SecurityFocus has two posts indicating that Cisco IOS 15.0 is vulnerable to attackers.
Both are DoS (Denial of Service) issues and at this time there is no solution being offered from Cisco to correct the problem.
Should your network be non-responsive, you could attempt to reboot your Cisco device to restore connectivity.
The original posts are as follows:
Homeland Secure IT is a Cisco partner and can apply the IOS updates when they become available. We are also partners with and offer sales & support of SonicWall, WatchGuard and other firewall/router manufacturers. If this issue becomes of concern for you or your business, we can provide alternative products which are not vulnerable. In Greenville / Upstate SC, call 864.990.4748 or email email@example.com