The Windows 10 update marks a major turning point for Microsoft customers who want to move to a cloud-based version of the classic operating system, but researchers say cybercriminals are acting quickly to dupe early adopters into installing ransomware as part of their upgrade.

Unlike previous versions of the OS that required a local installation, Microsoft is offering the Windows 10 update through a software-as-a-service (SaaS) model. However, Cisco revealed that cybercriminals are creating versions of the company’s email messages that, if clicked on, will inject the CTB Locker malware onto devices. A PC or tablet with CTB Locker installed will essentially be frozen unless the victim makes a cash payment within a four-day period. As CSO Online pointed out, the cybercriminals have replicated the Microsoft marketing materials very well, so be careful about what you open in your inbox regarding Windows 10.

The ransomware threat runs directly counter to Microsoft’s efforts to bolster its reputation for security with its Windows 10 update. For example, Trend Micro recently detailed major improvements in Edge, which will replace the Internet Explorer browser as part of the OS refresh. While IE had often been a target for attackers, Edge has done away with toolbars and Browser Helper Objects, which have proven vulnerable in the past. At the same time, Edge will include features such as an Enhanced Protected Mode sandbox, a way to deal with memory corruption and a tool for sweeping out what are known as use-after-free (UAF) exploits.

According to some experts, though, Microsoft may have overlooked a few other default features that could rile security professionals and put consumers at risk without being aware of it. The Hacker News reported that the Windows 10 update contains a mechanism called Windows Update Delivery Optimization (WUDO). While the concept of WUDO is well-intended — allowing those with a poor or limited Internet connection to get apps or updates quickly — the feature can also use up significant bandwidth. Throw in the way cybercriminals have traditionally used torrents to distribute malware, and it may be best in some cases to disable WUDO.

Of course, the Windows 10 update is the kind of thing that will trigger many computer users to make a significant change to their machines, so it’s not surprising that cybercriminals would want to find a way to get in on the action. And while Microsoft has suggested that in some ways this would be its last OS, don’t expect these to be the last security warnings you hear about it.