Surge in .HTM Phishing Emails: Evading Filters & Targeting Trust

Cybercriminals are once again exploiting a familiar — but highly effective — phishing tactic: emails containing .htm attachments designed to steal credentials or install malware. At MailGuard, we’ve detected a notable surge in this attack pattern over the past fortnight. Our AI & ML-powered threat detection engines are successfully neutralising these threats ahead of Microsoft’s own filters — providing a vital lead time for our partners and customers.

While this method isn’t new, it’s the volume, sophistication, and filter evasion strategy that make this resurgence worth paying attention to.

🧠 Why Attackers Are Using .HTM Attachments

HTML attachments (.htm or .html) have become an increasingly popular method of attack because:

• They evade traditional filters: Many secure email gateways and Microsoft 365’s built-in protection deprioritise scanning .htm attachments, especially if they pass SPF/DKIM checks and contain no known-malicious URLs.
• They support interactivity: HTML supports JavaScript, redirects, and embedded content — perfect for fake login pages and scripts that trigger malware downloads.
• They obscure intent: Because .htm files require user interaction to open and execute, they often bypass sandbox analysis and link detectors.
• They don’t raise user suspicion: A file titled “invoice.htm” or “receipt.htm” seems innocuous enough to open — especially if the email mimics a trusted brand.

This makes .htm attachments a powerful and under-detected delivery method for phishing and credential harvesting.

🔍 Example 1: QuickBooks Impersonation with .HTM Attachment

This email claims to be a payroll confirmation from QuickBooks Online. It includes a .htm attachment disguised as a legitimate document. Once clicked, the file launches a credential harvesting website or malware payload.

🔍 Example 2 & 3: Microsoft 365 Renewal Scams

We’ve seen a spate of attacks impersonating Microsoft’s billing or renewal services.

These emails pressure the user to “confirm renewal details” or “verify a transaction” — triggering a .htm attachment that simulates the Microsoft login experience.

🎭 The Deception: Fake Login Page in Action

Once the .htm file is opened, the user is taken to what appears to be a legitimate Microsoft 365 sign-in page.

This screen creates the illusion of a standard redirect to the organisation’s Microsoft login portal — a trusted interface that doesn’t raise immediate suspicion.

After the user enters their email address, they’re prompted to enter their password:

The moment the password is submitted, it's harvested by attackers and often used immediately to access email accounts, cloud storage, and downstream business systems.

⚠️ Potential Impact of a Successful Compromise

• 🔓 Account Takeover
  Attackers gain access to inboxes, calendars, and sensitive internal communications, opening the door to highly effective social engineering and man in the middle (MitM) attacks. 
• 📂 Data Breach
  Documents in SharePoint, OneDrive, and Teams are at risk — including IP, financial records, and customer data.
• 💣 Malware Infection
  Some versions of this attack serve as a delivery vector for ransomware or keyloggers.
• 📉 Brand & Reputation Damage
  Recovery is expensive — not just financially, but in lost trust and customer confidence.

🤖 Why MailGuard Stops What Others Miss

Most traditional security layers rely on known-signature detection or sandboxing, which struggles to identify threats that:

• Are hosted on previously clean infrastructure
• Use obfuscated code inside attachments, or
• Don’t include visible links

MailGuard’s real-time, AI & ML-driven threat detection inspects each message and attachment on behavioural cues and emerging pattern indicators — not just static reputation or rule-based filters.

That means we’re detecting and stopping threats like this before they cause harm, often hours ahead of Microsoft Defender or other major security vendors.

🛡️ What You Can Do Right Now

• Educate your staff about .htm. They’re rarely used in day-to-day business and should be treated with caution.
• Layer your defences. If you’re relying solely on Microsoft 365’s built-in protections, you’re likely exposed to these fast-breaking campaigns.
• Request an email security health check. Ask our team about a Microsoft 365 email security health check that will show where your current defences are lacking — and how to fix them.

📚 Related Reading

• DocuSign Impersonation Phishing Campaign
• Fake Adobe Invoice Attack
• Cybercriminals Mimic Xero Billing Emails

🔔 Stay informed. Stay protected.
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MailGuard advises all recipients of these emails to delete them immediately without clicking on any links. Responding or providing your personal details can result in your sensitive information being used for criminal activity and may have a severe negative impact on your business and its’ financial well-being.    

MailGuard urges users not to click links or open attachments within emails that:      

• Are not addressed to you by name.      
• Appear to be from a legitimate company but use poor English or omits personal details that a legitimate sender would include.      
• Are from businesses that you were not expecting to hear from, and/or      
• Take you to a landing page or website that is not the legitimate URL of the company the email is purporting to be sent from.     
• Many businesses turn to MailGuard after an incident or a near miss, often as a result of an email similar to the one shown above. If unwanted emails are a problem for your business, don’t wait until it’s too late.

Reach out to our team for a confidential discussion by emailing expert@mailguard.com.au or calling 1300 30 44 30.

One email is all that it takes     

All that it takes to devastate your business is a cleverly worded email message that can steal sensitive user credentials or disrupt your business operations. If scammers can trick one person in your company into clicking on a malicious link or attachment, they can gain access to your data or inflict damage on your business.     

For a few dollars per staff member per month, you can protect your business with MailGuard's specialist, 'zero zero-day' email security. Special Ops for when speed matters!  Our real-time 'zero zero-day', email threat detection amplifies our client’s intelligence, knowledge, security and defence. Talk to a solution consultant at MailGuard today about securing your company's inboxes.  

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