Low-cost phones surreptitiously install and reinstall unwanted apps.

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.