Email Thread Hijacking in 2026: Stop Reply-Chain BEC Scams

I see this exact problem three times a week. Somebody walks in (or calls) from Palm Beach County with a simple question: “Did our vendor really change their bank info?” And I already know the answer. No. Your vendor didn’t change anything. You got hit with email thread hijacking, the sneakier cousin of old-school phishing.

Back in my day, scams came in big flashing neon signs: broken English, weird attachments, and a “Nigerian prince” who somehow needed a wire transfer. Now? The email looks like it’s part of a real conversation you’ve been having for weeks. Same subject line. Same signature. Same tone. It’s like somebody slid a fake cassette tape into the case and hoped you wouldn’t notice until track 7.

This post is the boring-but-works guide to spotting reply-chain phishing, preventing business email compromise (BEC), and hardening Microsoft 365 or Google Workspace so your money doesn’t walk out the door wearing a fake mustache.

Email Thread Hijacking: What It Is and Why It Works

Email thread hijacking is when an attacker inserts themselves into a legitimate email conversation and uses the trust in that thread to push a financial request: wire transfers, ACH changes, “updated invoices,” gift cards (yes, still), or “new payment portal” links.

How attackers get into the conversation

• Compromised mailbox login: stolen password, weak MFA, reused password from some other breach. (If you reuse passwords, you’re basically leaving your keys under the doormat.)
• Malicious mailbox rules: auto-forwarding, auto-deleting, or moving messages so the real owner never sees the warning signs.
• OAuth app consent scam: user clicks “Allow” on a shady app that requests mailbox access. No password required after that. Just permission. Like handing a stranger your garage door opener because they wore a polo shirt.

Why reply-chain phishing beats “regular” phishing

• The message lands inside a real thread, so people don’t scrutinize it.
• It often references real invoices, projects, or names pulled from prior emails.
• It arrives at the worst possible time: end of day, payroll day, closing day, or when the boss is traveling and cranky.

And here’s what actually happens when you ignore this: the money goes out, the scammer launders it fast, and your bank says, “Sorry.” Then everybody argues about whose fault it was. Spoiler: it’s everyone’s fault if there was no verification process.

Reply-Chain Phishing Red Flags (The Subtle Stuff People Miss)

Don’t do this: don’t assume “it’s in the thread, so it must be legit.” That’s like assuming the VCR is set correctly because the clock is blinking 12:00. It’s not. It never was.

Red flags inside the email content

• Payment urgency: “Need this today,” “closing is delayed,” “we’ll incur fees.” Pressure is the scammer’s favorite tool.
• Quiet change requests: bank details, remittance address, “showing a new portal,” or “send to this alternate email.”
• Different writing rhythm: the “boss” suddenly types like a robot, or the vendor rep stops using their normal sign-off.
• Attachment swaps: a “revised invoice” PDF that you weren’t expecting, especially if it appears mid-thread with no explanation.

Red flags in the email headers and addresses

• Domain lookalikes: “vendor-co.com” vs “vend0r-co.com” (that’s a zero). Or a sneaky extra word like “-billing” added.
• Reply-to mismatch: the From looks normal, but Reply-To points somewhere else.
• New external sender inside internal-looking thread: a message appears “from” a known name but the address is off by one character.

Yes, this is nitpicky. Security is nitpicky. So is brake maintenance, and I reminded you anyway.

Business Email Compromise (BEC) Protection: The Payment Change Rule

If you want the single most effective BEC protection move for invoice fraud prevention, it’s this:

Never accept payment changes over email alone.

The verification process that actually works

1. Stop: if the email asks for bank changes, a new payee, gift cards, or “send it to this new account,” pause.
2. Verify out-of-band: call a known-good phone number from your vendor master file or prior contract. Not the number in the email.
3. Confirm with two people: one requester, one approver. Yes, even if you’re “too small for that.” Scammers love “too small.”
4. Document the change: who approved, when, and how verified.

Back in my day, you couldn’t “update banking” without paper forms, a fax machine, and a grumpy accountant. Turns out the grumpy accountant had a point.

Microsoft 365 Security: Lock Down the Usual Break-In Points

Most Palm Beach County businesses we help are on Microsoft 365. It’s fine. It’s also a big target. You don’t need fancy magic. You need the basics done correctly and checked regularly.

Turn on strong sign-in protection (MFA and conditional access)

• Require multi-factor authentication for all users, especially finance and executives. If you need a reference, see Microsoft Support on MFA in Microsoft 365.
• Use Conditional Access where available: block sign-ins from risky locations, require compliant devices, and reduce the “log in from a motel Wi-Fi in another country” problem.

