A vulnerability in the implementation of the Intermediate System&ndash;to&ndash;Intermediate System (IS&ndash;IS) routing protocol functionality in Cisco IOS XR Software could allow an unauthenticated attacker who is in the same IS&ndash;IS area to cause a denial of service (DoS) condition. The vulnerability is due to incorrect processing of crafted IS&ndash;IS link-state protocol data units (PDUs). An attacker could exploit this vulnerability by sending a crafted link-state PDU to an affected system to be processed. A successful exploit could allow the attacker to cause all routers within the IS&ndash;IS area to unexpectedly restart the IS&ndash;IS process, resulting in a DoS condition. This vulnerability affects Cisco devices if they are running a vulnerable release of Cisco IOS XR Software earlier than Release 6.6.3 and are configured with the ISâ€“IS routing protocol. Cisco has confirmed that this vulnerability affects both Cisco IOS XR 32-bit Software and Cisco IOS XR 64-bit Software.