MITRE ATT&CK · T1557

Adversary-in-the-Middle

MITRE · cited in 24 of our lessons

What attackers do. Attackers position themselves between two parties, such as a user and a website or two hosts on a network, to intercept or alter traffic. On local networks this can involve poisoning name resolution or ARP; on the internet it includes reverse proxy phishing kits that capture passwords and session cookies. It serves Credential Access and Collection.

How to defend. Disable legacy name resolution protocols such as LLMNR and NetBIOS where not needed, enforce SMB signing and encrypted protocols, and use network access control. Against phishing proxies, use phishing-resistant MFA such as FIDO2, which binds authentication to the real site. Monitor DHCP, ARP and DNS changes on internal networks.

Signs to look for. Look for duplicate or changing MAC to IP mappings, rogue DHCP servers, unexpected name resolution responses, and logins whose session is immediately reused from a different IP. The common gap is assuming MFA stops credential phishing when the kit relays the MFA step as well.

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The official text

Adversaries may attempt to position themselves between two or more networked devices using an adversary-in-the-middle (AiTM) technique to support follow-on behaviors such as Network Sniffing, Transmitted Data Manipulation, or replay attacks (Exploitation for Credential Access). By abusing features of common networking protocols that can determine the flow of network traffic (e.g. ARP, DNS, LLMNR, etc.), adversaries may force a device to communicate through an adversary controlled system so they can collect information or perform additional actions. For example, adversaries may manipulate victim DNS settings to enable other malicious activities such as preventing/redirecting users from accessing legitimate sites and/or pushing additional malware. Adversaries may also manipulate DNS and leverage their position in order to intercept user credentials, including access tokens (Steal Application Access Token) and session cookies (Steal Web Session Cookie). Downgrade Attacks can also be used to establish an AiTM position, such as by negotiating a less secure, deprecated, or weaker version of communication protocol (SSL/TLS) or encryption algorithm. Adversaries may also leverage the AiTM position to attempt to monitor and/or modify traffic, such as in Transmitted Data Manipulation. Adversaries can setup a position similar to AiTM to prevent traffic from flowing to the appropriate destination, potentially to impair defenses and/or in support of a Network Denial of Service.

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