The fact that your landlord is not permitted to sublease his apartment does not invalidate his sublease to you; it does, however, subject him (and through him, you) to potential eviction for violating his own lease. You may be able to recover the difference in rent for a comparable sublease, or whatever else you can establish as bona fide contract damages, if it comes down to that. If you sue, you can expect that you'll be providing the court with a copy of the sublease, and quite possibly the defendant as well.