Email Thread Hijacking in 2026: Stop BEC Scams That Look Internal

Back in my day, scams came by fax, or a guy in a tie tried to sell you a “maintenance contract” for your dot matrix printer. In 2026, the scam shows up as a perfectly normal reply inside a perfectly real email conversation. That’s email thread hijacking, and I see the aftermath way too often: “We changed the vendor bank info like the email said” and then the money vanishes like a sock in a laundromat.

This is a flavor of business email compromise (BEC) that works because it looks internal. Not a weird cold email. Not some prince with a suitcase of gold. It’s your ongoing thread with your bookkeeper, your vendor, your project manager, your HOA, your contractor, your CPA. Same subject line. Same history. Same tone. Sometimes even the same signature. And that’s why it’s dangerous.

If you run a business in Palm Beach County (West Palm, Palm Beach Gardens, Lake Worth, Boynton, Wellington, Jupiter, Boca, and all the places where people still think “wire transfer” sounds classy), this is your practical checklist. No hype. No “AI-powered synergy.” Just boring but works controls.

What Is Email Thread Hijacking (And Why It Beats Your Spam Filter)

Email thread hijacking usually starts with a stolen mailbox session. The attacker gets into somebody’s email (often through password reuse, weak MFA habits, or a successful phishing prompt). Then they sit quietly, reading like a nosy neighbor. When the timing is right, they reply inside an existing conversation to push a payment change or urgent request.

What it looks like in the real world

• Invoice fraud: “Here’s the updated invoice. Please send ACH to the new account.”
• Vendor payment change scam: “We switched banks. Update our routing/account info going forward.”
• Gift card nonsense: “Need you to buy gift cards for a client ASAP. I’m in a meeting.” (If your CEO is always in a meeting, maybe that’s the real problem.)

The attacker may use the real compromised mailbox to reply, or they may spoof the domain and impersonate the display name. Either way, the email lands in the thread you trust. Your brain sees “continuation,” not “attack.”

Business Email Compromise (BEC) Red Flags: The Subtle Stuff People Miss

Look, I’m not going to sugarcoat this: most BEC losses happen because somebody didn’t slow down for 30 seconds. Here are the tells I see three times a week.

Red flags inside the message

• Payment urgency: “Today only,” “before cutoff,” “ASAP,” “confidential.”
• Process change midstream: new bank, new email, new portal, new “accounting contact.”
• Odd phrasing: slightly off tone, missing inside jokes, too formal, or weird punctuation (like someone trying to sound human).
• Attachment or link pressure: “Open this PDF” that’s actually a credential trap.

Red flags in the sender identity

• Display name impersonation: It says “Mary Smith” but the address is not Mary’s real address.
• Domain spoofing: vendorname.com becomes vend0rname.com or vendorname.co.
• Reply-to mismatch: From looks normal, Reply-To goes somewhere else.

Here’s what NOT to do: don’t rely on “it’s in the same thread, so it must be fine.” Email threads are not tamper-proof. They’re more like a VCR tape. Anybody can record over the good stuff if you leave it in the machine.

Email Thread Hijacking Checklist: BEC Protection That Actually Works

You don’t need the newest thing. You need the thing that works. This is the “boring controls” list that prevents most invoice fraud and payment reroutes.

1) Lock down Microsoft 365 security (and stop trusting luck)

If you’re on Microsoft 365, treat it like your accounting system, not like a free email box that came with the internet.

• Require MFA for everyone (yes, everyone). If you need a refresher, read Microsoft Support: what multi-factor authentication (MFA) is.
• Use Conditional Access where it makes sense: block sign-ins from countries you don’t do business with, require compliant devices for admin accounts, and tighten access for finance mailboxes. (Yes, it’s a pain. So is losing $48,000.)
• Disable legacy authentication if it’s still hanging around. Attackers love old doors that never got locked.
• Separate admin accounts from daily email accounts. Admin should not be browsing coupons and opening attachments.

2) Lock down Google Workspace security (same idea, different buttons)

• Enforce 2-step verification for all users, and especially for finance and executives.
• Review third-party app access and remove anything sketchy or unused.
• Check forwarding and routing settings so mail isn’t silently copied out to an attacker-controlled address.

3) Audit mailbox rules and forwarding (because attackers love automation)

This is the part everyone forgets. The attacker gets in, then creates inbox rules like:

• Auto-forward any email containing “invoice,” “wire,” “ACH,” “payment” to an external address
• Auto-move vendor emails to RSS or Archive so nobody notices
• Auto-delete “security alert” messages

Do a mailbox rules audit for executives, accounting, and anyone who approves payments. If you find forwarding you didn’t set up, assume compromise until proven otherwise.

