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12/07/2026 12:21 pm
TL;DR: Email reply chain attacks abuse already trusted conversation threads after an email account takeover, making malicious attachments and links far harder to spot than ordinary phishing, according to SentinelOne. The real lesson is that human trust, mailbox rules, and endpoint controls fail together when identity compromise becomes the entry point.
NHIMG editorial — based on content published by SentinelOne: email reply chain attacks and how they work
Questions worth separating out
Q: What breaks when attackers hijack an existing email thread?
A: The normal warning signals break down because the message comes from a real account inside a trusted conversation.
Q: Why do reply chain attacks increase business email compromise risk?
A: They increase risk because recipients already trust the sender and the conversation context looks familiar.
Q: How can security teams detect hidden mailbox persistence?
A: Look for unusual forwarding rules, inbox filters, auto-deletes, and message redirection that change how the account handles mail.
Practitioner guidance
- Review mailbox rules and forwarding settings routinely Audit user mail rules, inbox filters, and forwarding destinations for unexpected redirects, hidden deletions, and auto-replies that suppress suspicious messages.
- Treat mailbox takeover as an identity incident When an account is suspected of compromise, reset credentials, revoke sessions, inspect delegated access, and validate whether the account has been used to reply within active threads.
- Restrict Office macros and script-enabled attachments Block or tightly control Office Macros, especially where email delivers documents that can trigger VBScript, PowerShell, or other payload loaders.
What's in the full article
SentinelOne's full analysis covers the operational detail this post intentionally leaves for the source:
- Specific malware family behaviour and plugin use in reply chain operations
- Examples of mailbox rule abuse and message diversion techniques used to hide the compromise
- Step-by-step defensive recommendations for email hardening, awareness training, and endpoint protection
👉 Read SentinelOne's analysis of email reply chain attacks and thread hijacking →
Email reply chain attacks: what IAM teams are missing in threads?
Explore further
12/07/2026 12:34 pm
Reply chain attacks are really mailbox identity abuses, not just phishing variants. The attacker does not need to win a spoofing contest if they can use a real, compromised account and inherit the trust attached to the thread. That shifts the control question from message authenticity alone to account integrity, mailbox governance, and user visibility. Practitioners should treat every compromised mailbox as a high-confidence fraud and malware relay.
A few things that frame the scale:
- The average estimated time to remediate a leaked secret is 27 days, despite 75% of organisations expressing strong confidence in their secrets management capabilities, according to The State of Secrets in AppSec.
- Only 44% of developers are reported to follow security best practices for secrets management, which helps explain why trust-based abuse often starts with weak identity hygiene in the first place.
A question worth separating out:
Q: How should organisations handle compromised government or law enforcement email accounts?
A: Treat them as privileged identity incidents, not simple mailbox abuse. Disable the account, revoke sessions and tokens, check connected portals and delegated access, and validate whether any legal, investigative, or takedown requests were issued from the identity. Then review how the account was obtained, because password reuse, phishing, and infostealers often affect more than one system.
👉 Read our full editorial: Email reply chain attacks expose the trust gap in IAM and EDR
