Searches for Rare Decays at CMS

\nRare decays are suppressed in the Standard Model, making them intriguing avenues for exploring new physics beyond the current theoretical framework. Many new physics models could potentially emerge from the study of these decays, providing the insights how the Standard Model could be extended. This proceeding presents the latest studies on rare B meson and τ lepton decays, specifically $$ {B_{(s)}^0} $$ → µ+µ− and τ → 3µ, using data collected by the CMS experiment. Additionally, recent observations of η 4µ and J/ψ 4µ decays are discussed.\n