AUSCERT External Security Bulletin Redistribution
                     ESB-2000.267 -- ISS Security Alert
                  Multiple vulnerabilities on all platforms
                   and versions of Check Point FireWall-1
                              28 September 2000


	AusCERT Security Bulletin Summary

Product:                VPN-1/FireWall-1
Vendor:                 Check Point
Ref:                    ESB-2000.196

- --------------------------BEGIN INCLUDED TEXT--------------------


Internet Security Systems Security Alert
September 27th, 2000

Multiple vulnerabilities on all platforms and versions of Check Point 


On July 26th, Thomas Lopatic, John McDonald, and Dug Song released 
vulnerability information at the Black Hat 2000 briefings that exposed the
following security holes in Check Point FireWall-1:

1. One-way Connection Enforcement Bypass
2. Improper stderr Handling for RSH/REXEC
3. FTP Connection Enforcement Bypass
4. Retransmission of Encapsulated Packets
5. FWA1 Authentication Mechanism Hole
6. OPSEC Authentication Spoof
7. S/Key Password Authentication Brute Force Vulnerability
8. GetKey Buffer Overflow


1. One-way Connection Enforcement Bypass 
Any source from any security region passing the rule-base may be able to make
unauthorized connections through the firewall that would otherwise be blocked
by this additional security layer of FireWall-1.  By using specially fragmented
TCP connection requests or by closing and re-opening one-way TCP connections
in conjunction with certain complex multi-connection protocols, an attacker
can bypass the directionality check.

All implementations of Check Point FireWall-1 prior to the new service packs
that allow protocols employing unidirectional data flow connections (such as
FTP and RSH STDERR) are vulnerable.

2. Improper stderr Handling for RSH/REXEC
A malicious external source can open an unauthorized connection to an internal
protected RSH/REXEC client by sending specially formatted stderr connection
requests (as an RSH/REXEC server) to the client.  These connection attempts
are mishandled by FireWall-1 and are allowed through, giving connection
access for possible exploitation.

This attack may lead to a compromise of integrity, including relay access on
targeted machines, by taking advantage of the connected service.

All implementations of Check Point FireWall-1 prior to the new service packs
that have explicitly enabled the VPN-1/FireWall-1 RSH/REXEC property are 

3. FTP Connection Enforcement Bypass
A malicious source can use this vulnerability to redirect connections to other
systems accessible to the FTP server "bounced" off of.  By exploiting the
standard "FTP Bounce" attack, while advertising a small maximal segment
size (server replies being split) in PASV handling of FTP, an attacker can
successfully "bounce" connections off the available FTP server through the
FireWall-1 daemon.  This form of exploit is often used as a form of IP
spoofing to obscure the source of an attack to other targets, and as a way to
gain access to assets that are accessible, passing the rule-base, to the 
targeted FTP server.  

All implementations of Check Point FireWall-1 prior to the new service packs
that have "FTP bounce" vulnerable FTP servers available for inbound connection
with write access and do not have the latest service packs or patches to this
vulnerability are susceptible to this attack.

4. Retransmission of Encapsulated Packets
Any source may be able to send packets that pass normal rule-based and
anti-spoofing checks, if setup and enabled, through the firewall without being
an FWZ client.  By sending a payload of specially encapsulated FWZ packets that
match a rule in the FireWall-1 rule base, a malicious source can effectively
spoof through the firewall as a FWZ client. 

All versions and implementations of Check Point FireWall-1 prior to the new 
service packs are vulnerable.

5. FWA1 Authentication Mechanism Hole
An attacker may successfully compromise the authentication mechanism (but not
compromise the connection encryption thereafter), and possibly flood the
mechanism with successful authentications, theoretically denying service.

As part of the FWA1 authentication, the server starts by sending the client a 
random number and a hash of the random number plus the shared secret key.  The
client is then required to send the server a different random number and a hash
of the random number sent by the server, XORed with the client's random
number's power, plus the secret key.  By using 0 as the client's random number,
a malicious attacker can subvert authentication (since the server's random
number XORed with zero will equal itself and therefore produce the same hash
sent to the server in the first step of the protocol).  Although this subverts
authentication, it doesn't expose the secret key and therefore prevents further

All implementations prior to the new service packs, having rules that allow
control connections from un-trusted sources or users.  In version 4.1, by
default, FW-1 control connections are allowed only from systems defined as
having FireWall-1 installed on them.  

6. OPSEC Authentication Spoof
A remote attacker can effectively authenticate to any OPSEC channel that it is
allowed to communicate with under the rules base, giving full access to any
services allowed therein.

For authentication, the OPSEC protocol sends a random number and a hash of the
random number plus the shared secret key to the client.  The client is then
expected to verify the key by adding the random number to the previously known
secret key, performing the same hash, and comparing it to the one received. The
client then authenticates by sending what is assumed to be a different random
number, in addition to their own hash of the secret key plus their random
number.  The server then performs the same verification, successfully
authenticating the client's knowledge of the secret key and granting access.
To compromise this authentication, an attacker initiates a connection,
receiving the initial random number and hash from the server.  The
authentication process of the server is skipped, because the secret key is
unknown, and the messages are replayed back to the server.  The server does not
verify if the random number chosen by the client is different and authenticates
the previously sent message successfully, granting access to the malicious

All implementations of Check Point FireWall-1 prior to the new service packs
and those that don't constrain OPSEC communication to specifically trusted
source/destinations pairs via the rule base are vulnerable to this attack.  

