivity tools, converters, etc.)
• Malicious or compromised extensions request excessive permissions, including access to cookies and session data
• Extensions monitor browser activity and extract authentication tokens from active sessions

Session Hijacking Without Credentials:

• Unlike traditional phishing attacks, these techniques do not require credential theft:
• Session tokens stored in the browser are captured and reused by attackers
• This enables direct access to authenticated Microsoft 365 and SaaS applications
• MFA protections are bypassed, as attackers inherit an already authenticated session

Stealthy Access Across Cloud Environments:

• Attackers use stolen session tokens to access M365 services such as Exchange Online, SharePoint, and Teams
• Activity often appears legitimate, making detection difficult without behavioral analytics
• Sessions persist until tokens expire or are revoked, allowing prolonged unauthorized access

Security Risks: MFA Bypass, Account Takeover & Data Exfiltration

• MFA Bypass: Session reuse eliminates authentication challenges
• Account Takeover: Full access to user sessions and associated services
• Data Exfiltration: Sensitive data from M365 and SaaS platforms can be accessed silently
• Detection Evasion: Activity mimics legitimate user behavior
• Cross-Service Exposure: Compromise may extend across integrated cloud applications

How to Prevent Session Hijacking in Microsoft 365 

Immediate Actions

• Review and remove unauthorized or high-risk browser extensions across endpoints
• Investigate anomalous session activity (e.g., impossible travel, token reuse) in Entra ID logs
• Revoke active sessions for affected users and enforce re-authentication

Short-Term Actions

• Implement policies to restrict browser extension installations to approved lists
• Enable Conditional Access policies to enforce device compliance and session risk checks
• Monitor Defender for Endpoint and identity logs for abnormal browser-related activity

Mid / Long-Term Actions

• Implement continuous session monitoring and risk-based access controls
• Reduce session lifetimes for high-risk applications
• Integrate identity, endpoint, and SIEM telemetry (e.g., Microsoft Sentinel) for correlation and detection
• Establish governance around browser usage and extension management

Conclusion

As organizations continue adopting cloud-first and SaaS-driven environments, session-based access becomes a critical attack surface. Threat actors are shifting from credential theft to session hijacking, exploiting gaps in visibility and control. Defending against these attacks requires strong identity monitoring, endpoint visibility, and proactive SOC response and deep Microsoft 365 visibility capabilities that require more than traditional security tools can offer.

As a Microsoft Intelligent Security Association (MISA) member, ProArch’s SOC monitors, hunts, and responds around the clock Proarch, delivering the MDR services, Identity Detection and Response, and Microsoft-native security expertise needed to detect and remediate these attacks before they cause lasting damage.