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MS Exchange-2K, Excel-XP security warnings

Malicious fun with SMTP and style-sheets

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First up we have a potentially crippling exploit with Exchange 2000, in which a malformed mail attribute can spike the system CPU to 100% load while the malicious message is being processed. Re-starting the service or re-booting the Exchange server will not correct it; the process will resume automatically as soon as the service is re-started. Depending on the attacker's ingenuity, a server could be taken down for anywhere from a few seconds to several hours.

The problem lies in the Store function that processes messages, which takes precedence over other Store functions which might otherwise be invoked to clear the offending message, MS says. It was discovered by a team of researchers from Johannes Gutenberg University in Mainz, Germany.

This isn't quite k1dd13 stuff. An attacker would have to create a raw message and establish a direct connection to the target server. It's not something one can accomplish by playing around with an e-mail client; a successful attacker would have to be quite familiar with SMTP, which narrows the threat field.

That being said, the potential for a crippling DoS attack is sufficient for MS to brand this one 'critical', a denomination which it avoids like the plague. As usual the MS bulletin is unbearably vague about what's going on here, with frequent recourse to meaningless generalities like 'a specially malformed attribute'.

On the positive side there's no potential for a system compromise here, Exchange 5.5 is not affected, and there is a patch for 2K posted here. The patch, we're told, will reject messages possessed of those 'specially malformed attributes', whatever they happen to be.

Next up we have a glitch in Excel-XP style sheets, discovered by Georgi Guninski. This is k1dd13 material, so beware. If a user views an .xls file using the XML style sheet, it's quite easy for an attacker to run or call executable code. Guninski gives a harmless example in his advisory, which will simply call a shell and list your directories for you, thus:

xls_sux.xls
<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="#?m$ux" ?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/TR/WD-xsl">
<xsl:script>
<![CDATA[
x=new ActiveXObject("WScript.Shell");
x.Run("%systemroot%\\SYSTEM32\\CMD.EXE /C DIR C:\\ /a /p /s");
]]>
</xsl:script>
<msux>
msux
written by georgi guninski
</msux>
</xsl:stylesheet>

As for the malicious potential here, we'll just leave that to the admirably evil imaginations of our beloved readers.

On the plus side, style sheets are not selected by default, so when the exploit runs, users should be asked if they want to view the file with the style sheet. How many will think, 'yeah, that sounds like an improvement' as opposed to those who will think, 'stuff that Redmond gimmick' we can't guess.

For a workaround, Guninski offers this bit of wisdom: "Power-off the poor Windoze box if you see Excel mentioning style sheets."

We can offer no better advice. ®

Reducing security risks from open source software

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