The perimeter router must be configured to have Proxy ARP disabled on all external interfaces.
Overview
Finding ID
Version
Rule ID
IA Controls
Severity
V-207166
SRG-NET-000364-RTR-000112
SV-207166r856651_rule
Medium
Description
When Proxy ARP is enabled on a Cisco router, it allows that router to extend the network (at Layer 2) across multiple interfaces (LAN segments). Because proxy ARP allows hosts from different LAN segments to look like they are on the same segment, proxy ARP is only safe when used between trusted LAN segments. Attackers can leverage the trusting nature of proxy ARP by spoofing a trusted host and then intercepting packets. Proxy ARP should always be disabled on router interfaces that do not require it, unless the router is being used as a LAN bridge.