Aladdin eSafe Gateway Script-filtering Bypass through Unicode Vulnerability

["eDvice Security Services" <support () edvicetech com>]

29 May 2001

This is the third of 3 sequential advisories we are issuing regarding
Aladdin eSafe Gateway.

Status
--------
The entire content of this advisory was reviewed and acknowledged by
Aladdin.

Product Background
--------------------------
eSafe Gateway is an Internet Content Security product.

You can configure eSafe Gateway to remove scripts (VBScripts, JavaScripts)
and other executable tags from incoming HTML documents. Alternatively, the
administrator can ban certain scripting commands from appearing inside
scripts. The banned commands will be removed, while the rest of the HTML
page is left intact.

Scope
--------
eDvice recently conducted a test of eSafe's ability to remove scripts from
HTML documents. Although scripts are widely used by many web-sites, some
organizations requesting to allow only limited use of Internet access from
their internal network, prefer to disable scripting capabilities in order to
avoid various known, as well as yet to be found, browser-based attacks.

The Findings
----------------
eSafe does not recognize scripting tags constructed using extended Unicode
notation. This allows an attacker to bypass eSafe script filtering mechanism
and introduce malicious code into an organization.

Details
---------
eSafe gateway analyzes the incoming HTML file and searches for the keyword
"<SCRIPT'. From the moment the keyword was found, eSafe looks for a
following "</Script>" keyword and then replaces the entire content between
these keywords with spaces.

However, browsers such as Internet Explorer accept extended Unicode
character representation within HTML files. If the string "<SCRIPT" is
replaced with some extended Unicode representation, then eSafe will not
filter the tag and the browser will run the script.

Exploit
----------
See Attached file (unzip it first).

To repeat this vulnerability, place the file on your web server and
configure eSafe to remove all scripts. Access the file using the browser and
you will see the message "hello" on your screen. This is a message generated
by a VBScript script that should have been filtered.

Solution
-----------
Do not rely on eSafe Gateway version 3.0 for HTML filtering until Aladdin
fixes the problem.
Aladdin will publish a workaround to avoid this vulnerability and will
address this issue in the next release of eSafe Gateway.


====================
Discovered by:
eDvice Security Services
support () edvicetech com
http://www.edvicetech.com
Tel: +972-3-6120133
Fax: +972-3-6954837


begin 666 script38a.zip
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MC\$*@D 0AN]![S#LW>P40:Y@M660"K4%'3?;<F%S3<>HY^AU(NS-4JQ.'_SS
MS\>,X_-@Z78[CL^\:<. <0]"+V"4S%G(5AZ/5@0F)D69(B6!BG-3F"/"5A6E
MT+#&\J ,#'I]\E^/?VV4-[03/.L1Q(G("XETPV?6D$""F%GR4JHK_;HM?L]D
MH^ +OF2N8[>L`_MWVSB:[FI6S_<CSE6&H$5Z*L5)TNO^&U4OH66.0'RIM2'5
MT_X/&M/8'.ZMLGW[`U!+`0(4`!0````(`$=QO2JS]/CJP@```/\````.````
M``````$`( "V@0````!S8W)I<'0S.&$N:'1M;%!+!08``````0`!`#P```#N
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`
end