Cisco Systems added a new feature called digitally signed IOS softwares starting on version 15.0(1)M. This protects against tampered Cisco IOS software distributed over the Internet. If someone downloaded this file and then loaded it on their Cisco devices that supports that image opens their network to attackers. Furthermore, attacker starts mapping your entire network, scanning open ports, running exploits and finding vulnerable systems. Unfortunately, this feature is only suppoted for Cisco ISR Generation 2 routers (19xx, 29xx and 39xx) and Cisco Catalyst 4500 E+ series switches running IOS XE software. Information about this feature is detailed on this link.