In addition to access-lists used to control access to the vty lines/snmp/ntp etc. you can also have a look at COPP - Control Plane Policing - see this attached link for details on how it works -
you may enable the IOS IPS feature on these routers and download the latest signature file from cisco's website. depending how big your network and applications you're using, it may eat up the router's cpu and memory. this feature may be used as an interim while you plan to purchase a dedicated appliance for security.
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