Many crackers run such tools continuously and sort data to identify vulnerable hosts. Most reconnaissance attacks can be deflected through conscientious router and server configurations to turn off unneeded services, especially where they are exposed to the Internet. Using such tools, the Cisco Secure Consulting Services group has been able to crack 53 percent of passwords obtained from compromised networks. Preventive actions against access attacks include using strong passwords that combine letters and numbers, protecting Internet mail servers using proxy devices such as firewalls, and filtering incoming TCP requests by allowing only those services that are needed. Both services are rarely deployed over the Internet but are often allowed through Internet filtering devices.
Web services are particularly vulnerable to access attacks.
You need visibility inside Web traffic to watch for violations. Recent vulnerabilities in such applications as Microsoft's IIS Web server allow attackers to imbed malicious commands inside proper Web requests. Somewhere on the Internet, a DoS is always under way, according to Edward Vielmetti, Consulting Engineer, Office of the Chief Strategy Officer at Cisco. A DoS attack sends a large amount of useless traffic to a particular host or port. While they do not compromise or damage hosts, well-executed DoS attacks can prevent any legitimate traffic from getting through, effectively shutting down its services.