Adversaries may bridge network boundaries by compromising perimeter network devices. Breaching these devices may enable an adversary to bypass restrictions on traffic routing that otherwise separate trusted and untrusted networks.
Devices such as routers and firewalls can be used to create boundaries between trusted and untrusted networks. They achieve this by restricting traffic types to enforce organizational policy in an attempt to reduce the risk inherent in such connections. Restriction of traffic can be achieved by prohibiting IP addresses, layer 4 protocol ports, or through deep packet inspection to identify applications. To participate with the rest of the network, these devices can be directly addressable or transparent, but their mode of operation has no bearing on how the adversary can bypass them when compromised.
When an adversary takes control of such a boundary device, they can bypass its policy enforcement to pass normally prohibited traffic across the trust boundary between the two separated networks without hinderance. By achieving sufficient rights on the device, an adversary can reconfigure the device to allow the traffic they want, allowing them to then further achieve goals such as command and control via Multi-hop Proxy or exfiltration of data via Traffic Duplication. In the cases where a border device separates two separate organizations, the adversary can also facilitate lateral movement into new victim environments.
|M1043||Credential Access Protection||
Some embedded network devices are capable of storing passwords for local accounts in either plain-text or encrypted formats. Ensure that, where available, local passwords are always encrypted, per vendor recommendations.
|M1037||Filter Network Traffic||
Upon identifying a compromised network device being used to bridge a network boundary, block the malicious packets using an unaffected network device in path, such as a firewall or a router that has not been compromised. Continue to monitor for additional activity and to ensure that the blocks are indeed effective.
Use multi-factor authentication for user and privileged accounts. Most embedded network devices support TACACS+ and/or RADIUS. Follow vendor prescribed best practices for hardening access control.
Refer to NIST guidelines when creating password policies. 
|M1026||Privileged Account Management||
Restrict administrator accounts to as few individuals as possible, following least privilege principles. Prevent credential overlap across systems of administrator and privileged accounts, particularly between network and non-network platforms, such as servers or endpoints.
|ID||Data Source||Data Component|
|DS0029||Network Traffic||Network Traffic Content|
|Network Traffic Flow|
Consider monitoring network traffic on both interfaces of border network devices with out-of-band packet capture or network flow data, using a different device than the one in question. Look for traffic that should be prohibited by the intended network traffic policy enforcement for the border network device.
Monitor the border network device’s configuration to validate that the policy enforcement sections are what was intended. Look for rules that are less restrictive, or that allow specific traffic types that were not previously authorized.