The correct answer to the question "where is my network vulnerable to attack?" is "everywhere." To some extent, that's the nature of the Internet beast; if you have a door open to the world, then it's inevitable that someone will try to open it up. And there's a good chance that they're not doing it just to say hello.
Dan Ingevalson, the director of professional security services at Internet Security Systems, says that enterprises have gotten better at managing security vulnerabilities, but the increasing complexity of networks and network-borne applications make perfect protection impossible. "There is always going to be some level of complexity in a network that will create a network security vulnerability," he says.
Having said that, some open doors are bigger and more common than others. A big part of maintaining network security, says Mark Curphey, senior director of consulting at Foundstone Services, a division of McAfee Inc., is knowing where these vulnerabilities are, and knowing how to plug them up.
Network edge devices: Though well-publicized, worms and viruses continue to be a common and, to some extent, under-appreciated network threat says Yankee Group senior analyst Jim Slaby. "We haven't seen a really big, really pervasive worm like Blaster or Slammer in some time, but they are waiting in the wings," he says. "It's not that people are complacent, but the problem with worms is that they're zero-day exploits. Signature defenses only work against things that you've seen before, or someone has seen before you, and they proliferate quickly."
Although the high-profile worms of the last years have trained network security personnel to respond quickly and apply patches diligently, penetration tests still find perimeter holes --- big, gaping holes, according to Curphey. "You see border routers with their admin interfaces open, so people can manage them from home," he notes. "But so can anyone else."