The vulnerability in Adobe Reader's browser plug-in, which was publicized Wednesday by several security companies, can let hackers force trusted Adobe PDF (Portable Document Format) files to run malicious JavaScript code on victimized PCs.

Early Wednesday, Symantec researchers insisted that only Firefox 1.5 and Opera 9.10 were vulnerable to a possible exploit; by Thursday, however, additional research had confirmed that some versions of Internet Explorer are at risk. According to an updated DeepSight threat network alert, IE 6.0 on XP SP2 equipped with Adobe Reader 6, as well as IE 6 on XP SP1 running Reader 7, are vulnerable. Also at risk: Firefox 1.5, Firefox 2.0, and Opera 9.10 when running either Reader 6 or 7.

"Version 6 of Internet Explorer is impacted," says David Cole, director of Symantec's security response group. "The best way for enterprises and users to protect themselves is to update Adobe Reader."