MHTML

MHTML
Filename extension
.mht, .mhtml
Internet media type
  • multipart/related
  • application/x-mimearchive
  • message/rfc822
Type of formatMarkup language
Extended fromHTML
StandardRFC 2557 (proposed 1999)

MHTML, an initialism of "MIME encapsulation of aggregate HTML documents", is a web archiving file format used to combine, in a single computer file, the HTML code and its companion resources (such as images) that are represented by external hyperlinks in the web page's HTML code. The content of an MHTML file is encoded using the same techniques that were first developed for HTML email messages, using the MIME content type multipart/related. MHTML files use an .mhtml or .mht filename extension.

The first part of the file is an e-mail header. The second part is normally HTML code. Subsequent parts are additional resources identified by their original uniform resource locators (URLs) and encoded in Base64 binary-to-text encoding. MHTML was proposed as an open standard, then circulated in a revised edition in 1999 as RFC 2557.

The .mhtml and .eml filename extensions are interchangeable: either filename extension can be changed from one to the other. An .eml message can be sent by e-mail, and it can be displayed by an email client. An email message can be saved using a .mhtml or .mht filename extension and then opened for display in a web browser or for editing other programs, including word processors and text editors.

Layout

The header of an MHTML file contains metadata such as a date and time stamp, page title, the source URL, and a unique randomized boundary string for separating resources contained within the file. The boundary string is defined at the beginning and used throughout the file.

From: <Saved by Blink>
Snapshot-Content-Location: https://en.wikipedia.org/wiki/Smartphone
Subject: Smartphone - Wikipedia
Date: Sat, 24 Sep 2022 00:34:32 -0000
MIME-Version: 1.0
Content-Type: multipart/related;
        type="text/html";
        boundary="----MultipartBoundary--GsIBda0vjy2AKIAIliwl7JMwezXDRjDAsLje9khd5l----"

Then, the page resources are contained sequentially, starting with the page's rendered HTML source code. Each resource has its own metadata header which specifies its MIME type and the original location.

------MultipartBoundary--GsIBda0vjy2AKIAIliwl7JMwezXDRjDAsLje9khd5l----
Content-Type: text/html
Content-ID: <frame-D968CEC8BB7E60A1859261A8CA5DFB4D@mhtml.blink>
Content-Transfer-Encoding: binary
Content-Location: https://en.wikipedia.org/wiki/Smartphone

<!DOCTYPE html>

The MHTML file ends with a boundary string that is not followed by any data.

MIME type

MIME type for MHTML is not well agreed upon. Used MIME types include:

Supporting apps

Web browsers

Some browsers support the MHTML format, either directly or through third-party browser extension.

Other apps

Exploits

In May 2015, a researcher noted that attackers could build malicious documents by creating an MHT file, appending an MSO object at the end (MSO is a file format used by the Microsoft Outlook e-mail application), and renaming the resulting file with a .doc extension. The delivery method would be by spam emails.

In April 2019, a security researcher published details about an XML external entity (XXE) vulnerability that could be exploited when a user opens an MHT file. Since the Windows operating system is set to automatically open all MHT files, by default, in Internet Explorer, the exploit could be triggered when a user double-clicked on a file that they received via email, instant messaging, or another vector, including a different browser.

Alternatives

Self-contained HTML files

Contemporary HTML files support storing images, style-sheets and scripts in-line using the data URI scheme. Like the embedded content within MHTML, data URIs use Base64 encoding of the external resources. All modern web browsers can open self-contained HTML files. However, As of February 2026, creating self-contained HTML pages requires browser extensions (such as SingleFile) or third-party app (such as pandoc).

Mozilla Archive Format

The Mozilla Archive Format (MAFF) is a legacy Web archive file format that was supported by Firefox from 2004 to 2018 through an add-on. Unlike both MHTML and data URIs, MAFF uses a ZIP container to preserve both the HTML file and its external elements. In October 2017 the add-on developer announced the format would no longer be supported in future versions of Firefox.

See also

References

  1. ↑ "Key Concepts of MHTML". Microsoft. 9 November 2009. Archived from the original on 21 February 2026. Retrieved 21 February 2026.
  2. ↑ Holden, Amanda. "Difference of HTML & MHTML". Archived from the original on 17 November 2017. Retrieved 17 November 2017.
  3. ↑ "2. The MHTML File Format - Hunchly Knowledge Base". support.hunch.ly. October 17, 2018. Retrieved 24 September 2022.
  4. ↑ "Bug 40873 - Save as rfc 2557 MHTML; complete webpage in one file".
  5. ↑ "NEWS · master · GNOME / Epiphany". 28 July 2023.
  6. ↑ Santambrogio, Claudio (10 March 2006). "…and one more weekly!". Opera Software. Archived from the original on 15 January 2010. Retrieved 2009-05-15.
  7. ↑ "Pale Moon Add-ons - MozArchiver". addons.palemoon.org. Retrieved 3 December 2025.
  8. ↑ février 6, Publié sur; Tetzchner, 2019-Par Jon von (2019-02-06). "Vivaldi Update | Auto-Stacking Tabs". Vivaldi (in French). Retrieved 2019-05-16.{{cite web}}: CS1 maint: numeric names: authors list (link)
  9. ↑ Kovacs, Eduard (May 11, 2015). "Attackers Hide Malicious Macros in MHTML Documents". SecurityWeek.Com. Retrieved April 19, 2019.
  10. ↑ Mosuela, Lordian (July 10, 2015). "New Tricks of Macro Malware". Cyren. Archived from the original on September 8, 2024. Retrieved April 19, 2019.
  11. ↑ Cimpanu, Catalin (April 12, 2019). "Internet Explorer zero-day lets hackers steal files from Windows PCs". ZDNet. Retrieved April 19, 2019.
  12. ↑ "data: URLs". MDN Web Docs. Mozilla Foundation. 31 October 2025. Browser compatibility.
  13. ↑ Brinkmann, Martin (2 February 2026). "Save any webpage as a single file in Chrome or Firefox". gHacks Tech News. gHacks.
  14. ↑ "Mozilla Archive Format Add-on - File Format Overview". amadzone. Retrieved April 2, 2023.
  15. ↑ "Firefox Addon: MAF - Mozilla Archive Format". Archived from the original on 2 November 2017. Retrieved 2 April 2023.