Microsoft warns of critical flaws in Internet Explorer

Microsoft released a patch for a number of flaws in its Internet Explorer (IE) Web browser Wednesday, including two it rated critical for some versions of the browser, which could enable an attacker to take control of a user's computer.

The company also released a patch for a flaw, rated important, in the MDAC (Microsoft Data Access Components) element of its Windows OS (operating systems).

The critical flaws affect IE versions 5.01, 5.5, 6.0 and 6.0 SP1 (6.0 with Service Pack 1 installed), and could allow an attacker to run arbitrary code on a user's system if the user either visited a Web site or read an e-mail message in HTML (Hypertext Mark-up Language) designed to exploit the flaw, Microsoft said.

The flaw also affects IE 6.0 for Windows Server 2003, but Microsoft only rated the flaws on this platform "moderate," its third-highest danger rating, because the Windows Server 2003 version of IE is delivered in a default configuration which prevents exploitation of these flaws. Nevertheless, users who have changed the default configuration may have made their systems vulnerable.

Microsoft urged systems administrators to immediately install the patch, described in Microsoft Security Bulletin MS03-032. The patch brings together all previously released fixes for the affected versions of IE.

The first of the two critical flaws relates to a potential buffer-overrun vulnerability in a now-obsolete ActiveX control in part of the Windows HTML help system. The patch sets the Kill Bit on the BR549.DLL ActiveX control, which disables the control and prevents it from being reinstalled. The control was part of the Windows Reporting Tool, no longer supported in IE.

The second critical flaw is due to IE's mishandling of object tags in HTML pages. When the browser encounters an object tag and requests a file from a Web server, it does not check that the file returned is of the correct type. This could allow an attacker to make IE execute a program of their choice, using the security privileges of the user.

The patch also fixes a vulnerability rated important in IE's cross-domain security model, which is intended to prevent browser windows accessing Web sites in different security zones from interfering with one another, according to the security bulletin. This allows scripts in the Internet security zone -- for example, on Web sites or in HTML-format e-mail, to access files in the My Computer zone by exploiting an error in the way IE retrieves files from the browser cache.

In a separate security bulletin, MS03-033 the company warned of an important vulnerability, its second-highest danger rating, in the MDAC element of its Windows OS.

The flaw affects MDAC versions 2.5 through 2.7, which are included by default with Windows XP, Windows 2000, and Windows Millennium Edition. MDAC software is also part of other Microsoft software, including Windows NT 4.0 Option Pack, and the Access and SQL (Structured Query Language) Server databases.

MDAC 2.8, the version which ships with Windows Server 2003, does not have the vulnerability, Microsoft said.

The flaw in MDAC could allow an attacker to run arbitrary code on a vulnerable system -- but to do so, they would need to set up a fake SQL server on the same subnet as the target system, Microsoft said. It encouraged systems administrators to install the patch, which also includes a fix for an earlier vulnerability, reported in security bulletin MS02-040.

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Peter Sayer

IDG News Service
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