  Silva et al. proposed a universal secure network coding scheme based on MRD
codes, which can be applied to any underlying network code. This paper
considers a stronger eavesdropping model where the eavesdroppers possess the
ability to re-select the tapping links during the transmission. We give a proof
for the impossibility of attaining universal security against such adversaries
using Silva et al.'s code for all choices of code parameters, even with
restricted number of tapped links. We also consider the cases with restricted
tapping duration and derive some conditions for this code to be secure.
