Cracking preshared keys

From: Michael Thumann (
Date: 04/23/03

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    Date: Wed, 23 Apr 2003 11:35:15 +0100
    From: Michael Thumann <>


    we would like to announce the publication of a proof of concept paper 'PSK
    cracking using IKE Aggressive Mode'. Paper can be downloaded from .

    The theoretical vulnerability about this topic is not new. While we were
    preparing a talk about VPN hacking we configured the lab that is described
    in the paper to do some kind of demonstration. We were able to capture and
    crack successfully PSKs of a cisco router due to the issue that the cisco
    router switches automatically to aggressive mode if the initiating clients
    demands it.

    This attack depends on some conditions on the vpn gateway.

    1. Preshared keys are use for authentication
    2. The vpn gateway must allow any ip address to establish a vpn connection
    3. The vpn gateway must support aggressive mode
    4. Of course the psk must be weak to crack it in an acceptable amount of time

    Under these circimstances it's possible to capture and crack the preshared
    key of a vpn gateway and gain unauthorized access to private networks.

    To prevent your vpn gateways from being compromised we suggest the
    following actions:
    - Don't use preshared key for authentication, if its possible. Otherwise
    ensure that you use very strong keys.
    - If possible, don't allow vpn connections from any ip address (difficult,
    I know).
    - Disable aggressive mode, if it's supported (for example in Checkpoint
    Firewall-1 where it's disabled per default).


    ERNW Enno Rey Netzwerke GmbH - Zaehringerstr. 46 - 69115 Heidelberg
    Tel. +49 6221 480390 - Fax +49 6221 419008 - Mobil +49 173 6745903

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