This is a Phase 1, open-label, multicenter clinical study evaluating ERAS-801 as a monotherapy. The study will initially commence with dose escalation of ERAS-801 in study participants with recurrent GBM. Once the MTD and/or RD has been determined from dose escalation, then dose expansion of ERAS-801 may commence in study participants with recurrent GBM harboring alterations in epidermal growth factor receptor (EGFR).