Microsoft has released a security patch to fix three known vulnerabilities in its Internet Explorer Web browser that have been exploited to attack Internet users.

The patch also includes a change in the basic authentication functionality in IE that Microsoft announced last week. After installing the patch, the browser no longer supports handling usernames and passwords embedded in Web URLs using the "@" symbol, Microsoft said in a statement.

The security update was released outside of Microsoft's regular monthly patch cycle because of the seriousness of the issues, said Mike Reavey, a security program manager at Microsoft in Redmond, Washington. Microsoft's official patch day this month is Tuesday, 10 February.

One of the three newly patched issues is rated "critical" by Microsoft, while two are "important." By taking advantage of two of the security flaws, attackers can run, or save, arbitrary code on a user's computer. Another flaw allows an attacker to spoof a website address and potentially trick users into providing personal information, Microsoft said.

The spoofing issue received wide publicity late last year and Microsoft has been criticized for not delivering a fix sooner. The company said Monday that it is providing the security update as soon as possible after completing development and testing.

In Microsoft's rating system for security issues, vulnerabilities that could allow a malicious Internet worm to spread without any action required on the part of the user are rated critical. Issues that will not lead to the spread of a worm without any action taken by the user, but could still expose user data or threaten system resources, are rated important.

The problems affect all currently supported versions of Internet Explorer, on all currently supported operating systems. Users are urged to install the patch immediately, Microsoft said in Security Bulletin MS04-004.

The Internet Explorer 6 Service Pack 1 version of the patch also works on Windows 98, Windows 98 Second Edition and Windows Millennium Edition, which normally would only get patches by request because the products are in what Microsoft calls the Extended Support phase of their lifecycle, Reavey said.