Richard Hicks' Forefront TMG Blog

Consider a scenario in which you have published your Exchange 2010 Outlook Web App servers using Forefront TMG 2010 and are using Active Directory or LDAP authentication along with Forms-based Authentication (FBA). In an effort to gain access to the system, an attacker may perform a brute force password attack by either manual or programmatic means. The attacker will attempt to guess the password for a given user until they reach the configured account lockout threshold as defined in Active Directory. Once this happens, the attacker will have to wait for the password to unlock automatically or be unlocked by an administrator depending on your security policy. Effectively this results in a Denial of Service (DoS) because the legitimate user is unable to authenticate when this happens.

To address this concern, Forefront TMG SP2 includes a feature that allows administrators to enforce an account lockout policy on the Forefront TMG firewall itself. When configured with thresholds lower than those configured in Active Directory, this feature provides valuable protection from DoS that result from unsuccessful password guessing attempts. To enable this feature, install Forefront TMG 2010 SP2, and then follow these detailed instructions.