Examples of design failures include accidentally bringing third-party network connections into a network without implementing a firewalling mechanism, not including proper access controls between tiers in e-commerce applications and failing to implement cryptography to protect critical data sets.
Implementation failures may include forgetting to enable the ACLs (access-control lists) on a router, not patching a new Web server or forgetting to scrub user data in a Web form. Any of these vulnerabilities could expose sensitive information, allow unauthorized access or, in the case of worms and viruses, wreak digital carnage.
Design problems typically are harder to identify than implementation errors because few tools can replicate the abilities of a professional. This is why including security teams in the design life cycle is so critical--experienced humans can identify potential design failures quickly, avoiding costly long-term mitigation efforts. Implementation problems can also be costly, of course, but fortunately there are more tools and technology solutions that can reduce these risks.
Regardless of the type of vulnerability, the tactical process remains the same--identify, then respond. However, there may be multiple approaches to the response phase, some more proactive than others. An organization might choose to fix the problem directly with a software patch, or it might deploy a device to reduce the chances of exploitation. Some might even decide to do nothing and assume the level of risk associated with that particular vulnerability. Let's apply this concept to a real-world example:
Scenario: A critical flaw has been found in Microsoft's Internet Explorer Web browser (not much of a stretch). This flaw lets attackers execute arbitrary code on a victim's (now) vulnerable desktop.