Mo. Rev. Stat. § 441.150: Attornment to stranger void — exceptions.

The attornment of a tenant to a stranger shall be void, and shall not in any wise affect the possession of his landlord, unless it is made:

(1) With the consent of the landlord; or

(2) Pursuant to or in consequence of a judgment at law, or a decree in equity, or sale under execution or deed of trust; or

(3) To a mortgagee, after the mortgage has been forfeited.