Proxy: Multi-hop Proxy

To disguise the source of malicious traffic, adversaries may chain together multiple proxies. Typically, a defender will be able to identify the last proxy traffic traversed before it enters their network; the defender may or may not be able to identify any previous proxies before the last-hop proxy. This technique makes identifying the original source of the malicious traffic even more difficult by requiring the defender to trace malicious traffic through several proxies to identify its source. A particular variant of this behavior is to use onion routing networks, such as the publicly available TOR network. [1]

In the case of network infrastructure, particularly routers, it is possible for an adversary to leverage multiple compromised devices to create a multi-hop proxy chain within the Wide-Area Network (WAN) of the enterprise. By leveraging Patch System Image, adversaries can add custom code to the affected network devices that will implement onion routing between those nodes. This custom onion routing network will transport the encrypted C2 traffic through the compromised population, allowing adversaries to communicate with any device within the onion routing network. This method is dependent upon the Network Boundary Bridging method in order to allow the adversaries to cross the protected network boundary of the Internet perimeter and into the organization’s WAN. Protocols such as ICMP may be used as a transport.

ID: T1090.003
Sub-technique of:  T1090
Tactic: Command And Control
Platforms: Linux, Network, Windows, macOS
Data Sources: Netflow/Enclave netflow, Network protocol analysis, Packet capture
Version: 2.0
Created: 14 March 2020
Last Modified: 21 October 2020

Procedure Examples

Name Description

A backdoor used by APT29 created a Tor hidden service to forward traffic from the Tor client to local ports 3389 (RDP), 139 (Netbios), and 445 (SMB) enabling full remote access from outside the network.[2]


Attor has used Tor for C2 communication.[3]


Dok downloads and installs Tor via homebrew.[4]


FIN4 has used Tor to log in to victims' email accounts.[5]


GreyEnergy has used Tor relays for Command and Control servers.[6]


Inception used chains of compromised routers to proxy C2 communications between them and cloud service providers.[7]


Keydnap uses a copy of tor2web proxy for HTTPS communications.[8]


MacSpy uses Tor for command and control.[4]


StrongPity can use multiple layers of proxy servers to hide terminal nodes in its infrastructure.[9]


Traffic traversing the Tor network will be forwarded to multiple nodes before exiting the Tor network and continuing on to its intended destination.[10]


Ursnif has used Tor for C2.[11][12]


WannaCry uses Tor for command and control traffic.[13]


Mitigation Description
Filter Network Traffic

Traffic to known anonymity networks and C2 infrastructure can be blocked through the use of network allow and block lists. It should be noted that this kind of blocking may be circumvented by other techniques like Domain Fronting.


When observing use of Multi-hop proxies, network data from the actual command and control servers could allow correlating incoming and outgoing flows to trace malicious traffic back to its source. Multi-hop proxies can also be detected by alerting on traffic to known anonymity networks (such as Tor) or known adversary infrastructure that uses this technique.

In context of network devices, monitor traffic for encrypted communications from the Internet that is addressed to border routers. Compare this traffic with the configuration to determine whether it matches with any configured site-to-site Virtual Private Network (VPN) connections the device was intended to have. Monitor traffic for encrypted communications originating from potentially breached routers that is addressed to other routers within the organization. Compare the source and destination with the configuration of the device to determine if these channels are an authorized Virtual Private Network (VPN) connections or other encrypted modes of communication. Monitor ICMP traffic from the Internet that is addressed to border routers and is encrypted. Few if any legitimate use cases exist for sending encrypted data to a network device via ICMP.