Tex. Water Code § 53.178

When the attorney general approves the bonds, the comptroller shall register them in a book kept for that purpose. The comptroller shall record the certificate of the attorney general as to the bonds' validity. The bonds are then prima facie valid in any action, suit, or proceeding. In a suit to enforce collection of the bonds and interest on the bonds, the only defense against the validity of the bonds is forgery or fraud.