Sending an SMTP message with a malformed attachment, it is possible for malicious code to avoid detection by Trend Micro's InterScan SMTP scanner version 3.0.1 for Solaris. Other versions may be affected as well, but were not tested.
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<FONT COLOR="#CC0000">COMMAND</FONT>
InterScan VirusWall
<FONT COLOR="#CC0000">SYSTEMS AFFECTED</FONT>
Trend Micro's InterScan VirusWall version 3.0.1 for Solaris.
<FONT COLOR="#CC0000">PROBLEM</FONT>
Following is based on Alcatel Security Advisory. The NewApt Worm
is currently exploiting this bug to avoid detection. By sending
an SMTP message with a malformed attachment, it is possible for
malicious code to avoid detection by Trend Micro's InterScan SMTP
scanner version 3.0.1 for Solaris. Other versions may be affected
as well, but were not tested.
RFC2045 describes the number of padding characters needed at the
end of a base64 encoded MIME attachment. InterScan VirusWall does
not properly handle incorrectly padded attachments. Upon
receiving such an attachment, InterScan fails to scan the
attachment properly and the message is allowed to pass through;
however, InterScan does log the following message to its system
logs:
<FONT COLOR="#00FF00">
base64: Unexpected EOF seen
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Note: This modification of the padding does not appear to affect
mail clients such as Netscape Communicator.
Alcatel noticed this bug while testing the product with live
viruses. The NewApt Worm replicates by replying to emails in the
victim's mailbox. The above error message was a clear indication
that this particular attachment was problematic. It was
determined that an extra "=" character at the end of the base64
encoding was the cause of the problem. Further investigation
revealed that if the correct number of "=" characters (as per
RFC2045) were not present, InterScan failed to catch the virus.
This was tested with several other viruses such as Melissa and
Shankar.
To exploit this vulnerability, create a new message with the virus
of your choice attached. Save this message to your local disk.
Edit the message and add any number of "=" characters to the
end of the base64 encoded attachment. This message will now pass
through the InterScan VirusWall, and the virus will remain
undetected and intact.
John Lampe added following. Along a similar vein numbers 1
through 3 below will also allow virus-infected attachements to
pass right by Interscan Viruswall.
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1) adding a "-" to the end of base64 message
2)changing content-type application type in the header Example,
Content-type: Application/FOO;
name="whatever.doc"
3) Adding an extra "-" at end of base64 boundary
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3 methods above were tested and verified on NT running the latest
engine from Trend Micro, along with the latest patch. At least
one of the methods above (Number 1) was tested and verified on a
Solaris box by Kris Herrin (the original poster). 3 methods
above were chosen *at random* from RFC 2045. Vendor was
notified.
<FONT COLOR="#CC0000">SOLUTION</FONT>
Trend Micro has posted a fix for this bug. The patch is can be
downloaded from the following URL:
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http://www.antivirus.com/download/patches.htm
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The patch is titled isvwsol301a_u2.tar.
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