From the Highlights: "On January 2, 2017, a 4-hour system outage disrupted Custom and Border Protection's (CBP) processing of incoming international travelers at airports nationwide. We conducted this review to determine the effectiveness of CBP's efforts to address this system outage, as well as the sufficiency of its plans for minimizing the possibility and impact of similar system outages in the future. [...] We are recommending that CBP implement improvements to its software testing, vulnerability patching, and disaster recovery capabilities."
Department of Homeland Security, Office of Inspector General, Report No. OIG-18-19 (revised)
Department of Homeland Security Office of Inspector General: https://www.oig.dhs.gov/