Tex. Water Code § 55.406

After bonds are approved by the attorney general and registered with the comptroller, they shall be held valid and binding in any action, suit, or proceeding in which their validity is questioned. In any action brought to enforce collection of the bonds, the certificate of approval by the attorney general, or a certified copy of the certificate, shall be admitted as evidence of the validity of the bonds. The only defense which can be offered against the validity of the bonds is forgery or fraud.