Si votre serveur ftp ProFtpd dispose d’un accès anonyme, lors d’une attaque si l’authentification échoue le serveur va ouvrir le compte anonymes. Ainsi dans /var/log/xferlog il apparait :

Mar 17 22:48:43 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): FTP session opened.
Mar 17 22:48:44 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): no such user 'Administrator'
Mar 17 22:48:44 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): USER Administrator: no such user found from ::ffff:219.140.61.17 [::ffff:219.140.61.17] to ::ffff:192.168.1.253:21
Mar 17 22:48:45 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): no such user 'Administrator'
Mar 17 22:48:45 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): USER Administrator: no such user found from ::ffff:219.140.61.17 [::ffff:219.140.61.17] to ::ffff:192.168.1.253:21
Mar 17 22:48:46 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): no such user 'Administrator'
Mar 17 22:48:46 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): USER Administrator: no such user found from ::ffff:219.140.61.17 [::ffff:219.140.61.17] to ::ffff:192.168.1.253:21
Mar 17 22:48:46 ubuntuserv proftpd[29240] ubuntuserv.lan (::ffff:219.140.61.17[::ffff:219.140.61.17]): Maximum login attempts (3) exceeded, connection refused

(tiens j’en profite pour balancer un adresse IP qui fait de la force-brute… Christine Albanel va pouvoir en faire quelque chose!)

ces lignes ne correspondent à aucune des règles fournies dans /etc/fail2ban/filter.d/proftpd.conf. Voici une nouvelle règle à ajouter dans le fichier proftpd.conf:

\(\S+\[<HOST>\]\)[: -]+ USER \S+ \(Login failed\): No such user found.$

Ainsi le fichier complet devient:

<pre># Fail2Ban configuration file
#
# Author: Yaroslav Halchenko
#
# $Revision: 677 $
#

[Definition]

# Option: failregex
# Notes.: regex to match the password failures messages in the logfile. The
#          host must be matched by a group named "host". The tag "<HOST>" can
#          be used for standard IP/hostname matching and is only an alias for
#          (?:::f{4,6}:)?(?P<host>\S+)
# Values: TEXT
#
failregex = \(\S+\[<HOST>\]\)[: -]+ USER \S+: no such user found from \S+ \[\S+\] to \S+:\S+$
           \(\S+\[<HOST>\]\)[: -]+ USER \S+ \(Login failed\): Incorrect password\.$
           \(\S+\[<HOST>\]\)[: -]+ SECURITY VIOLATION: \S+ login attempted\.$
           \(\S+\[<HOST>\]\)[: -]+ Maximum login attempts \(\d+\) exceeded$
           \(\S+\[<HOST>\]\)[: -]+ USER \S+ \(Login failed\): No such user found.$

# Option:  ignoreregex
# Notes.:  regex to ignore. If this regex matches, the line is ignored.
# Values:  TEXT
#
ignoreregex =
</pre>

Mise à jour 5-07-2009: si fail2ban a des problème pour reconnaitre ces attaques dans les logs vérifier que vous ne modifiez pas leur format avec une instruction de type logformat dans /etc/proftpd/proftpd.conf reseemblant à:

LogFormat write "%h %l %u %t \"%r\" %s %b"

de nombreux exemples de configuration de proftpd sur le net modifient le format des fichiers de log….

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