4) Turn on DMARC, SPF, and DKIM (so spoofing gets harder)

These are not magic shields, but they matter. SPF helps indicate which servers can send for your domain. DKIM signs mail so it’s harder to tamper with. DMARC tells receiving systems what to do when SPF/DKIM fail (monitor, quarantine, reject).

Two blunt truths:

• DMARC does not stop an attacker who is already inside a real mailbox. That’s why you still need MFA and sign-in controls.
• DMARC helps a lot with domain spoofing and some impersonation attempts. It’s worth doing correctly.

5) Train people on display name impersonation (because humans read names, not addresses)

Most users glance at the display name and move on. That’s how “CEO Name <random-gmail-address>” gets paid. Teach staff to click the sender and verify the actual address, especially on payment requests.

Invoice Fraud and Vendor Payment Change Scam: The One Policy That Saves You

If you only do one thing after reading this, do this: implement an out-of-band payment verification process. That means you verify payment changes using a method that is not the email thread you’re currently staring at.

A simple payment verification process (boring, effective)

1. No bank detail changes by email alone. Ever. Not for ACH, not for wire, not for “new remittance instructions.”
2. Call a known-good phone number from your vendor master record, not the number in the email.
3. Require two-person approval for new payees or bank changes (accounting + owner/CFO).
4. Do a small test payment for new accounts when feasible, then confirm receipt by phone.
5. Document the verification in your accounting system or ticketing system. If it’s not written down, it didn’t happen.

Back in my day, people yelled at the TV because the VCR clock blinked 12:00. Now we’re wiring five figures based on a reply that says “Sent from my iPhone.” Progress.

Microsoft 365 and Google Workspace: What to Check After You Suspect a Hijack

Here’s what actually happens when you ignore this: the attacker stays in the mailbox, learns your payment rhythms, and strikes again later. So if you suspect email thread hijacking, don’t just delete the message and move on.

Immediate actions (first hour stuff)

• Reset the user’s password and revoke active sessions (sign out everywhere).
• Check MFA methods for changes (new phone, new app, new number).
• Review inbox rules, forwarding, and delegated access.
• Search for similar emails sent to other staff or vendors.
• Warn accounting and pause payments tied to that thread until verified.

Follow-up actions (the stuff people skip, then regret)

• Review sign-in logs for unusual IPs, locations, impossible travel, and repeated failures.
• Check OAuth app consents (attackers sometimes add “helpful” apps that keep access even after password changes).
• Harden finance accounts with stricter access rules than general staff.

If you need help doing this without breaking your day, that’s literally what our managed cybersecurity services for businesses are for. We’d rather prevent the mess than clean it up after your bank says “sorry.”

Where Virus Removal and Backups Fit (Yes, This Is Still Related)

People hear BEC and assume it’s “just email.” Sometimes it is. Sometimes the initial compromise came from malware stealing sessions or credentials. Either way, you should treat a compromised mailbox like a sign your whole device hygiene might be sloppy.

Don’t do this

• Don’t keep using the same laptop after you “changed the password” if it’s infected.
• Don’t assume OneDrive or Google Drive means you have a real backup.

Do this instead

• Run a proper cleanup and verification with professional virus removal and malware checks, especially for the user who got compromised.
• Set up boring, tested backups for critical business data with managed business backups. If you don’t have a backup, you don’t have data. You’re just borrowing it.
• If you already lost files during the scramble, stop “trying things” and call for data recovery services before you turn a small problem into a cooked hard drive.

And if you want ongoing reading that isn’t sales fluff, I tell folks to browse reputable threat write-ups like Malwarebytes threat research and scam write-ups. Not because they’re perfect, but because they live in the muck and report what’s actually happening.

Cybersecurity in Palm Beach County: What We See Locally (And Why It Matters)

Palm Beach County businesses are a sweet spot for BEC scammers: lots of real estate transactions, construction vendors, medical offices, legal offices, and small-to-mid companies where one person wears ten hats. That’s not an insult. That’s reality. And reality is what scammers eat for breakfast.

The pattern is consistent: one mailbox gets compromised, then the attacker targets payments. If you have a clear verification process and locked-down email, the scam fizzles. If you rely on “we’ll notice,” you won’t. Not until the money’s gone.

Quick Recap: Email Thread Hijacking Defense in Plain English

• Assume email threads can be tampered with. Verify payment changes out-of-band.
• Enforce MFA and tighten access with Conditional Access (Microsoft 365) or equivalent controls (Google Workspace).
• Audit mailbox rules and forwarding regularly, especially for finance and execs.
• Implement DMARC/SPF/DKIM to reduce spoofing and impersonation.
• Clean compromised devices and get your backups in order.