  Software patching is a common method of removing vulnerabilities in software
components to make IT systems more secure. However, there are many cases where
software patching is not possible due to the critical nature of the
application, especially when the vendor providing the application guarantees
correct operation only in a specific configuration. In this paper, we propose a
method to solve this problem. The idea is to run unpatched and patched
application instances concurrently, with the unpatched one having complete
control and the output of the patched one being used only for comparison, to
watch for differences that are consequences of introduced bugs. To test this
idea, we developed a system that allows us to run web applications in parallel
and tested three web applications. The experiments have shown that the idea is
promising for web applications from the technical side. Furthermore, we discuss
the potential limitations of this system and the idea in general, how long two
instances should run in order to be able to claim with some probability that
the patched version has not introduced any new bugs, other potential use cases
of the proposed system where two application instances run concurrently, and
finally the potential uses of this system with different types of applications,
such as SCADA systems.
