Office 365 MFA compromise through token theft

Token theft in the context of Azure Active Directory (Azure AD) refers to the unauthorized access or use of a security token that has been issued to a legitimate user. Security tokens in Azure AD are digital credentials used to access resources and services, and they contain claims that provide information about the user, such as authentication status, roles, and permissions.

When a token is stolen, an attacker can potentially use it to gain access to resources without needing to know the user’s password or perform Multi-Factor Authentication (MFA). This can happen in various ways, such as:

• Phishing attacks: An attacker tricks a user into providing access to a token.
• Malware: Malicious software extracts tokens stored on a compromised device.
• Man-in-the-middle attacks: An attacker intercepts the token during transmission.

MFA Requirement Satisfied by Claim in the Token

When Azure AD requires MFA for accessing a resource, it typically challenges the user to provide a second form of authentication (e.g., a code sent to their phone). However, if a valid security token already contains a claim indicating that MFA has been previously satisfied, Azure AD may skip the additional MFA challenge and allow access based on the existing token.

How Token Theft Relates to “MFA Requirement Satisfied by Claim in the Token”

When a token is stolen, an attacker can use it to access resources as if they were the legitimate user. If this stolen token contains an MFA claim (indicating that MFA was already completed by the legitimate user), Azure AD might not prompt the attacker for MFA again. Instead, it would consider the MFA requirement as “satisfied by claim in the token.” This scenario is risky because the attacker can bypass MFA entirely, leveraging the stolen token to access resources.

Why This Is Concerning

• Bypassing MFA: MFA is a critical security measure designed to protect against unauthorized access, even if a password is compromised. If an attacker can use a stolen token with an MFA claim, they effectively bypass one of the key layers of security.
• Persistent Access: Tokens often have a certain validity period. During this time, an attacker can repeatedly use the token without needing to re-authenticate, giving them persistent access to resources.
• Undetected Breach: If the attack is sophisticated enough, it might go undetected for some time, allowing the attacker to operate within the environment without raising immediate alarms.

Mitigation Strategies

To mitigate the risk of token theft and the subsequent misuse of “MFA requirement satisfied by claim in the token,” organizations can implement several strategies:

1. Token Lifetime Management: Shortening token lifetimes or implementing continuous access evaluation to revoke tokens in real-time if suspicious activity is detected.
2. Conditional Access Policies: Using conditional access policies to enforce MFA more dynamically based on risk signals, such as the user’s location or device.
3. Monitoring and Alerts: Implementing monitoring and alerting for unusual sign-ins or activities, especially from unfamiliar IP addresses or locations.
4. Privileged Identity Management (PIM): Using Azure AD Privileged Identity Management to enforce just-in-time access and require MFA for privileged roles on every access attempt.
5. Device Compliance: Ensuring that only compliant devices can access resources, reducing the risk of tokens being used from untrusted devices.
6. DNS filtering – this blocks access to malicious sites that may steal credentials from your browser, hence preventing theft of your token in the first place.
7. Conduct user training to gain cyber security awareness.

Understanding these concepts is crucial for securing Azure AD environments and protecting against unauthorized access through token theft.

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This Microsoft article contains further information Token tactics: How to prevent, detect, and respond to cloud token theft | Microsoft Security Blog