A $50 program that installs secretly on cell phones to monitor calls and text messages of wayward spouses was dubbed a Trojan horse spy by a security company Wednesday.

FlexiSpy, said its distributor, Thailand-based Vervata, is "a remote activity logger for mobile phones…[that] will sliently [sic] retrive [sic] and report all phone activity to an email address that you specify."

It markets the $50 program as a way to snoop on spouses' and kids' phone conversations and text messaging. FlexiSpy captures data, then transmits it to Vervata's servers, from where users can retrieve call logs and full text of all SMS messages.

Finnish security firm F-Secure calls it a Trojan.

"This application installs itself without any kind of indication as to what it is. And when it is installed on the phone it completely hides itself from the user," wrote F-Secure researcher Jarno Niemela on the company's blog.

Niemela warned that those traits made it a perfect hacker tool. "The application could easily be used by malware installing it as part of its payload, or a hacker could simply send it to a victim over Bluetooth and trust that there are enough curious people to install it," he said.

Vervata was not immediately available for comment, but it defended FlexiSpy on its own Web site. "FlexiSpy is not a Virus or Trojan. FlexiSpy requires to be consciously installed and configured by someone, unlike a Virus or Trojan which spreads automatically without any action."

Nonetheless, FlexiSpy, which works on Nokia cell phones running the Symbian S60 operating system, has been tagged as a Trojan by F-Secure; a signature to detect the software has been added to the company's database. The signature does not delete FlexiSpy, but does warn the cell phone's owner that his or her conversations are being monitored.

According to its Web site, Vervata is working on a more advanced version of the surveillance software. FlexiSpy Pro will offer up information on e-mail sent via cell phone, and will include a real-time eavesdrop feature that lets users "listen in on what [the cell phone's user is] doing from anywhere in the world!"