The best answer is that it is active, though this is a genuinely confusing issue, because it is not 100% clear whether 'interested' is functioning as an adjective or as a past participle of the verb 'to interest'. It feels like an adjective, but I imagine that this is an issue over which even sophisticated grammarians could have some debate. I am fairly confident that the majority view would be that it is used here adjectivally, and hence would be in the active voice.
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