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Cumulative fix

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Microsoft yesterday released a cumulative patch for Internet Explorer with rolls up previous fixes with a couple of extra damage limitation measures.

Dubbed critical by Redmond, the patch includes fixes for two newly discovered vulnerabilities involving IE's cross-domain security model.

The first of these two flaws arises because incomplete security checking means a Web site can potentially access information from another domain through certain dialog boxes.

An attacker would exploit this flaw by tricking users into visiting a malicious constructed Web site. Thereafter it would be possible for an attacker to run malicious script by misusing a dialog box to allow a malicious script to access information in a different domain.

"In the worst case, this could enable the web site operator to load malicious code onto a user's system," Microsoft warns. In addition, this flaw could also enable an attacker to invoke an executable that was already present on the local system."

This flaw affects only IE 5.5 and 6.0. A second vulnerability affects IE 5.5, 6.0 and IE 5.01.

This second (related) cross-domain vulnerability allows IE's showHelp() (HTML help) function to execute without proper security checking. The flaw arises because the function allows more types of pluggable protocols than necessary, Redmond admits.

"This could potentially allow an attacker to access user information, invoke executables already present on a user's local system or load malicious code onto a user's local system," its advisory adds.

The exploit scenario here is the same as for the first vulnerability: tricking users into visiting a maliciously-constructed Web site.

Microsoft's cumulative patch is designed to fix these two flaws as well as previous bugs with IE 5.01, 5.5 and 6.0. Users are urged to consider applying the fix, even though it has certain drawbacks.

The patch disables window.showHelp( ) function so users need to also install the latest HTML Help update, through Windows update, too. Thereafter window.showHelp( ) will function again, "but with some limitations" necessary to block the exploit.

Microsoft's advisory explains the issue in greater depth. ®

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