Md. Code, Tax-Prop. § 14-842
In any proceeding to foreclose the right of redemption, it is not necessary to plead or prove the various steps, procedure and notices for the assessment and imposition of the taxes for which the property was sold or the proceedings taken by the collector to sell the property. The validity of the procedure is conclusively presumed unless a defendant in the proceeding shall, by answer, set up as a defense the invalidity of the taxes or the invalidity of the proceedings to sell or the invalidity of the sale. A defendant alleging any jurisdictional defect or invalidity in the taxes or in the proceeding to sell, or in the sale, must particularly specify in the answer the jurisdictional defect or invalidity and must affirmatively establish the defense.
Md. Code, TP § 14-842