What are the signs that an attacker-in-the-middle compromise is underway?

How to Recognise an Attacker-in-the-Middle Session Path

The clearest signs are a login path that does not fit the user’s normal behaviour and a session that remains valid after the user appears to be interacting from a different place. An attacker-in-the-middle often sits between the user and the service, so the artefacts you see are usually mismatched location, reused session state, and control-plane changes that the attacker can make after authentication.

What makes this pattern different from ordinary suspicious logins is the sequence. A hosting provider or anonymised network is often the first visible source, but the more telling clue is that the same authentication event does not line up with a corresponding interactive login history from that IP. In practice, that gap points to credential interception or session capture rather than a normal user login.

Mailbox or tenant changes are also strong indicators when they appear immediately after the suspicious authentication. New inbox rules, forwarding changes, altered recovery settings, consent grants, or unexpected token reuse all suggest the attacker has moved beyond observation and is now using the session to persist or redirect messages and alerts.

What Evidence Usually Confirms the Compromise

Confirmation comes from correlating identity, network, and mailbox evidence rather than relying on one alert. DNS logs or firewall telemetry that show a connection to a phishing domain tied to the login path are especially valuable because they connect the login event to the interception infrastructure. That is the difference between a suspicious event and a likely active compromise.

In a mature investigation, the supporting evidence should answer three questions: was the login source unusual, was the session state reused in a way that bypassed normal interactive behaviour, and did the user’s post-authentication state change in a way that the user did not initiate? When all three line up, the case is no longer just anomalous, it is operationally active.

For practitioners looking for broader breach patterns, NHIMG’s The 52 NHI breaches Report is useful for understanding how credential abuse and compromise paths show up in real incidents, while Ultimate Guide to NHIs provides the broader lifecycle and visibility context for why weak control over secrets and access paths makes compromise easier to sustain.

Practitioner Response When the Indicators Start to Line Up

What to prioritise: Correlate the first suspicious login with mailbox, token, and DNS or firewall events before deciding whether this is a user mistake or an active interception. The priority is to preserve the sequence, because attacker-in-the-middle cases are often missed when teams examine the login event in isolation.

What to verify: Check whether the source IP, user agent, and session timing match the user’s normal pattern, then verify whether any mailbox rules, forwarding destinations, or recovery settings changed after authentication. If the session was valid but the post-login state changed unexpectedly, treat that as evidence of adversary control rather than harmless noise.

Decision rule: If you can tie the login to a phishing domain, abnormal session reuse, or mailbox persistence changes, move directly to containment, token revocation, and rule cleanup instead of waiting for additional user confirmation. The attacker’s value comes from the still-valid session, not only from the initial credential capture.

Practitioner takeaway: The most reliable judgement is not whether a login looked odd, but whether the login, session, and mailbox state form a single attack chain that explains how the attacker stayed inside.

Related resources from NHI Mgmt Group

• What are the signs that residential proxy activity may be hiding an identity compromise?
• How do attackers turn a supply-chain incident into wider NHI compromise?
• What did Shai Hulud 2.0 actually compromise?
• How do attackers turn stolen npm secrets into broader compromise?