In recent years, Microsoft 365 (M365) has become a cornerstone of productivity for organisations in the UK, providing seamless access to email, collaboration tools, and cloud storage. But with its popularity comes a growing threat: token theft. This blog explores how attackers exploit M365 access tokens, why it’s a lucrative attack vector, and how modern authentication methods like FIDO2 and Windows Hello can protect organisations from falling victim.
What Is Token Theft?
M365 Token Theft is a sophisticated cybersecurity attack where hackers steal authentication tokens used in Microsoft 365 environments to gain unauthorized access to corporate resources. Unlike traditional password theft, token theft bypasses multi-factor authentication by capturing validated session tokens, allowing attackers to impersonate legitimate users without triggering security alerts.
Why Attackers Target M365 Tokens
- Wide Attack Surface
M365 is widely adopted, making it a high-value target. A compromised token can grant access to emails, files, and sensitive information stored in SharePoint or OneDrive. - Persistent Access
Tokens often remain valid for hours or even days. Some refresh tokens can extend access indefinitely if not properly managed. This allows attackers to maintain access for longer periods without detection. - Bypassing MFA
Multi-factor authentication (MFA) is a robust defence, but it’s not foolproof. Token theft enables attackers to sidestep MFA entirely, as they don’t need the credentials used to generate the token.
How Attackers Steal Tokens
- Phishing
Phishing remains the most common method for stealing tokens. Attackers craft realistic-looking login pages to trick users into entering their credentials. Once authenticated, they intercept the resulting access token via a malicious redirect or browser session hijacking. - Session Hijacking
Session hijacking occurs when attackers exploit insecure communication channels or session cookies to steal tokens. This often happens in environments where users access M365 over unsecured public Wi-Fi. - Malicious Applications
Attackers can trick users into granting permissions to a malicious OAuth application. Once authorised, the app gains access to the user’s M365 environment, including email, calendar, and files, without ever needing to steal passwords. - Man-in-the-Middle (MitM) Attacks
In MitM attacks, attackers intercept traffic between the user and M365 servers. By exploiting vulnerabilities in network security, they can extract tokens in real-time. - Token Replay Attacks
In token replay attacks, attackers reuse stolen tokens in environments where token validation mechanisms are weak or poorly implemented.
Why Are These Attacks So Successful?
- Human Error: Users can be tricked by sophisticated phishing techniques or accidentally approve malicious apps.
- Token Lifecycle: Tokens have a lifespan that can be exploited if not properly managed.
- Lack of Visibility: Many organisations don’t monitor token usage, allowing attackers to operate unnoticed.
- Poor Configuration: Default or lax security settings, like long token expiry periods, increase the risk.
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Practical Steps for Organisations
- Adopt Passwordless Authentication
Deploy FIDO2 security keys or devices with Windows Hello to replace passwords entirely. This not only improves security but also reduces the administrative overhead of managing passwords. - Monitor Token Activity
Use tools like Microsoft Sentinel to monitor and analyse token usage. Configure alerts for suspicious behaviour, such as token usage from unexpected locations or devices. - Reduce Token Lifespans
Configure conditional access policies in Entra ID to reduce token expiry times and enforce device compliance checks before granting access. - Educate Users
Regularly train users to recognise phishing attempts, malicious app requests, and the importance of securing their devices. - Implement Conditional Access
Leverage Entra ID’s conditional access policies to enforce location, device compliance, and user risk checks before granting access to M365 services.
Closing Thoughts
Token theft in M365 is a growing threat, but with modern security tools and techniques, it’s a battle organisations can win. By embracing passwordless authentication with FIDO2 and Windows Hello, IT teams can not only mitigate token theft but also improve the overall user experience.
As IT professionals, it’s our responsibility to stay ahead of attackers. The technology is here, and it’s time we fully embrace it. Don’t let token theft compromise your organisation’s productivity – secure your M365 environment today.