Except as permitted by RCW 84.68.010 through 84.68.070 and chapter 84.69 RCW, no action shall ever be brought or defense interposed attacking the validity of any tax, or any portion of any tax: PROVIDED, HOWEVER, That this section shall not be construed as depriving the defendants in any tax foreclosure proceeding of any valid defense allowed by law to the tax sought to be foreclosed therein except defenses based upon alleged excessive valuations, levies or taxes.