Cisco put fear into the hearts of many network admins when they announced the discovery of three exploits inside the extremely popular IOS, which powers many Cisco devices. Depending on the configuration being used, certain devices running IOS could be vulnerable to a denial of service attack or even be compromised, allowing an attacked to execute arbitrary code. The flaws include an IPV6 vulnerability, a memory leak and a flaw in packet handling.
Cisco has already updated and released a version of IOS that fixes the problems. As IOS powers many firewalls and routers, the impact of a flaw there can have a greater impact than a single machine being compromised.