Audit mailbox rules (the quiet hiding place)

Reply-chain BEC attackers love inbox rules because rules don’t complain. Rules don’t ask questions. Rules just do what they’re told, like an old microwave with a sticky keypad.

• Look for rules that auto-forward externally.
• Rules that move invoices to odd folders (RSS, Archive, Deleted Items).
• Rules that mark messages as read or delete security alerts.

If you want help doing this properly, that’s exactly what our business cybersecurity services and email security audits are for. Because doing it once is good. Doing it monthly is better.

Check OAuth app consents (the “Allow” button problem)

OAuth app consent scams are the modern version of letting a stranger “borrow your car for a minute.” Users click “Allow” on an app that requests access to mail, files, or contacts. Then the attacker can read mail or send as the user without stealing the password again.

• Review which apps have access to Microsoft 365 accounts.
• Remove anything unknown, unnecessary, or “we tried it once in 2023 and forgot.”
• Limit user consent where appropriate, so random apps can’t get mailbox access with one click.

Google Workspace Security: Same Scam, Different Paint Job

Google Workspace shops get hit too. The scam doesn’t care which logo is on your login page.

What to lock down in Google Workspace

• Enforce 2-Step Verification for all users.
• Review third-party app access and remove suspicious or unused apps.
• Watch for forwarding addresses and routing rules that leak mail externally.

And no, “we’re on Google so we’re safer” is not a security plan. That’s a vibe. Vibes don’t stop invoice fraud.

DMARC, SPF, and DKIM: Boring Email Plumbing That Saves Real Money

Don’t do this: don’t ignore DMARC/SPF/DKIM because it sounds like alphabet soup. This is the part that helps stop attackers from spoofing your domain and makes it easier to trust legitimate mail.

What each one does (plain English)

• SPF: says which mail servers are allowed to send mail for your domain.
• DKIM: adds a cryptographic signature so recipients can verify the message wasn’t altered.
• DMARC: tells receiving servers what to do if SPF/DKIM checks fail, and gives you reports.

Set them up correctly, then move DMARC toward enforcement (meaning failures get quarantined or rejected). If that sentence made you tired, good. This is “good refrigerator” work: quiet, unglamorous, effective.

Invoice Fraud Prevention: Process Beats Panic

Tools help. Process prevents. If your process is “pay whatever looks normal,” you’re going to lose money. Not maybe. Eventually.

Simple controls that stop most BEC losses

• Vendor change control: bank detail changes require out-of-band verification and management approval.
• Two-person approval for wires: especially for new payees or changed instructions.
• Limit who can create payees in your accounting system.
• Use a known-good vendor list with verified phone numbers.

Also, if you don’t have a backup, you don’t have data. You’re just borrowing it. If a mailbox compromise turns into ransomware or mass deletion, you’ll wish you had managed business backups that are tested, not just “set and forget.”

Security Awareness Training: Teach People What to Do, Not Just What to Fear

Security awareness training isn’t about scaring staff with hacker boogeymen. It’s about giving them a short checklist they can follow when they’re busy and distracted (which is always).

What to train for reply-chain BEC scams

• Payment-change requests are always verified by phone or in-person.
• Hover and inspect email addresses and links, especially inside threads.
• Report fast: if someone clicked or replied, report immediately. Speed matters.
• No shame reporting: the faster we know, the more we can contain.

And if you think “our people would never fall for that,” I’ve got a box of old Windows XP driver CDs to sell you. Smart people get tricked when they’re rushed.

What To Do If You Suspect Email Thread Hijacking

Look, I’m not going to sugarcoat this. Time is everything.

Immediate steps

1. Stop payments: call your bank immediately if money moved.
2. Change passwords and revoke sessions for the affected account(s).
3. Review mailbox rules and forwarding settings.
4. Review OAuth app access and remove suspicious consents.
5. Scan endpoints: if the compromise started on a PC, clean it. Our professional virus removal service is built for exactly this kind of mess.

If data is missing or deleted

If mail, files, or accounting data got wiped, don’t “try a bunch of random fixes” first. That’s how people turn a recoverable problem into a bonfire. Start with data recovery help and get the situation assessed before more damage happens.

For a solid overview of how BEC works in the real world, Malwarebytes has a good explainer: business email compromise basics and protections.

Palm Beach County Reality Check: “It Won’t Happen to Us” Is Expensive

We help businesses across Palm Beach County, including West Palm Beach, Palm Beach Gardens, Lake Worth Beach, Boynton Beach, Wellington, Royal Palm Beach, Jupiter, and Delray Beach. And the pattern is always the same: the scam works because it blends into normal work.

You don’t need the newest thing. You need the thing that works: verified payment procedures, hardened email settings, and regular audits. Boring. Effective. Like a good refrigerator.