Microsoft's JavaScript Update for Excel Is a Gift to Cryptominers

Microsoft announced at its Build 2018 conference this week -- and in a blog post -- that it was going to add custom functionality locally in Excel spreadsheets through JavaScript.

The idea here, according to Redmond, is that it will be used for functions such as calculating math operations (like whether a number is prime), bringing information from the web (like a bank account balance) or streaming live data (like a stock price).

That might be true, but a merry band of pranksters out in the wild haa already figured out how to make this pay off for them through cryptomining scheme. (See Cryptocurrency Crime: The Internet's New Wild West.)

Security expert Charles Dardaman wrote in his blog that he figured out a way to do exactly that.

(Source: Security Now)

Of course, Microsoft made it sort of easy to come up with JavaScript functions for Excel by publishing most of the steps to activate it.

Not only that, the company published several files on GitHub that are essential to it.

Dardaman outlines the simple edits that are necessary to glue everything together.

The Coinhive script is pathed as a "script src" in HTML as the first step. A wrapper is then specified as a function, and finally a JSON file for the actual miner function used inside Excel is outlined.

All three files need to be hosted on an accessible server -- a local server will do just fine.

An XML file that serves as a manifest for these hosted files must also be specified.

If all is done in this manner, the mining function will show up when Excel is opened. It can then be inserted into a cell, and then start running.

The function has persistence, so if the XLSX document with the inserted miner is reopened then the mining will activate.

While the Excel JavaScript custom functions feature is currently available in the Developer Preview edition of Office 365 for subscribers enrolled in the Office Insiders program, it can be assumed that it will be rolled out to all users over time.

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It seems reasonable to also assume that the setup for this sort of function will be much simpler as the general release date approaches. Some of the manual editing that now goes into establishing the JavaScript functionality will no doubt be unnecessary.

That means Joe User will be able to do this sort of thing on his own.

That worries Dardaman. As a matter of fact, his opinion is that JavaScript should be blocked from Excel in its entirety once this new version hits general release.

His fears are understandable, but there may be use cases for a JavaScript-enabled spreadsheet in the organization that would stop this kind of Draconian solution.

Unfortunately, we will need to take another look at the entire situation as the release date grows nearer, in order to come up with something that protects both the organization, as well as the user.

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— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.