Access Piggy Backing via NAT Addresses

Packet filters are generally good at filtering traffic from the transport layer and below, but they are not as good at interpreting the application layer. As a result, the filtering criteria an SPA daemon applies to accept an incoming connection (after it receives a valid SPA packet) can only realistically contain the source IP address, the requested Internet protocol, and the port number. That is, when an SPA packet instructs the SPA server to "open TCP port 22 for some source IP address for 30 seconds," the SPA server configures the packet filter to accept packets from anyone that can connect from the source IP address to TCP port 22 during that 30-second time window. If the IP address within the SPA packet is the external NAT address (which is necessary if the SPA client is behind a NAT device), then anyone on the same internal network as the legitimate client will have the same access during the allowed time window.7

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