Rare and forbidden decays

Recent measurements of rare decays involving the b-flavor changing neutral current at the CMS and ATLAS experiments are presented. The full set of optimized CP-averaged observables is measured in the angular analysis of the decay ${\mathrm{B}^0} \to \mathrm{K^{\ast 0}(892)} \mu^+\mu^-$ using CMS Run 2 data. The results are among the most precise experimental measurements, and clear tensions are observed between the observed values and theoretical predictions in P2 and P5' observables. The $\mathrm{B_s}^0 \to \mu^+\mu^-$ lifetime is also measured using partial ATLAS Run 2 data and is found to be consistent with the Standard Model prediction.