Microsoft Exchange Autodiscover Username Enumeration Scanner

Microsoft Exchange is a widely used email server solution employed by corporations globally to manage their email, calendar, and contacts. It enables organizations to achieve a centralized communication platform, offering access to emails, address books, and documents through shared calendars and folders. The software is typically used by IT departments in medium to large businesses for efficient collaboration and improved communication. Exchange services support various kinds of mail clients and provide robust tools for seamless integration with other corporate systems. Many organizations choose Microsoft Exchange for its reliability, security, and broad range of features that cater to business requirements. The software facilitates internal and external business communication effectively, ensuring email delivery and data protection.

User Enumeration is a process of discovering valid usernames on a system. In this particular scenario, Microsoft Exchange's Autodiscover feature inadvertently reveals user information through misconfigurations. It's crucial because it allows attackers to verify the existence of accounts, forming the basis for further attacks, like password spraying or phishing campaigns. This vulnerability primarily resides in the AutodiscoverV2 endpoint, where improper handling of responses can disclose sensitive user data. It's a significant issue, as user enumeration often precedes more damaging attacks, providing a foothold for malicious activity. The ability to identify and correct such exposures can greatly improve an organization's security posture.

Technically, this enumeration manifests through Microsoft's Exchange AutodiscoverV2 endpoint, enabling attackers to enumerate users via the ActiveSync protocol. When an attacker sends a crafted request to the endpoint, it provides responses indicative of whether an email address is valid. This happens because of how the server handles authentication attempts, especially when improper response sanitization occurs. The system inadvertently leaks username confirmations, which can assist attackers in refining their attack vectors. It's an exploitation of the Misconfiguration category because it capitalizes on default or erroneous settings, revealing more information than intended. Monitoring such interactions and appropriately configuring servers can mitigate these risks.

Exploitation of this vulnerability can lead to several negative outcomes. Once attackers confirm valid usernames, they can move on to password attacks or phishing schemes, targeting those users with malicious intent. It reduces the entropy an attacker faces, narrowing down potential targets and improving their chances of a successful breach. User enumeration also compromises user privacy, potentially leading to further disclosure of sensitive information. The vulnerability highlights the importance of hardening security controls to prevent information leaks. Regular audits and adherence to security best practices can prevent such exposures.

REFERENCES

• https://www.msxfaq.de/exchange/autodiscover/autodiscover_v2.htm
• https://github.com/righettod/toolbox-pentest-web/blob/master/docs/4-HINTS_ARCHIVED.md#user-enumeration-techniques-for-microsoft-exchange