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Microsoft 365, Malware 👾, and your responsibilities

· 4 min read
Nočnica Mellifera

a clone of the game 'space invaders' Cover image By Lee Robinson -, MIT,

When we evaluate malware threats, we often think mainly of protecting our users. The biggest concern is always going to be lost availability and leaked data if malware affects our system. But like any threat with an infection model, part of the story is about your responsibilities as an operations engineer to keep others safe.

Microsoft-hosted Malware

Research earlier in 2023 showed that Microsoft OneDrive was host to about 30% of all malware. OneDrive is a popular platform for hosting malware because the malicious actor can get a legitimate looking URL that will increase the chance of their payload being downloaded or executed.

This malware hosting is usually done on accounts created by malicious actors, but it's even more effective if a compromised account within a legitimate organization can be used

The responsibility for addressing this issue lies more with administrators than with Microsoft. Data on OneDrive is customer data and it will be intrusive and disruptive for Microsoft to automatically start taking down files.

Anyone running OneDrive and SharePoint should take measures to detect and remove malware - to protect their own users and the broader community.

If you accept that as Microsoft 365 and OneDrive user you should ‘be part of the solution’, how can you take a stand against malware?

Scan for Malware

Sophisticated malware is difficult to engineer. Threats like BazarLoader, which use a Trojan horse to create an ISO which waits for the user to open an innocent-looking ‘Documents’ folder, aren’t being developed from scratch every day. Therefore, it’s possible to scan for malware and find most threats before they affect large numbers of systems.

While there are a number of tools to scan backups, attachments, and other file locations, I’m pleased to say that Corso has implemented malware scanning for your backups as of V0.5.0. Corso aims to prevent content already flagged as malware from making it in your backups. Since Corso is free and open-source, admins can take advantage of this and take action (for example delete, extract for forensic analysis) against files flagged by Corso.

See it in Action: Create a Malware-Free Backup with Corso

We hope that the first time you use a tool like Corso to scan your backups, you will have no malware detected. This however begs the question: how do we know it’s working?

Good news: there are long-standing resources to grab ‘known bad’ files that should set off any malware or virus scanner. The European Institute for Computer Anti-Virus Research (EICAR) have made such a file available. With this rather choice paragraph about why a non-virus ‘known bad’ file is useful for security practices:

Using real viruses for testing in the real world is rather like setting fire to the dustbin in your office to see whether the smoke detector is working. Such a test will give meaningful results, but with unappealing, unacceptable risks.

Download the EICAR test file here. Any scanner worth its salt will alert on at least the first two versions of the file ( and and should notice malware inside a .zip as well. When using Corso with any of these files, the feedback is quite clear:

Corso giving feedback

Any detected files will be listed as 'skipped' and the rest of the backup will complete as normal.

What to do when Corso Detects Malware

Files that Corso detects as malware will be skipped from backups, but you should take steps to delete these files and do some analysis of their source within your OneDrive instance. When Corso detects malware, it will log the fact (Corso’s log location is displayed when the CLI runs).

Image of Corso logging errors and exceptions, with one item of malware detected

Lines for detected malware will show up marked as malware detected and will even have a malware_description parameter.

Monitor for new reports

The landscape for malware is shifting, and it’s vital you stay on top of new reports. Three sources of updates I’d recommend:

If you keep these practices in place in your organization, not only are you less likely to suffer from malware attacks, but the danger of your playing host to malicious files and attacks on others will be greatly reduced!