OpenVPN before 2.0.1, when running in "dev tap" Ethernet bridging mode, allows remote authenticated clients to cause a denial of service (memory exhaustion) via a flood of packets with a large number of spoofed MAC addresses.
http://www.mandriva.com/security/advisories?name=MDKSA-2005:145
http://www.debian.org/security/2005/dsa-851
http://secunia.com/advisories/17103