I would use "is". The clause "what is in it" is not the part of the sentence that is hypothetical. " I know what is in this cake.
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Anonymous"Who would eat a cake without knowing what is in it?""Who would eat a cake without knowing what was in it?"Pardon my butting in. I find them both plausible. Without even stopping to analyze it, I'd use "was", but "is" sounds just fine, too. A would-clause without an explicit if-clause is a bit tricky. There can be multiple analyses.