# [fetchmail-devel] Security vulnerability in APOP authentication

Gaëtan LEURENT gaetan.leurent at ens.fr
Wed Mar 14 15:55:08 CET 2007

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    Hello,

    I found a security vulnerability in the APOP authentication.  It is
    related to recent collision attacks by Wang and al. against MD5.  The
    basic idea is to craft a pair of message-ids that will collide in the
    APOP hash if the password begins in a specified way.  So the attacker
    would impersonate a POP server, and send these msg-id; the client will
    return the hash, and the attacker can learn some password characters.

    The msg-ids will be generated from a MD5 collision: if you have two
    colliding messages for MD5 "<????@????>x" and "<¿¿¿¿@¿¿¿¿>x", and the
    message are of length two blocks, then you will use "<????@????>" and
    "<¿¿¿¿@¿¿¿¿>" as msg-ids.  When the client computes MD5(msg-id||passwd)
    with these two, it will collide if the first password character if 'x',
    no matter what is next (since we are at a block boundary, and the end of
    the password will be the same in the two hashs).  Therefore you can
    learn the password characters one by one (actually you can only recover
    three of them, due to the way MD5 collisions are computed).

    This attack is really a practical one: it needs about an hour of
    computation and a few hundred authentications from the client, and can
    recover three password characters.  I tested it against fetchmail, and
    it does work.

    However, using the current techniques available to attack MD5, the
    msg-ids sent by the server can easily be distinguished from genuine ones
    as they will not respect the RFC specification.  In particular, they
    will contain non-ASCII characters.  Therefore, as a security
    countermeasure, I think fetchmail should reject msg-ids that does not
    conform to the RFC.

    The details of the attack and the new results against MD5 needed to
    build it will be presented in the Fast Software Encryption conference on
    March 28.  I can send you some more details if needed.

    Meanwhile, feel free to alert any one that you believe is concerned.
    I am already sending this mail to the maintainers of Thunderbird,
    Evolution, fetchmail, and mutt.  KMail already seems to do enough checks
    on the msg-id to avoid the attack.

    Please CC me in any reply.

    --
    Gaëtan LEURENT

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