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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Yes—a file named with a .doc extension can actually contain an MHTML web archive rather than a conventional Word document. In a technique reported in 2015, attackers hid macro content in the archive’s MIME data and relied on the recipient to enable macros before the infection chain continued. The extension alone did not reveal the file’s true format.
How could a .doc file actually be an MHTML document?
MHTML is a format for saving a web page and its associated resources together in one file. A file using it may contain HTML and MIME-encoded content, even if its name ends in .doc or .xls.
SecurityWeek’s May 11, 2015 report described a bogus invoice attachment presented as a Microsoft Word .doc. The sample was actually an MHTML file. The report said its construction included an MSO object containing an OLE object, and noted a simpler route: save a macro-enabled Word document as MHTML, then rename the resulting .mht file with a .doc extension. SecurityWeek’s account of the 2015 sample.
Trustwave described the broader historical pattern in its Global Security Report 2016: attackers saved Word or Excel files as MHTML, renamed them with Office-looking extensions, and placed a malicious macro in a Base64-encoded MIME attachment. The file could appear as expected in Word or Excel while its underlying content was HTML-based. Trustwave, Global Security Report 2016.
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How did the 2015 infection chain work?
- Delivery: A bogus invoice email carried the disguised MHTML attachment, named to look like a Word document.
- Opening and consent: The recipient opened it and saw a prompt telling them to enable macros.
- Macro execution: After macros were enabled, the macro downloaded a VBS script.
- Further download: The VBS script downloaded and ran a Trojan downloader.
SecurityWeek said VirusTotal links suggested the final payload was a banking Trojan and quoted the researcher describing Dyre as likely—not confirmed. This account describes a specific historical sample, not proof of a current widespread campaign or a present-day detection rate.
Why did the unusual file structure matter to defenders?
Traditional email security tools that looked for macros in conventional Office documents might not find macro content carried in an unusual MIME location. Trustwave described that as a limitation of some tools at the time. It should not be generalized into a claim that current scanners routinely miss MHTML documents or that all security products behave alike.
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The practical lesson is narrower and durable: a filename extension does not establish what a file contains. A familiar-looking document name is not, by itself, evidence that an attachment is a normal Word or Excel file.
How is the 2021 CVE-2021-40444 attack different?
CVE-2021-40444 also involved MHTML, but it was a distinct, vulnerability-based attack—not the 2015 technique of placing a macro inside a disguised MHTML document.
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| Aspect | 2015 macro-in-MHTML technique | 2021 CVE-2021-40444 case |
|---|---|---|
| Where the malicious content fit | Macro content was stored in MIME data inside a disguised MHTML document, according to Trustwave’s 2016 account. | An Office document used an external OLE relationship to load remotely hosted MHTML content as part of an exploit chain, according to Microsoft. |
| How execution was triggered | The reported sample’s chain proceeded after the recipient enabled macros. | The observed attack exploited a vulnerability; Microsoft said content without a Mark of the Web could execute without the user interaction normally associated with Protected View. |
| Defensive response cited | Keep macros disabled and use caution with suspicious attachments. | Microsoft recommended applying security updates and documented an attack-surface-reduction rule to block Office applications from creating child processes for the observed attack. |
Microsoft’s analysis explains the separate 2021 vulnerability and its mitigations: Analyzing attacks that exploit the CVE-2021-40444 MSHTML vulnerability.
What should users and administrators do?
- Leave macros disabled for unexpected attachments. Microsoft’s macro-malware guidance recommends keeping macros disabled and avoiding suspicious messages and attachments. Microsoft: Macro malware.
- Be wary of prompts that ask you to enable macros. In the 2015 sample, enabling macros was the step that allowed the reported download chain to proceed.
- Set an organizational macro policy. Security researcher Bart Blaze recommended considering a Group Policy that blocks all macros or allows only digitally signed ones. Administrators should assess the impact on legitimate workflows before applying a policy.
- Check guidance for the deployed Office product and channel. Microsoft’s 2018 VBA and AMSI article described runtime inspection of macro behavior, including behavior obscured in source code. At publication, Microsoft said the integration was enabled by default for Office 365 client applications in the Monthly Channel; that dated statement is not a guarantee for every current Office installation. Microsoft: Office VBA + AMSI.
- For CVE-2021-40444 exposure, follow the vulnerability-specific guidance. Apply the relevant security updates and consult Microsoft’s documented attack-surface-reduction rule for the observed exploit. These measures address the separate vulnerability case, not the 2015 macro technique.
What is established—and what is not?
The cited accounts document a 2015 sample and a broader evasion pattern described by Trustwave in 2016. They do not provide a current prevalence estimate or a comparative detection rate. They also do not establish that every present-day security scanner misses this format. The evidence supports understanding the technique and applying basic macro precautions, not assuming a particular level of current threat activity.
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