Buyer beware. Sascha Kohlmann/Flickr (CC) Some tablets being sold on Amazon are apparently coming with pre-installed malware on them, and can delete users' apps and force them to look at unwanted adverts.

That's according to an investigation from Chinese mobile company Cheetah Mobile.

Cheetah's researchers have been looking into a particular Trojan called "Cloudsota," and say that it is frequently coming pre-installed on more than 30 different tablet brands.

These aren't iPads or Google Nexus tablets, of course — they're low-end devices from no-name manufacturers, typically built in China. The most affected, Cheetah says, are "no brand" devices that use Allwinner chips.

The company says that devices loaded with the trojan are being sold on Amazon, highlighting 10 allegedly affected listings. In total, Cheetah says that more than 17,000 infected devices have been sold, though not all through Amazon — the tablets are also available for purchase elsewhere on the web.

Cloudsota "can install adware or malware on the devices and uninstall anti-virus applications silently," according to Cheetah. "With root permission, it is able to automatically open all installed applications. Furthermore, we found that the Trojan replaces the boot animation and wallpapers on some devices with advertisements. Cloudsota also changes the browser’s homepage and redirects search results to strange ad pages."

On a $59.99 (£40) tablet currently on sale on Amazon US, a customer complains that it is "loaded with malware/adware that can't be removed. Shows full screen pop ads all the time and suspends app you have running." Another device on Amazon US — this one on sale for $79.99 (£53)— has a review claiming that "the tablet itself and keyboard cover are great, but it came pre-installed with Trojan.coudw.a in 2 system files ... I also had issues with advertisement pop-ups coming up on the desktop -- even before I opened the browser and before I installed anything! This is another symptom that this thing came infected -- and it seemed new when I got it!"

Both of these devices are sold by third-party sellers, but "fulfilled" by Amazon — meaning that Amazon holds the stock in its warehouses.

Business Insider has reached out to Amazon for comment, and will update this story when it responds.

This isn't the first time something like this has happened. In June 2014, eBay banned listings of the Star N9500 smartphone after a security firm discovered that it was coming pre-loaded with a trojan that tracked unwitting users' activity. And realistically, this won't be the last time, either.

But it serves as a cautionary measure that if you're buying something extremely cheaply, then it often comes with a cost.