  The new measurement of the $W$-boson mass from the CDF collaboration shows a
significant tension with the Standard Model prediction. We quantify this
discrepancy within a state-of-the-art analysis of electroweak precision data
and scrutinize the leading deformations of the Standard Model Effective Field
Theory arising at dimension six. We find evidence for a non-zero value of the
$T$ parameter, i.e. for a novel source of violation of custodial symmetry,
pointing to physics beyond the Standard Model at the 4.5$\sigma$ level. We
contextualize the implications of our findings in light of other present
anomalies in Particle Physics.
