[NEWS] Thompson (Alcatel) SpeedTouch Home ADSL Modem Predictable TCP ISN Generation

From: SecuriTeam (support_at_securiteam.com)
Date: 08/09/04

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      Thompson (Alcatel) SpeedTouch Home ADSL Modem Predictable TCP ISN
    Generation
    ------------------------------------------------------------------------

    SUMMARY

    The Thompson (formerly Alcatel) <http://www.speedtouchdsl.com/>
    SpeedTouch is an ADSL router for home and business providing a
    continuously available, "always on," connection.

    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.

    DETAILS

    Vulnerable Systems:
     * Thompson's SpeedTouch firmware version GV8BAA3.270 (1003825), possibly
    prior

    CVE Information:
     <http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2004-0641>
    CAN-2004-0641

    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.

    Disclosure Timeline
    06/08/04 Initial vendor contact - no response
    06/08/04 iDEFENSE clients notified
    06/18/04 Secondary vendor contact - no response
    08/05/04 Public disclosure

    ADDITIONAL INFORMATION

    The information has been provided by
    <mailto:idlabs-advisories@idefense.com> iDEFENSE Security Labs.

    ========================================

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