D-link DIR-825AC G1 devices have Insufficient Compartmentalization between a host network and a guest network that are established by the same device. A DHCP Request is sent to the router with a certain Transaction ID field. Following the DHCP protocol, the router responds with an ACK or NAK message. Studying the NAK case revealed that the router erroneously sends the NAK to both Host and Guest networks with the same Transaction ID as found in the DHCP Request. This allows encoding of data to be sent cross-router into the 32-bit Transaction ID field.
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