Date: Wed, 20 Feb 2002 21:04:28 -0000
From: "Martin O'Neal" <BugTraq@corsaire.com.>
To: [email protected]Subject: Symantec Enterprise Firewall (SEF) SMTP proxy inconsistencies
-- Corsaire Limited Security Advisory --
Title: Symantec Enterprise Firewall (SEF) SMTP proxy inconsistencies
Date: 18.01.02
Application: Symantec Enterprise Firewall (SEF) 6.5.x
Environment: WinNT, Win2000
Author: Martin O'Neal [[email protected]]
Audience: General distribution
-- Scope --
The aim of this document is to clearly define some issues related to
a some SMTP proxy inconsistencies within the Symantec Enterprise
Firewall (SEF) environment as provided by Symantec [1].
-- History --
Vendor notified: 18.01.02
Document released: 21.02.02
-- Overview --
The SEF firewall product uses an application proxy strategy to provide
enhanced security features for a variety of common protocols. For the
SMTP proxy, part of this additional functionality allows the firewall
to restrict the sender / recipient domains and to hide internal
infrastructure information from external recipients.
However, when the firewall is configured to provide network address
translation (NAT) to an SMTP connection (effectively hiding the internal
server behind a publicly routable address), this might not always be
conducted as desired.
-- Analysis --
The SMTP proxy works by analysing the SMTP format and optionally rewriting
some of the headers to achieve the desired aim.
When an inbound or outbound SMTP connection is NATed to an address other
than the one assigned to the physical firewall interface, then the SMTP
proxy still uses the physical interface name and address within the SMTP
protocol exchange.
This has two potential issues:
The first issue is that there is now a potential information leak;
additional information is contained within the SMTP protocol exchange that
could aid an attacker in analysing the firewall configuration.
The second issue is that any receiving / sending host that is configured
to enforce strict checks on the SMTP protocol exchange, might not accept
the connection due to the inconsistencies in the fields.
-- Proof of concept --
To reproduce this issue, place an SMTP host on an internal interface of the
SEF firewall. Create a rule that allows inbound and outbound traffic to this
host, then create an address translation and redirection entry that maps
SMTP connections to and from an external address other than the physical
interface address.
smtp address (internal) 1.1.1.1 (twist.dance.org)
firewall address (external physical) 2.2.2.2 (waltz.dance.org)
firewall address (external SMTP NAT) 2.2.2.254 (foxtrot.dance.org)
firewall name tango
firewall domain dance.org
redirect 2.2.2.254 -> 1.1.1.1 (for SMTP only)
NAT 1.1.1.1 -> any (use 2.2.2.254)
NAT any -> 1.1.1.1 (use client address)
rule 1.1.1.1 -> any (for SMTP only)
rule any -> 1.1.1.1 (for SMTP only)
For inbound connections to 2.2.2.254:
-> 220 tango.dance.org Generic SMTP handler
For outbound connections from 2.2.2.254:
<- 220 ...
-> HELO waltz.dance.org
<- 250 ... talking to foxtrot.dance.org ([2.2.2.254])
-- Recommendations --
The SMTP proxy behaviour should be revised to resolve the information
within the protocol exchange to the correct host / address combination.
-- References --
[1] http://enterprisesecurity.symantec.com/products/products.cfm?ProductID
=47&PID=9674250&EID=0
-- Revision --
a. Initial release.
Copyright 2002 Corsaire Limited. All rights reserved.
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