Dozens of low-cost Android phone models come preinstalled with apps that covertly download and install adware and other unwanted programs, researchers said.

At least 26 phone models come preinstalled with a downloader dubbed Android.DownLoader.473.origin, according to a blog post published Monday by antivirus provider Doctor Web. Doctor Web researchers described the app as a downloader trojan that can download not only benign applications but also malicious and unwanted ones. One such app, known as H5GameCenter, displays ads on top of running applications. The image can't be removed, and infected users report that when they uninstall the app, Android.DownLoader.473.origin quickly downloads and installs it again.

Another preinstalled downloader Doctor Web detected is known as Android.Sprovider.7 and comes encrypted inside another app. It has the ability to automatically download Android application files and install them when users click on a confirmation button, make phone calls to certain numbers, and show ads on top of apps.

"It is known that cybercriminals generate their income by increasing application download statistics and by distributing advertising software," the security researchers wrote. "Therefore, Android.DownLoader.473.origin and Android.Sprovider.7 were incorporated into Android firmware because dishonest outsources who took part in creation of Android system images decided to make money on users."

The list of affected models included:

MegaFon Login 4 LTE

Irbis TZ85

Irbis TX97

Irbis TZ43

Bravis NB85

Bravis NB105

SUPRA M72KG

SUPRA M729G

SUPRA V2N10

Pixus Touch 7.85 3G

Itell K3300

General Satellite GS700

Digma Plane 9.7 3G

Nomi C07000

Prestigio MultiPad Wize 3021 3G

Prestigio MultiPad PMT5001 3G

Optima 10.1 3G TT1040MG

Marshal ME-711

7 MID

Explay Imperium 8

Perfeo 9032_3G

Ritmix RMD-1121

Oysters T72HM 3G

Irbis tz70

Irbis tz56

Jeka JK103

Lenovo A319

Lenovo A6000

The downloaders reported Monday probably don't pose as big a threat as the powerful backdoors that were recently found pre-installed on more than 3 million Android handsets . Still, their ability to surreptitiously install any app chosen by their unaccountable operators, combined with their embrace of adware, make the phones worth avoiding.