Catalyzes endohydrolysis of 1,4-beta-D-glucosidic linkages in xyloglucan with retention of the beta-configuration of the glycosyl residues. Specific for xyloglucan and does not hydrolyze other cell wall components (By similarity). xyloglucan + H2O = xyloglucan oligosaccharides. Belongs to the glycosyl hydrolase 12 (cellulase H) family.