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Re: "Strip Script Tags" in FW-1 can be circumvented

From: james () JEDITECH COM (James Lin)
Date: Tue, 1 Feb 2000 11:55:02 -0800

On Mon, 31 Jan 2000, Jonah Kowall wrote:


      I don't consider this a bug in FW-1, but a bug in the products
navigator, and internet explorer.  These tags shouldn't be parsed, because


        Perhaps a bug or feature - they are adhering to the principle of
"flexible in what you accept." Browsers have always given a lot of leeway to
poorly written HTML and scripts, and authors expect them to behave that way
(whether that is good or bad is another debate)

The firewall should be just as flexible in order to recognize all errors. In
this case I expect firewall to either strip the SCRIPT tag, or deny access to
this document because it contains illegal HTML - just as it would if the user
tried to access a malformed URL.

Keep in mind exploits often take advantage of bugs or deficiences in
protocols, and isn't that what a firewall is supposed to protect
against?:=)

-James


-----Original Message-----
From: Arne Vidstrom [mailto:arne.vidstrom () NTSECURITY NU]
Sent: Saturday, January 29, 2000 8:52 AM
To: BUGTRAQ () SECURITYFOCUS COM
Subject: "Strip Script Tags" in FW-1 can be circumvented


Hi all,

The "Strip Script Tags" in FW-1 can be circumvented by adding an extra <
before the <SCRIPT> tag like in this code:

<HTML>
<HEAD>
<<SCRIPT LANGUAGE="JavaScript">
alert("hello world")
</SCRIPT>
</HEAD>
<BODY>
test
</BODY>
</HTML>

This code will pass unchanged, and still execute in both Navigator and
Explorer. I tried this on version 3.0 of FW-1 (on Windows NT 4.0) but I'm
not able to check it on version 4.0 since I don't have access to it.


/Arne Vidstrom

http://ntsecurity.nu
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