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Authentication Weaknesses
The first section of this chapter discussed scanning for usernames. Even if you have
usernames that are difficult to guess, it is critical that you have strong passwords as
well. If an attacker is able to obtain a valid username, they will likely attempt to brute-
force the password. Strong passwords make this much more difficult.
The default authentication scheme of the SIP protocol is weak. Authentication is done
using an MD5 challenge-and-response mechanism. If an attacker is able to capture
any call traffic, such as a SIP call made from a laptop on an open wireless network, it
will be much easier to work on brute-forcing the password, since it will not require
authentication requests to the server.
Use strong passwords. There are countless resources available on
the internet that help define what constitutes a strong password.
There are also many strong password generators available. Use
them!
Fail2ban
The previous two sections discussed attacks involving scanning for valid usernames
and brute-forcing passwords. Fail2ban is an application that can watch your Asterisk
logs and update firewall rules to block the source of an attack in response to too many
failed authentication attempts.
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Chapter 22: Security
Use Fail2ban when exposing Voice over IP services on untrusted
networks. It will automatically update the firewall rules to block the
sources of attacks.
Installation
Fail2ban is available as a package in many distributions. Alternatively, you can install
it from source by downloading it from the Fail2ban website. To install Fail2ban on
RHEL, you must have the EPEL repository enabled (which was handled during
Chapter 3). You can install Fail2ban by running the following command:
$ sudo yum install fail2ban
The installation of Fail2ban from a package will include a startup
script to ensure that it runs when the machine boots up. If you
install from source, make sure that you take the necessary steps to
ensure that Fail2ban is always running.
Configuration
First up, we’ll want to configure the security log in Asterisk, which Fail2ban is able to
make use of.
$ sudo vim /etc/asterisk/logger.conf
Uncomment the (or add a) line that reads security => security, and edit the date
format so Fail2ban understands the logfile.
[general]
exec_after_rotate=gzip -9 ${filename}.2;
dateformat = %F %T
[logfiles]
;debug => debug
security => security
;console => notice,warning,error,verbose
console => notice,warning,error,debug
messages => notice,warning,error
full => notice,warning,error,debug,verbose,dtmf,fax
Then reload the Asterisk logger:
$ sudo asterisk -rx 'logger reload'
Since current versions of Fail2ban already come with an Asterisk jail definition, all we
need to do is enable it:
The current best practice is to create a file /etc/fail2ban/jail.local for this purpose
(technically you can put it in /etc/fail2ban/jail.conf, but this is more likely to be
overwritten):
Fail2ban
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$ sudo vim /etc/fail2ban/jail.local
[asterisk]
enabled = true
filter = asterisk
action = iptables-allports[name=ASTERISK, protocol=all]
sendmail[name=ASTERISK, dest=me@shifteight.org, sender=fail2ban@shifteight.org]
logpath = /var/log/asterisk/messages
/var/log/asterisk/security
maxretry = 5
findtime = 21600
bantime = 86400
We’ve set up the ban for 24 hours, but you can do longer or shorter times as well if
you prefer (the bantime is defined in seconds, so calculate accordingly). Since most
attacking hosts move on after a few hours, there’s no harm in unblocking an IP after