  Cloud applications expose - besides service endpoints - also potential or
actual vulnerabilities. Therefore, cloud security engineering efforts focus on
hardening the fortress walls but seldom assume that attacks may be successful.
At least against zero-day exploits, this approach is often toothless. Other
than most security approaches and comparable to biological systems we accept
that defensive "walls" can be breached at several layers. Instead of hardening
the "fortress" walls we propose to make use of an (additional) active and
adaptive defense system to attack potential intruders - an immune system that
is inspired by the concept of a moving target defense. This "immune system"
works on two layers. On the infrastructure layer, virtual machines are
continuously regenerated (cell regeneration) to wipe out even undetected
intruders. On the application level, the vertical and horizontal attack surface
is continuously modified to circumvent successful replays of formerly scripted
attacks. Our evaluations with two common cloud-native reference applications in
popular cloud service infrastructures (Amazon Web Services, Google Compute
Engine, Azure and OpenStack) show that it is technically possible to limit the
time of attackers acting undetected down to minutes. Further, more than 98% of
an attack surface can be changed automatically and minimized which makes it
hard for intruders to replay formerly successful scripted attacks. So, even if
intruders get a foothold in the system, it is hard for them to maintain it.
