The IEEE 802.11 standard for WLANs is one of the most widely adopted standards for broadband Internet access. However, it is vulnerable to many attacks such as DoS attacks, shared-key authentication attacks, replay attacks, .. In order to enhance security of IEEE 802.11, a new standard, called the IEEE 802.11i, was developed. The 802.11i standard extends 802.11 with new security mechanisms to ensure message confidentiality and integrity. 802.11i includes the 802.1x port authentication algorithm to provide strong mutual authentication and key management; new data encryption and data integrity methods using Temporal Key Integrity Protocol and Counter mode/CBC-MAC Protocol. Thus, 802.11i can address some of the security vulnerabilities in 802.11, such as shared-key authentication, message modification and replay, and web keystream recovery attacks. Unfortunately, some recent studies have pointed out that 802.11i also has some flaws on both its authentication protocol and key management mechanism. There exists vulnerabilities because of the lack of mutual authentication or the prevention of key exchange from DoS attacks.