Sanctuary in the Sky — John Brunner

A cold war among the stars was growing hotter by the minute. As Pag and Cathrodyne struggled for domination, a hot war threatened which would rend and annihilate whole planetary systems. The two master races would have consumed one another long ago, but for one single factor:

Waystation. It was a stupendous synthetic world, famed throughout this galaxy. For Waystation was controlled by a neutral people, and until the greater powers could seize this strategic wonder planet and ferret out its secrets, they were doomed to fretful inactivity.

But as a Cathrodyne vessel drew near to Waystation, the all-important balance of power stood in sudden peril. The ship in itself was routine. But on board was a stranger, a man of undiscovered race, who spoke too little, and, it appeared, knew too much...