Sunday, September 19, 2021

Home windows zero-day MSHTML assault – how to not get booby trapped! – Bare Safety

.Details are scarce so far, but Microsoft is warning Office users about a bug that’s dubbed CVE-2021-40444, and described as Microsoft MSHTML Remote Code Execution Vulnerability.

The bug doesn’t have a patch yet, so it’s what’s known as a zero-day, shorthand for “the Good Guys were zero days ahead of the Bad Guys with a patch for this vulnerability.”

In other words: the crooks got there first.

As far as we can tell, the treachery works like this:

  1. You open a booby-trapped Office file from the internet, either via an email attachment or by downloading a document from a criminal-controlled web link.
  2. The document includes an ActiveX control (embedded add-on code) that ought not to have unrestricted access to your computer.
  3. The ActiveX code activates the Windows MSHTML component, used for viewing web pages, exploits a bug in it to give itself the same level of control that you yourself would have right from the Windows desktop, and uses it to implant malware of the attacker’s choice.

MSHTML isn’t a full-on browser, like Internet Explorer or Edge, but is a part of the operating system that can be used to create browsers or browser-like applications that need or want to display HTML files.

Even though HTML is most closely associated with web browsing, many apps other than browsers find it useful to be able to render and display web content, for example as a convenient and good-looking way to present documentation and help files, or to let users fill in and submit support tickets.

This “stripped down minibrowser” concept can be found not only on Windows but also on Google’s Android and Apple’s iOS, where the components Blink and WebKit respectively provide the same sort of functionality as MSHTML on Microsoft platforms. Mozilla products such as Firefox and Thunderbird are based on a similar idea, known as Gecko. On iOS, interestingly, Apple not only uses WebKit as the core of its own browser, Safari, but also mandates the use of WebKit in browsers or browser-like apps from all other vendors. That’s why Firefox on iOS is the only version of that product that doesn’t include Gecko – it has no choice but to use WebKit instead.