The CMS experiment is a privileged test bench for rare decays involving heavy flavors, based on the highly efficient dimuon triggers and the large integrated luminosity provided by the LHC collider. The most recent studies from CMS collaboration based on the data collected during LHC Run-1 and 2016 are presented, including the measurement of $B_s^0$ and $B^0$ mesons decaying into dimuons and the search for $\tau$ lepton decaying into three muons.