Could Python in Excel be a boon for cryptocurrency miners?

Microsoft has unveiled Python in Excel, which allows a user to execute Python calculations using Azure Container instances. Has the company learned nothing from the Azure Pipelines cryptomining fiasco?

On the surface, giving Excel users the power to natively combine Python and Excel analytics in the same workbook is a good idea.

Typing Python into a cell, running calculations in the cloud, and receiving results in the worksheet is handy, where complex calculations and visualizations could otherwise stress a spreadsheet. As long, of course, as you have a robust connection, and you don’t mind beaming your work to the cloud.      

Unless we’re very much mistaken, however, it doesn’t take a huge leap of the imagination to spot the potential for abuse. After all, one needs only look to Microsoft’s recent past to see how people have exploited a certain naivete to mine cryptocurrency.

Microsoft cryptomining abuse: Is this a case of history repeating itself? 

At first glance, the implementation – currently a public preview in the Excel beta channel – looks secure. The Python code runs on a hypervisor in isolated containers built on Azure Container instances. It doesn’t have network or access to the local computer. It can’t get hold of a user token or access the user’s Microsoft 365 account. 

However, Excel performs calculations and executes Python in the Microsoft Cloud, which is where the problems could begin. 

Cryptocurrency mining is easily accomplished in Python; you just need hefty computational power to generate the relevant hashcode – which is where the Microsoft Cloud comes in.

Bitcoin mining, for example, is relatively simple to run in Python, although you’d need network access to synchronize transactions. Microsoft has stated the code will not have network access, but it’s not hard to imagine a threat actor finding a workaround and using Microsoft’s cloud resources for at least part of the process.

The new Excel feature requires only a Microsoft 365 account. No additional setup is required. And although workbooks from the internet or untrusted sources are subject to extra checks, there appears to be no restriction on the code you run other than the content of the Anaconda Distribution for Python, on which the system is based.

Mitigating abuse when using Python in Excel

Microsoft is keen to emphasize the security measures it’s taken, detailing this in a dedicated page on its site, Promising enterprise-level security and container isolation, however, means little if somebody decides to execute Python code in the Microsoft Cloud for nefarious purposes. There’s also scope for abuse via excessive compute usage, for example. 

We’re really hoping we’re wrong here, though, and Microsoft has a well thought-out mitigation strategy. Because, after all, Python in Excel makes use of the Anaconda distribution, which includes libraries for data manipulation and modelling – we can’t imagine it being too difficult to set it a challenge that could cause issues.

The company has also published a list of registry settings so IT admins (or anyone with access) can disable all security warnings for Python in Excel. One such instruction is: “Use this command to disable all security warnings. This is the default setting for Python in Excel”. But hey – this is all beta code, right?

Based on the information available at the time of writing, there appear to be no guardrails to prevent excessive resource usage.

Microsoft will need to ensure these are in place or face another Azure Pipelines incident, where cyber criminals abuse good intentions – and perhaps naivete.

There’s every chance Microsoft will make changes to this feature between now and general availability. The company has also warned that, although the preview is bundled with a user’s Microsoft 365 subscription, “after the Preview, some functionality will be restricted without a paid license”. This could be a critical line of defense. 

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