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Posted Sep 12, 2002
Authored by Noam Rathaus | Site SecuriTeam.com

Outlook Express allows users to bypass many SMTP content protection programs by enabling the 'message fragmentation and re-assembly' feature. Vulnerable filters include GFI, Symantec, Trend Micro, and more.

tags | bypass
SHA-256 | f4e74ad446badf4dfeb8df4ef5e09926ea7b4179e2a15b6eeb976e5f55953a98


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Bypassing SMTP Content Protection with a Flick of a Button

Article reference:


Forget underground hacking tools. How about using Outlook Express as
your attack platform?

Beyond Security's SecurITeam has discovered a new method of bypassing
many SMTP-based content filter engines.
This discovery is alarming since it requires from the attacker nothing
more than an Outlook Express client and employs a rarely-used feature
called 'message fragmentation and re-assembly' that is available in
Outlook Express. Using this feature, an attacker can send e-mails that
will bypass most SMTP filtering engines including gateway Virus
scanners, content filters, Firewalls that do SMTP checking, etc.


One of the least known features of Outlook Express allows Internet and
Intranet users to split up sent messages. This allows slow connecting
users to send smaller segments of a larger email in multiple emails,
whereas the receiving client will automatically join them into a single
message. This RFC documented feature called "Message Fragmentation and
Reassembly" (RFC2046, section allows anyone to bypass most of
the security restrictions imposed on email messages, due to the fact
that messages are spliced into smaller segments that will not be
detected by virus scanners or other content testing mechanisms.

Possibly affected:
Any email filtering, virus checking, and content checking mechanism that
is unable to assemble a fragmented email to its complete form.

Technical details:
The main idea behind the RFC 2046 message fragmentation is to enable
users to send large files as several partial messages, while making it
transparent to the recipient, who will receive a single message rather
than multiple smaller files.

Fragmentation and Reassembly example:
If a binary attachment is broken into two pieces, the first piece might
look something like this:

X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
Subject: First mail (part 1 of 2)
MIME-Version: 1.0
Content-type: message/partial; id="ABC@host.com";
number=1; total=2

X-Weird-Header-1: Bar
X-Weird-Header-2: Hello
Subject: Audio mail
MIME-Version: 1.0
Content-type: application/binary
Content-transfer-encoding: base64


And the second half might look something like this:

From: Bill@host.com
To: joe@otherhost.com
Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
Subject: Second mail (part 2 of 2)
MIME-Version: 1.0
Content-type: message/partial;
id="ABC@host.com"; number=2; total=2


When the fragmented message is reassembled, the resulting message will
look something of the sorts of:

X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
Subject: Mail
MIME-Version: 1.0
Content-type: application/binary
Content-transfer-encoding: base64


Since the emails traversing though the product will be the first email
and the second email, and not the completed form, any product looking
for the phrase "VIRUS SIGNATURE" will fail to detect the Virus, and the
message will pass undetected. Similarly, if compressed files are
involved, a product will try to decompress them in order to look into
its content, but will be unable to do so since each email contains only
a fragment of the compressed file.

So far, the only client that we found to support this feature with a
"flick of a button" is Microsoft's Outlook Express. This mail client
supports an option that allows fully transparent fragmentation and
reassembly of messages. The reassembly feature is enabled by default,
while the fragmentation feature is not. Note though, that it can be
easily enabled by going to: Tools -> Accounts -> Choose your email
account -> Advanced -> Sending / Break apart messages larger than [...].
No other mail client we have checked supports this feature including
Microsoft Outlook. However, Outlook Express is widely used in both
Cooperate and Home environments making this issue a possible high-risk

Anyone wishing to bypass SMTP filtering engines can utilize the
mentioned method to bypass most types of content checking, and deliver
its payload to the end-client without any trouble, whether it is a
Virus, Trojan or a file type that is not allowed by the corporate

It seems that by embedding email footer (company disclaimer, privacy
note, etc) to each outgoing email traversing though the content filter
it is possible to completely hamper the effective usage of this attack.
However, since this is an RFC documented feature that may be used in
Outlook Express for legitimate purposes, this legitimate usage will be
hampered as well.

