Reply Chain Attacks: How Hackers Hijack Trust Inside Your Mailbox

A project email thread with your vendor; you are in the middle of it. Files, updates, and approvals are all being exchanged in the thread.

All of a sudden, you receive a message: 

“Here’s the updated contract. Please review and sign.”

The subject line, formatting, and signatures are all the exact same. Even the conservation history remains the same, and you download the file without a second thought.

You have no idea that the file is fake and the sender is an attacker. The outcome? Your trusted thread just became a weapon. 

So What is a Reply Chain Attack?

A reply chain attack is a type of thread hijacking phishing. It refers to a scenario where an attacker inserts themselves into a legitimate or historical email conversation, but the underlying motive is to deliver malware, steal credentials, or execute financial fraud.

Key Characteristics

• The attacker tries to keep the context legitimate. For this, they use existing subject lines, quoted text, and prior conversation history. And in this way, they more naturally blend seamlessly into genuine correspondence. 
• Account Compromise or Spoofing is also one of the key characteristics of a reply chain attack. It may come from a hacked mailbox within the organization or from a lookalike domain that appears legitimate at first glance. And to make sure that the impersonation is convincing, the attackers often research the parties involved.
• Delivery Mechanisms:

• Malicious attachments (PDF, DOCX, XLSX) often appear to be contracts, invoices, or project updates
• Credential-harvesting links are created in a way that mimics login portals
• Fake invoices or payment requests timed to align with real project milestones

As these spoofed emails almost look credible and blend with the original thread, it is easier for them to bypass many security defenses and human suspicion. This makes them even more dangerous than cold-call phishing attempts, and this increases the chances of achieving the attacker’s intended outcome.

Why Reply Chain Attacks Work So Well

Attackers are well aware that the most powerful phishing email is one you don’t recognize as phishing at all. These are not something like random blasts, rather you can consider them as precision strikes designed to hide in plain sight.

Pre-Built Trust

This is one of the most important factors, as the email thread already contains trusted senders. Also, it contains the project context. For this reason, the employees don’t re-verify the legitimacy of the ongoing conversations. They assume that they are already from trustworthy sources. Moreover, the presence of familiar names and internal language reduces the suspicion.  

Visual Familiarity

From logos and email signatures to past correspondence, everything looks very authentic, and this makes the employee believe that it is just a part of the routine messages. A single malicious link may seem harmless, as the entire conversation history serves as a built-in credibility shield. 

Contextual Relevance

The attacker’s main aim is to blend in seamlessly, and for this, he uses relevant project details like timelines and jargon. They may give reference to specific milestones or documents mentioned earlier in the thread to appear engaged and knowledgeable.

Security Blind Spots

Ongoing threads from known senders are often whitelisted by many filters, and this is exactly what creates a safe lane for the attackers. Inspection of the ‘middle’ of a conversation is difficult for the detection engines, and they fail to assume any kind of danger or threat.

Common Entry Points for Attackers

Vector	How It’s Exploited
Compromised Email Account	Stolen credentials allow direct access to real email threads.
Spoofed Reply-To Headers	Spoofed email threads appear to be part of an existing conversation.
Lookalike Domains (vendor impersonation phishing)	Example: vendor-support.com → vendor-supp0rt.com.
Forwarded Threads	Attackers inject malicious edits before sending.

Visual Cues Users Often Miss

Email Element	Legitimate?	Risk
Thread Subject	Same as original	Reinforces trust
Signature	Copied from old thread	Easy to forge
Attachments	PDF/DOCX	Can contain malicious macros
Sender Name	Familiar	The domain may differ subtly

Real-World Example: Emotet Reply Chain Campaigns

When it comes to reply chain phishing, you can’t miss out on the Emotet malware group. They compromised a mailbox, and then they harvested the entire conversation histories. Next, they replied with a malicious attachment that appeared just like a legitimate document update. 

As the email came from the actual account and included full context, open rates were higher than traditional phishing attempts. In some campaigns, nearly 45% of targeted recipients opened the attachments, and this is something very rare in standard phishing metrics.

Security Blind Spots in Reply Chain Attacks

1. Filter Overconfidence
   • Security tools may trust ongoing threads and skip deep inspection.
     
2. User Workload
   • Employees skim emails during busy hours and overlook anomalies.
   • Long email chains condition recipients to “click first, think later.”
     
3. Historic Thread Abuse
   • Attackers may reply to a thread from months ago, catching recipients off guard.
   • The sudden reappearance of an old conversation often triggers curiosity rather than suspicion.
     
4. Multiple Participant Risk
   • A new “stakeholder” added mid-thread may be an attacker.
   • Users may not question why someone new is suddenly part of the discussion.

Threatcop’s Framework for Preventing & Detecting Reply Chain Attacks

1. Assess (TSAT)

• Simulate thread hijack scenarios:
  • Fake invoice approvals
  • “Updated” document delivery
  • Old project email continuation
    
• Analyze click, open, and report behavior to measure team resilience.
• Use analytics to identify departments most vulnerable to trust-based deception.

2. Aware (TLMS)

• Train employees to:
  • Inspect email headers and reply-to addresses
  • Question unexpected attachments; even in trusted threads
• Promote the “trust reset” mindset: Every email is a new verification opportunity, even inside an old conversation.

3. Protect (TDMARC)

• Enforce domain-based authentication to block spoofing.
• Detect and flag lookalike domains attempting to impersonate vendors or internal staff.

4. Empower (TPIR)

• Enable one-click reporting for suspicious messages—especially within trusted threads.
• Encourage context-based anomaly reporting:
  • “The tone in this email feels different.”
  • “Why is there a sudden payment request?”

Trust Decay Checklist for Users

Before you take any action on an email inside a reply chain, be cautious and ask:

1. Is the sender domain an exact match?
2. Is there a sudden urgency or a payment request?
3. Are there new or unexpected attachments/links?
4. Did the tone or language style change?
5. Have I verified the request via another channel?
6. Does the attachment require macros or unusual permissions to open?

Zero Trust Takeaway

In a reply chain attack, trust is already built, and this is exactly the plus point for the attackers. So, now is no longer the time to rely solely on spam filters or attachment scanners. Rather, organizations need to combine people-centric vigilance with simulation, training, authentication, and rapid reporting.

Now you are aware of how a single compromised reply chain can trigger a chain of breaches. It affects not only your business but also every partner and client in the conversation. So now is the time to get in touch with cybersecurity experts for the right assistance!