Glioblastoma (GBM) and gliosarcoma (GS) are the most common and aggressive forms of malignant primary brain tumor in adults and can be resistant to conventional therapies. The purpose of this Phase Ib study is to evaluate how well a recurrent glioblastoma or gliosarcoma tumor responds to one injection of DNX-2401, a genetically modified, conditionally replicative and oncolytic human-derived adenovirus. DNX-2401 is delivered directly into the tumor where it may establish an active infection by replicating in and killing tumor cells.