Mo. Rev. Stat. § 535.180: Reservation in favor of mortgagee of lease in such case.

A mortgagee of such lease, not in possession of such demised premises, who within three months after execution of any such judgment shall pay all rent in arrear, and all costs, and the charges incurred by the landlord, and shall perform all the agreements which ought to be performed by the first lessee, shall not be affected by the recovery of the possession of the demised premises.