  This paper is devoted to measuring the security of cyber networks under
advanced persistent threats (APTs). First, an APT-based cyber attack-defense
process is modeled as an individual-level dynamical system. Second, the dynamic
model is shown to exhibit the global stability. On this basis, a new security
metric of cyber networks, which is known as the limit security, is defined as
the limit expected fraction of compromised nodes in the networks. Next, the
influence of different factors on the limit security is illuminated through
theoretical analysis and computer simulation. This work helps understand the
security of cyber networks under APTs.
