These discoveries are publicly announced through various forums, such as Bugtraq, as well as non-public avenues. Insufficient patching of systems prior to placement on public networks make it possible for intruders to break into system through known vulnerabilities. These problems are fixed in patch clusters offered by software and hardware vendors, and are usually available from the vendor's public web offerings. In many enterprises, network security practices are not adequate in relation to threat. Problems with design, vulnerability and patch information and tracking, and intrusion detection make it possible for attackers to infiltrate networks unhindered and unnoticed.
Network design plays a major factor.
The risk and intelligence of doing such a thing are inversely proportional. Risk mitigating tools such as network firewalls, access control entries on routers and switches, and other filtration systems on the network can significantly lower risk. Even with security infrastructure in place, it's possible for an attacker to go unnoticed after a machine has been compromised. A legitimate service with a vulnerability could allow an attacker to gain unauthorized access. Once machines have been compromised, backdoors are installed on the systems, allowing the attacker to return uninhibited at any time, even if the original hole is later patched.