Cisco ASA Unicast Reverse Path Forwarding (uRPF) Bypass Vulnerability

A vulnerability in the Unicast Reverse Path Forwarding (uRPF) feature in the Cisco Adaptive Security Appliance (ASA) could allow an unauthenticated, remote attacker to bypass the uRPF validation checks.

The vulnerability is due to incorrect uRPF validation where IP packets from an outside interface, whose IP address is both in the ASA routing table and associated with an internal interface, are not dropped. An attacker could exploit this vulnerability by sending spoofed IP packets to the ASA in a subnet range that should be dropped. An exploit could allow the attacker to bypass uRPF validation on the ASA and the packet will be incorrectly forwarded on the internal network.

Cisco has confirmed the vulnerability; however, software updates are not available.

To exploit this vulnerability, the targeted device must have the ip verfiy reverse-path interface command set on an outside interface and the interface must have a default route configured. In addition, the attacker must know a subnet range set on the targeted device that is configured to drop packets prior to sending spoofed IP packets to the device.

Cisco indicates through the CVSS score that functional exploit code exists; however, the code is not known to be publicly available.