[AlotOfReading]

A microwave oven is an excellent (30-40dB) attenuator specifically around 2.4GHz. The legal requirements essentially mandate 30dB + manufacturing safety margin. Your mileage will vary for other frequencies, as they're not actually faraday cages, so it's usually easier to simply use aluminum foil or a copper mesh/PVC box depending on your needs.

[PeterisP]

Thing is, 30-40dB is not sufficient to properly block wireless connections, they can still work with 40dB attenuation.

[PeterisP]

To add on to this - I just tested with my home microwave - I get 35-38dB attenuation, but I can still quite reliably get wifi packets to/from an ESP32 in that microwave.