7. S/Key Password Authentication Brute Force Vulnerability
An attacker can use this vulnerability to gain user intermodule access through
VPN-1 or any other access vector using this authentication in FireWall-1.  As
part of the FireWall-1 S/Key authentication protocol, an index number is sent
to the client.  The client then authenticates by sending the secret key, hashed
the index number of times, back to the server.  By utilizing brute force
techniques, an attacker could determine the secret key by intelligently trying
all possible secret keys in the given keyspace.  Upon determining the secret
key, the attacker will also gain authentication to all available assets.

All implementations of Check Point FireWall-1 prior to the new service packs and
those that use S/Key for inter-module authentication are vulnerable.  Note that
all 4.1 installations use FWA1 by default and are therefore not vulnerable
unless the administrator specifically weakened the inter-module
authentication mechanism from FWA1 to S/Key.

8. GetKey Buffer Overflow
An attacker can use this vulnerability to terminate the firewall daemon, leaving
the current policy intact.  This represents a possible threat of integrity
compromise to all assets available via policy that would otherwise not be
available given other checks and services available with the firewall daemon
actively running.  Due to insufficient bounds checking and handling in the
intermodule communication protocol, specifically within the GetKey procedure,
an attacker can exploit this vulnerability by sending a specially-crafted
instruction to overflow the buffer and cause the firewall daemon to terminate,
leaving policy enforcement operational.

All implementations of Check Point FireWall-1 prior to the new service packs
or patches to this vulnerability are vulnerable.

Affected Platforms:

All platforms on which Check Point's FireWall-1 product is available are
vulnerable to these attacks. They are platform independent, existing completely
within the FireWall-1 application.


Check Point has released service packs VPN-1/FireWall-1 4.0 SP7 and
VPN-1/FireWall-1 4.1 SP2 that eliminate each of these vulnerabilities.  For
VPN-1 Appliances (IPSO) running version 4.0, the service pack is version 4.0
SP5 Hotfix.
Service Pack information, download access, and a description of how these
vulnerabilities are addressed can be accessed at: 

The ISS X-Force will provide additional functionality to detect these
vulnerabilities in upcoming X-Press Updates for Internet Scanner and

Additional Information:

Supplemental technical notes on these vulnerabilities are available at:

The Common Vulnerabilities and Exposures (CVE) project has assigned the 
following names to these issues.  These are candidates for inclusion in the
CVE list (http://cve.mitre.org), which standardizes names for security problems.

CAN-2000-0804  One-way Connection Enforcement Bypass
CAN-2000-0779  Improper stderr Handling for RSH/REXEC
CAN-2000-0813  FTP Connection Enforcement Bypass
CAN-2000-0805  Retransmission of Encapsulated Packets
CAN-2000-0806  FWA1 Authentication Mechanism Hole
CAN-2000-0807  OPSEC Authentication Spoof
CAN-2000-0808  S/Key Password Authentication Brute Force Vulnerability
CAN-2000-0809  GetKey Buffer Overflow


About Internet Security Systems (ISS) 
Internet Security Systems (ISS) is a leading global provider of security
management solutions for the Internet. By providing industry-leading SAFEsuite
security software, remote managed security services, and strategic consulting
and education offerings, ISS is a trusted security provider to its customers,
protecting digital assets and ensuring safe and uninterrupted e-business.
ISS' security management solutions protect more than 5,500 customers worldwide
including 21 of the 25 largest U.S. commercial banks, 10 of the largest
telecommunications companies and over 35 government agencies.  Founded in
1994, ISS is headquartered in Atlanta, GA, with additional offices
throughout North America and international operations in Asia, Australia,
Europe, Latin America and the Middle East. For more information, visit the
Internet Security Systems web site at www.iss.net or call 888-901-7477. 

Copyright (c) 2000 Internet Security Systems, Inc. 

Permission is hereby granted for the redistribution of this Alert
electronically. It is not to be edited in any way without express consent
of the X-Force. If you wish to reprint the whole or any part of this Alert
in any other medium excluding electronic medium, please e-mail xforce@iss.net
<mailto:xforce@iss.net> for permission. 

The information within this paper may change without notice. Use of this 
information constitutes acceptance for use in an AS IS condition. There are
NO warranties with regard to this information. In no events shall the author
be liable for any damages whatsoever arising out of or in connection with
the use or spread of this information. Any use of this information is at
the user's own risk. 

Version: 2.6.3a
Charset: noconv


- --------------------------END INCLUDED TEXT--------------------

This security bulletin is provided as a service to AusCERT's members.  As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content.  The decision to use any or all of this information is
the responsibility of each user or organisation, and should be done so in
accordance with site policies and procedures.

NOTE: This is only the original release of the security bulletin.  It may
not be updated when updates to the original are made.  If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the original authors to ensure that the information is still current.

Contact information for the authors of the original document is included
in the Security Bulletin above.  If you have any questions or need further
information, please contact them directly.

Previous advisories and external security bulletins can be retrieved from:


If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security

Internet Email: auscert@auscert.org.au
Facsimile:	(07) 3365 7031
Telephone:	(07) 3365 4417 (International: +61 7 3365 4417)
		AusCERT personnel answer during Queensland business hours
		which are GMT+10:00 (AEST).
		On call after hours for emergencies.

Version: 2.6.3i
Charset: noconv
Comment: ftp://ftp.auscert.org.au/pub/auscert/AUSCERT_PGP.key