Public Vulnerability Reports

Thompson SpeedTouch Home ADSL Modem Predictable TCP ISN Generation



The Thompson (formerly Alcatel) SpeedTouch is an ADSL router for home
and business providing a continuously available, "always on,"
connection. More information about the product can be found at


Remote exploitation of a design error vulnerability in Thompson's
SpeedTouch Home ADSL modem allows attackers to spoof TCP traffic on
behalf of the device.

The problem specifically exists due to the predictable nature of the TCP
Initial Sequence Number (ISN) generator on the device. The following
sanitized tcpdump output demonstrates the existence of the vulnerability
when 10 consecutive TCP connection requests are generated for the telnet
server (port 23) on the Thompson device:

48.3 host_a.1096   > host_b.telnet: S
48.3 host_b.telnet > host_a.1096: S 4081040897:4081040897(0) ack
48.3 host_a.1096   > host_b.telnet: R
48.4 host_a.1096   > host_b.telnet: S
48.4 host_b.telnet > host_a.1096: S 4081104897:4081104897(0) ack
48.4 host_a.1096   > host_b.telnet: R
48.6 host_a.1096   > host_b.telnet: S
48.6 host_b.telnet > host_a.1096: S 4081232897:4081232897(0) ack
48.6 host_a.1096   > host_b.telnet: R
48.7 host_a.1096   > host_b.telnet: S
48.7 host_b.telnet > host_a.1096: S 4081296897:4081296897(0) ack
48.7 host_a.1096   > host_b.telnet: R
48.9 host_a.1096   > host_b.telnet: S
48.9 host_b.telnet > host_a.1096: S 4081360897:4081360897(0) ack
48.9 host_a.1096   > host_b.telnet: R
49.0 host_a.1096   > host_b.telnet: S
49.0 host_b.telnet > host_a.1096: S 4081488897:4081488897(0) ack
49.0 host_a.1096   > host_b.telnet: R
49.2 host_a.1096   > host_b.telnet: S
49.2 host_b.telnet > host_a.1096: S 4081552897:4081552897(0) ack
49.2 host_a.1096   > host_b.telnet: R
49.3 host_a.1096   > host_b.telnet: S
49.3 host_b.telnet > host_a.1096: S 4081616897:4081616897(0) ack
49.3 host_a.1096   > host_b.telnet: R
49.5 host_a.1096   > host_b.telnet: S
49.5 host_b.telnet > host_a.1096: S 4081744897:4081744897(0) ack
49.5 host_a.1096   > host_b.telnet: R
49.6 host_a.1096   > host_b.telnet: S
49.6 host_b.telnet > host_a.1096: S 4081808897:4081808897(0) ack
49.6 host_a.1096   > host_b.telnet: R

In the above example, host_a is the querying host and host_b is the
Thompson device. A clear pattern in ISN generation can be seen as the
value increases by approximately 64,000 each millisecond.


Successful exploitation of weak ISNs for the purpose of connection
spoofing is not a trivial task. Successful exploitation allows an
attacker to generate traffic on behalf of the affected device. Such an
ability is most dangerous when trust paths exist between the affected
device and another remote system.


iDEFENSE has verified the existence of this vulnerability in Thompson's
SpeedTouch firmware version GV8BAA3.270 (1003825). It is suspected that
earlier versions are susceptible to exploitation as well.



Untrusted traffic should be filtered at the network perimeter.


The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2004-0641 to this issue. This is a candidate for inclusion in
the CVE list (, which standardizes names for
security problems.


06/08/2004    Initial vendor contact - no response
06/08/2004    iDEFENSE clients notified
06/18/2004    Secondary vendor contact - no response
08/05/2004    Public disclosure


The discoverer wishes to remain anonymous.

Get paid for vulnerability research


Copyright © 2004 Verisign, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
written consent of iDEFENSE. If you wish to reprint the whole or any
part of this alert in any other medium other than electronically, please
email for permission.

Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.