Anti-spyware company Webroot Software said Tuesday that CoolWebSearch self-installs malicious HTML applications and exploits security flaws in IE.
"This has vexed all of us," said Nick Lewis, managing director of Boulder, Colo.-based Webroot. "For consumers, CoolWebSearch is probably one of the most vicious programs in terms of how nasty it is. It completely hijacks the browser so you can't do anything."
CoolWebSearch is the most dangerous program on Webroot's latest list of the 10 worst spyware and adware threats. Webroot's list of top 10 threats also includes:
PurityScan, a program that displays pop-up ads and claims that it can delete pornographic images on the person's computer.
Transponder (vx2), an Internet Explorer "browser helper object" that monitors Web browsing and sends relevant advertisements.
KeenValue, an adware program that collects personal information and sends advertisements to computer users.
Perfect Keylogger, a monitoring tool that records Web sites visited, keystrokes and mouse clicks. It logs passwords, account numbers and other sensitive information.
"The people who write this stuff are gaining sophistication in their coding practices, as they attempt to evade detection and removal," said Richard Stiennon, Webroot's vice president of threat research. "These 10 are the most insidious programs in terms of prevalence and effect."
Webroot recommends that people install Microsoft security patches, avoid using freeware and disable downloads via ActiveX in Internet Explorer.
Dan Ilett of ZDNet UK reported from London.