Is Cross Contamination permissible ?
بسم الله الرحمن الرحيم
1) Yes, it would be permissible to use such a microwave/oven; however, to cover your food with a lid would be best, to avoid any chance of haram food contaminating your food. Generally speaking, the chances are very low of food from the inner walls of a microwave/oven entering one’s plate, however, it’s always best to exercise precaution. In addition, there is no need to unnecessarily put oneself through hardship by cleaning the microwave/oven each time before use; a lid would suffice. The Prophet صلى الله عليه و سلم has stated, “Leave that which makes you doubtful for that which does not make you doubtful…” (Narrated by Tirmizi). By adopting the way of precaution, we will maintain our peace of mind, contrary to indulgence in doubtful matters.
2) In principle, if it has been confirmed that a certain product does not contain any haram ingredients, then one may consume it. Nevertheless, if there is fear of cross-contamination, then one should avoid such food items and search for a halal, pure, and doubt-free alternative.
And Allah تعالى knows best.