Wireless networks, especially those based on 802.11, have found widespread use in domestic, commercial, educational, military and public-safety environments. The security of these wireless networks is assuming an increasing importance as users come to rely on the availability and correct functioning of wireless network services. This paper investigates the cryptographic denial-of-service (DoS) attack against the 802.11i TKIP security protocol. We have conducted a laboratory study and show that it takes very little effort to bring TKIP-protected network traffic to a complete halt. This attack maybe used not just to compromise availability but is also an effective means of conducting a security-level rollback to the insecure WEP protocol. We use a testbed network to evaluate a remedial measure that eliminates the vulnerability on which the attack is based.