The Microsoft browser, the government warned, cannot protect against vulnerabilities in its Internet Information Services (IIS) 5 server programs, which a team of hackers allegedly based in Russia has exploited with a JavaScript that is appended to Web sites.

The particular virus initiated this week inserts JavaScript into certain Web sites. When users visit those sites, it initiates pop-up ads on home and office computers, and allows keystroke analysis of user information. The target is believed to be credit card numbers. CERT estimated that as many as tens of thousands of Web sites may be affected.

CERT said vulnerabilities in IIS and IE could include MIME- type determination, the DHTML object model, the IE domain/zone security model and ActiveX scripts. Alternative browsers such as Mozilla or Netscape may not protect users, the agency warned, if those browsers invoke ActiveX control or HTML rendering engines.

The only defense may be completely disabling scripting and ActiveX controls.

Microsoft said earlier in the week it is working with law enforcement officials to identify the source of the latest Internet virus.