3com NBX IP Phone Call manager Denial of Service - Update Apr 27 2003 01:37AM
Michael Scheidell (scheidell secnap net)
Revision Date: April 25, 2003
Reason for Revision: 3com updated nbx firmware to 4_1_21, Add bugtraq-id

Systems: 3com NBX IP Phone Call manager, FW Versions through 4_1_21
Severity: Critical
Category: Denial of Service
Classification: Boundary Condition Error
BugTraq-ID: 6297
CERT VU#:[VU#317417]
Vendor URL: www.3com.com, www.windriver.com
Author: Michael S. Scheidell, SECNAP Network Security
Original Release date: December 2nd, 2002
Notifications: (3com, WindRiver and CERT) Notified October 31st, 2002
Last contact with 3com: November 22nd, 2002
Attempted contact with 3com: April 15th, 2003
Last contact with WindRiver: December 6rd, 2002

Discussion: (From 3com's and WindRiver's web site)

3Com® SuperStack® 3 NBX® and 3Com NBX 100 networked telephony solutions
offer wide-ranging price/performance alternatives to fit your business
needs today and tomorrow. 3Com® SuperStack® 3 NBX® Networked Telephony
Solution Delivers robust, full-featured business communications for up to
1500 devices (lines/stations) Ensures high system availability with the
Wind River VxWorks real-time operating system (also used in pacemakers and
artificial hearts), so server and PC downtime does not impact your
telephone service.

VxWorks and pSOSystem are the most widely adopted real-time operating
systems (RTOSs) in the embedded industry -- for good reason. They are
flexible, scalable, reliable, and available on all popular CPU platforms.
They are also, by most measures, the fastest RTOSs available today.

Exploit: It was possible to make the remote FTP server crash by issuing
this command :

CEL aaaa[...]aaaa where string is 2048 bytes long. This can be done with
netcat, a windows client by telnetting to the nbx server on port 21 or by
running the vxworks_ftpd.nasl test in nessus (www.nessus.org)

The 3com NBX uses VXWORKS Embedded Real time Operating system and what
appears to be their own internal ftp server. This buffer overflow problem
seems to be one similar to the AIX ftpd reported in CVE 1999-0789 and has
been assigned bugtraq id 6297

By sending a specific string of data to the ftp server, an attacker can
not only disable the ftp server, but the integrated web based
administrative console and the call manager preventing diagnostics,
control and all incoming, outgoing or internal calls. Any calls in
progress cannot be disconnected, and in the case of long distance calls,
could result in excessive long distance bills and extended loss of use of
the phone system.

This condition is not recovered without a Hard reboot (power off/on).
Since the 3com nbx is based on an embedded Unix operating system
(vxworks), an abrupt power off could cause loss of data, including
corruption of voice mails in progress or logs.

A company who uses the VoIP features for remote locations, and who has the
call manager located on the outside of their firewall, or has no firewall
can have their voice communications disrupted easily. Even if the company
has call manager located on internal network, people with internal network
access can also disrupt communications. We have tested 3com nbx firmware
version 4_0_17 (with ftpd version 5.4) and nbx firmware version 4_1_4 and
4_1_21 (ftpd version 5.4.2) and this bug seems to be present in all three

3com Response:
3com confirmed problem and received a field patch, TSR(296292) from
WindRiver to address the problem. Neither WindRiver nor 3com has provided
a test bed or access to a fixed system for us verify fix. 3com will be
working to integrate this TSR into a future release of the nbx build but
has no date yet for release. Also, since ftpd is only used for debugging
and diagnostics, a future firmware will allow the administrator the
ability to turn off ftpd if not used.
We contacted 3com on Febuary 28th and March 14th to obtain above firmware
and then again on April 15th to report that Firmware 4_1_21 did not yet
fix the problem.
No response from 3com yet.

VxWorks Response:
The defect has been corrected. A solution has been provided to and is
being used by 3COM. We will be providing an update to CERT. We also plan
to issue patches to our population of customers for Tornado 2.0.2 /
VxWorks 5.4 and Tornado 2.2 / VxWorks 5.5. Testing on all CPU architecture
platforms for T2.0.2 and T2.2 is started now and we anticipate the patch
for T2.0.2 to be completed before Christmas and T2.2 by mid-Jan, 2003

CERT Response:
Cert has assigned VU #317417 and stated that as soon as they get vendor
confirmation they will publish report.

Please contact vendor once new software is released

There appears to be on way to turn off the built in ftp server in this
version of the software, nor anyway to do ip address limits via tcp
wrapper or acls. The only way we know of to prevent a denial of service
attack on the 3com nbx is to place it behind its own firewall. If call
manager is placed on the Internet side of the firewall or in the DMZ, care
should be taken to prohibit any access to ftp port (tcp port 21) This may
be impossible on an internal network unless 3com nbx is itself placed
behind a firewall, or on a separate VLAN or network segment. Care should
be taken in this approach, since some firewalls may interfere with the
VoIP operations.

see "Firewall limits vex VoIP users" at Nwfusion

This problem was originally found during a routine security audit by
Michael Scheidell, SECNAP Network Security, www.secnap.net using the
Nessus vulnerabilities scanner, www.nessus.org.,
Additional assistance by Leonid Rosenboim [lr24 (at) actcom.co (dot) il [email concealed]] in verifying
that this may be an old bug, already fixed in current vxworks sources but
might not have been tested by 3com yet.

Additional Information:
A tcpdump/pcap packet of the exploit and ftpd/nbx response can be found at
If you have snort or ISS's trons IDS, a signature to detect this attack
can be found at on www.snort.org

To test your systems for this vulnerability, you can use Nessus at
www.nessus.org. Either update your signatures, or download this nessus
signature: vxworks_ftpd.nasl at

For a report on Security Risk Factors with IP Telephony based Networks
Also reference article "is VoIP vulnerable ?"on NWfusion.com

Original copy of this report can be found here

Above Copyright(c) 2002, 2003, SECNAP Network Security, LLC. World rights

This security report can be copied and redistributed electronically
provided it is not edited and is quoted in its entirety without written
consent of SECNAP Network Security, LLC. Additional information or
permission may be obtained by contacting SECNAP Network Security at

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