Study Evaluating Cemiplimab Alone and Combined With RP1 in Treating Advanced Squamous Skin Cancer

Key Inclusion Criteria: * Histologically confirmed locally advanced or metastatic cutaneous squamous cell carcinoma * Patients with locally advanced disease who are not suitable candidates for surgical or radiological treatment of lesions or have refused those treatments * At least 1 lesion that is measurable and injectable by study criteria * Eastern Cooperative Oncology Group (ECOG) performance status ≤1. Patients with ECOG PS 2 at baseline may be allowed to enroll if PS 2 status is only related to the CSCC disease under study * Anticipated life expectancy \>12 weeks * All patients must consent to provide archived or newly obtained tumor material for central pathology review for confirmation of diagnosis of CSCC. Key Exclusion Criteria: * Prior treatment with an oncolytic therapy * Patients with active significant herpetic infections or prior complications of HSV-1 infection (e.g. herpetic keratitis or encephalitis) * Patients who require intermittent or chronic use of systemic (oral or intravenous) anti-virals with known anti-herpetic activity (e.g. acyclovir) * Ongoing or recent (within 5 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments * Prior treatment with an agent that blocks the PD-1/PD-L1 pathway. * Prior treatment with other immune modulating agents other than as adjuvant or neoadjuvant therapy within 3 years. * Untreated brain metastasis(es) that may be considered active. * Acute or chronic active hepatitis B or known history of hepatitis B or hepatitis C or human immunodeficiency virus (HIV) infection * History of ILD/pneumonitis within the last 5 years or a history of ILD/pneumonitis requiring treatment with systemic steroids. * Any major or surgical procedure ≤ 28 days before randomization * Administration of live vaccines ≤ 28 days before randomization Note: Other protocol defined Inclusion/Exclusion criteria apply.