The landlord and your roommate have a written agreement. The lease will control the terms of the tenancy, as long as those terms are not in contravention of applicable law or so restrictive as to violate public policy. If your roommate has not complied with the terms of the agreement, then the lease should provide for the consequences potentially available for his violation of the agreement. You have not stated how many people are living at your place but you did indicate you have more than one roommate. You also stated there is only two months remaining on your roommate’s lease and that the rent has been paid in full and on time. While the landlord may be able to terminate your roommate’s lease based upon his failure to follow the agreement, in my opinion what the landlord will do will likely depend upon how many more people are living at your place beyond those authorized in the agreement. Unless the size of your place and the people and/or pets are disproportionate, in my opinion it would not be in the interest of your landlord to begin an eviction process. More likely, the landlord will seek an accommodation from your roommate for the additional and unauthorized use of the premises. You state you live in Chicago, Illinois. There are very specific laws governing the proper distribution and/or application of the security deposit. I would recommend you attempt to work through your situation with your landlord. If you cannot reach an accord, then contact an experienced attorney who with more information will be able to properly advise you of your rights and responsibilities.