A class of curved spacetimes with global parabolic isometries (GPI) is introduced. These isometries have fixed point sets on two-dimensional null surfaces which can be interpreted as worldsheets of massless cosmic strings... Back reaction effects of the strings in such spacetimes can be described exactly, in terms of a nontrivial holonomy at the worldsheet. We show that the GPI spacetimes are type $N$ geometries of the Petrov classification. We describe a number of features of these spacetimes, including properties of Killing horizons associated to GPI. As an example, we consider a circular massless cosmic string in the de Sitter universe and present the metric in new coordinates centered at the string worldsheet. (read more)