The strongest magnetic fields on the Sun are routinely detected at dark sunspots. The magnitude of the field is typically about 3000 G, with only a few exceptions that reported the magnetic field in excess of 5000 G. Given that the magnetic field decreases with height in the solar atmosphere, no coronal magnetic field above similar to 2000 G has ever been reported. Here, we present imaging microwave observations of anomalously strong magnetic field of about 4000 G at the base of the corona in solar active region NOAA 12673 on 2017 September 6. Combining the photospheric vector measurements of the magnetic field and the coronal probing, we created and validated a nonlinear force-free field coronal model, with which we quantify the record-breaking coronal magnetic field at various coronal heights.