A vendor solution to this vulnerability would be to include a
reassembling agent at the server that will not allow any non-reassembled
message to traverse through it.

Vendor response - Check Point:
"Neither the latest 4.1 nor the latest NG versions of FW-1 are
vulnerable to this problem. A few details follow:

1. FW-1 does not directly analyze the body of attachments. In that
respect, the vulnerability is not applicable to FW-1.

2. FW-1 has the capability to easily filter these types of messages, by
specifying "message/partial" in the "Strip MIME of type:" section of the
resource definition.

3. FW-1 does serve as a platform for third party vendors to check
attachments for viruses via the "CVP" OPSEC mechanism. When defining a
CVP server, a message box is presented to the administrator (when
approving the resource) that says:

"When CVP server is used it is recommended to strip MIME of type
'message/partial'. Do you want to add 'message/partial'?"

Pressing "Yes" will automatically add 'message/partial' to the
appropriate place in the resource definition.

We therefore believe is safe to say that not only are we not vulnerable
to this problem ourselves, we also protect 3rd party opsec partners from
falling for this pitfall."

Vendor response - GFI:
GFI MailSecurity for Exchange/SMTP 7.2 has been updated to detect this
exploit as "fragmented message" through its email exploit detection
engine and quarantines it at server level.

GFI patch URL:
GFI's latest version of MailSecurity for Exchange is patched against
this problem. For more information, see:
<http://www.gfi.com/mailsecurity/> http://www.gfi.com/mailsecurity/

Vendor response - Symantec:
"Symantec has been aware for some time of the potential malicious use of
this email feature. As a result, all currently supported Symantec
gateway products, by default, block multi-part MIME messages at the
gateway. While this is a configurable feature of Symantec gateway
products and can be enabled if multi-part email is required, the
rejection of segmented messages should be a part of a company's
comprehensive security policy to restrict potentially harmful content
from the internal network.

Additionally, should known malicious code be delivered to a client
computer in this manner, the Symantec and Norton AntiVirus scanning
products will detect it when it is reassembled and downloaded to the
client computer and/or during attempted execution on the targeted
computer. As always, if previously unknown malicious code is being
distributed in this manner, Symantec Security Response will react and
send updated virus definitions via LiveUpdate to detect the new threat."

A full formal response from Symantec should be shortly available at:

(Under "Multiple SMTP bypass")

Vendor response - TrendMicro:
"We have confirmed that our product InterScan VirusWall 3.5x for NT is
affected by the vulnerability mentioned by Beyond Security Ltd.
regarding fragmented e-mails. In order to resolve this problem, we have
released a patch in order to address this particular concern for
InterScan VirusWall for NT. The said patch can be downloaded from the
following FTP server:

The said hotfix is named:

The hotfix mentioned above contains a Readme file which should include
the necessary instructions on how to apply the patch.

Our other mail gateway product, InterScan MSS v5.01 is not affected by
this vulnerability provided that you apply the latest hotfixes which can
be downloaded from our website at:
<http://www.antivirus.com/download> www.antivirus.com/download

Vendor response - SonicWALL:
We could not assert whether SonicWALL is vulnerable to this attack and
were unable to receive a response from SonicWALL despite several contact

Vendor response - NAI:
We could not assert whether any of NAI's products is vulnerable to this
attack and were unable to receive a response from NAI despite several
contact attempts.

Vendor response - Cisco:
The following response was received from Cisco's security contact on
September 1st:
"We are still working on this issue, and I do not have the latest
information. We will follow up in a few days."

We have received a response from CERT indicating that they have informed
several vendors about the issue, but were unable to receive an updated
status in the last few weeks. CERT is tracking this issue as VU#836088.


GFI Email Security Testing Zone:
GFI has set up an email security testing engine that can be used to test
your system for the mentioned vulnerability. This testing zone is
available at:

The information has been provided by <mailto:noamr@beyondsecurity.com>
Noam Rathaus, Beyond Security Ltd.

Aviram Jenik
Beyond Security Ltd.

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