As far as I know, the virtual particles that appear in Feynman diagrams are allowed to be outside of their mass hyperbola, namely \begin{equation} k^2 = \omega^2 - (\vec{p})^2 = m^2 \end{equation} So that the conservation laws are verified.
If this in mind (which I am not entirely sure it is), how can we know the mass of the interaction bosons?