It depends on what you mean by "corrupt".
If "corrupt" means "willing to cheat", then obviously that is not limited to the rich. If "corrupt" means "willing to abuse power bestowed on them by society", then it is literally impossible for the poor and disenfranchised, whom society has given nothing, to be corrupt, because they cannot abuse what they do not have.
People of all social classes participate in corruption, and have to do so, to survive. It is only those who have power who can be said to be abusing power, and when we talk about corruption, we're generally talking not about all bad human behaviors, but more specifically about the corrosion of institutions and social trust that occurs when those in charge use their positions toward unjust ends.