The Palner Group, Inc.

Kamailio, Asterisk, VoIP, and IT Consulting

Category: Security

Explaining Sip Brute Force Attacks to Non-Techs

Today we received a call from a federal employee investigating a “hack” on a client’s system. Basically, the client suffered a SIP Brute Force attack on their elastix system. Besides the shock of a call from the feds (why did they ignore those Amazon attacks?), the realization of explaining a sip attack to someone not familiar with SIP, telephony, networking, or servers posed a little challenge.

So, how do we start?

First step: We will no longer use the words SIP, Brute, Force, and Attack. =)

What we’re talking about is a scheme to make expensive calls through your phone system. Of course, this isn’t true for all scenarios, but the vast majority simply want to make expensive calls on your dime.

How does it work?

The bad guys trick your phone system into thinking they are a valid user.

How can they do that?

When phones connect to your phone system, the system replies with different messages. Based on those messages, the bad guys can figure out phone names. Think of your phone system as the receptionist. An attempt might be similar to…

Bad Guy: “Hi, is Alice there?”
Receptionist: “No, there is no Alice here. You have the wrong number.”
Bad Guy: “Hi, is Bob there?”
Receptionist: “Yes, who may I say is calling?”

Basically, there’s a different response based on if that person exists in the company. Same thing with the phones. Once the Bad Guys find out phone names, they then use their computers to crack the phone password.

Once the password is detected, they connect their phone to your system and begin making calls.

What can I do to stop this?

If the person in charge of your phone system doesn’t understand what this attack is, you need to hire a consultant to help you and/or train your administrator. If you or your administrator understand this attack, then you need to make sure you are following the best practices for SIP security (here’s a good link for asterisk best practices).

If you’re running asterisk, you might wish to install a script that checks for attacks and blocks those connections.

Even better… consider Kamailio.

Kamailio (pronounced KAMA-ILLY-OH) is an open-source SIP proxy, registrar, application that is extremely robust and powerful. The software includes anti-flood features that really help protect your system and truly helps to minimize these annoying attacks.

Remember, the Internet is like a big city. Sure there’s great museums and entertainment, but there’s also bad, bad places filled with bad, bad people. If you’re going to leave your BMW unlocked in Hell’s Kitchen, don’t be surprised when it’s been taken around the block a few times.

Automatically Block Failed SIP Peer Registrations

Previously we posted a little script for quickly checking your asterisk log for failed peer registrations. Building on that script, and with the use of iptables and cron, you can easily (and automatically) block flooding traffic from your system. Iptables, a linux command line program to filter IP traffic, provides high level packet filtering before the traffic can be used to corrupt a program. Cron, the linux time scheduler, enables you to automatically run commands at scheduled time periods.

Set up IP Tables

We will not be discussing the intricacies of iptables in this post. There are excellent tutorials on iptables, and with most things linux, help is only a google away. To help identify the traffic blocked as asterisk related, a new chain will be created appropriately called… asterisk.

Here’s how to add the new chain:

iptables -N asterisk
iptables -A INPUT -j asterisk
iptables -A FORWARD -j asterisk

This will help identify hosts blocked for failed registrations.

Asterisk’s Log for Failed Registrations

In most cases of a sip flood attack, the host attempts registration to Asterisk. These hosts are identified in the Asterisk log (/var/log/messages) as “No matching peer found.” The following perl script scans /var/log/messages for these patterns, strips the IP address, and puts the IP address into an array.

After the file has been read, the IP addresses are counted (each count is a failed attempt), compared against the existing blocked hosts, and new occurrences are blocked. With this script we are blocking any host after the 4th failed attempt.

Here’s the script (last updated 05 SEP 2010):

#!/usr/bin/perl -w
use strict;
use warnings;
my (@failhost);
my %currblocked;
my %addblocked;
my $action;

open (MYINPUTFILE, "/var/log/asterisk/messages") or die "\n", $!, "Does log file file exist\?\n\n";

while (<MYINPUTFILE>) {
	my ($line) = $_;
	chomp($line);
	if ($line =~ m/\' failed for \'(.*?)\' - No matching peer found/) {
		push(@failhost,$1);
	}
	if ($line =~ m/\' failed for \'(.*?)\' – Wrong password/) {
		push(@failhost,$1);
	}
}

my $blockedhosts = `/sbin/iptables -n -L asterisk`;

while ($blockedhosts =~ /(.*)/g) {
	my ($line2) = $1;
	chomp($line2);
	if ($line2 =~ m/(\d+\.\d+\.\d+\.\d+)(\s+)/) {
		$currblocked{ $1 } = 'blocked';
	}
}

while (my ($key, $value) = each(%currblocked)){
	print $key . "\n";
}

if (@failhost) {
	&count_unique(@failhost);
	while (my ($ip, $count) = each(%addblocked)) {
		if (exists $currblocked{ $ip }) {
			print "$ip already blocked\n";
		} else {
			$action = `/sbin/iptables -I asterisk -s $ip -j DROP`;
			print "$ip blocked. $count attempts.\n";
		}
	}
} else {
	print "no failed registrations.\n";
}

sub count_unique {
    my @array = @_;
    my %count;
    map { $count{$_}++ } @array;
    map {($addblocked{ $_ } = ${count{$_}})} sort keys(%count);
}

Schedule the script with cron

The final step is to schedule your script to run every X minutes in cron. We’ve chosen to run our script every 2 minutes, but you can change this to 1 minute or any other time period you choose. Just remember… you can receive thousands of attempts within 2 minutes.

If you have named your script check-failed-regs.pl and placed it in your /usr/local/bin directory, your cron statement would look like this:

*/2 * * * * perl /usr/local/bin/check-failed-regs.pl &> /dev/null

Questions? Comments? We love feedback. Or, contact us for more information.

Perl Script for Asterisk Failed Peer Registrations

I guess this might be better titled as the Quick and Dirty Perl Script… but here we go:

#!/usr/bin/perl -w
use strict;
use warnings;
my (@failhost);

open (MYINPUTFILE, "/var/log/asterisk/$ARGV[0]") or die "\n", $!, "Does log file file exist\?\n\n";

while (<MYINPUTFILE>) {
	my ($line) = $_;
	chomp($line);
	if ($line =~ m/\' failed for \'(.*?)\' - No matching peer found/) {
		push(@failhost,$1);
	}
}

if (@failhost) {
	&count_unique(@failhost);
} else {
	print "no failed registrations.\n";
}

sub count_unique {
    my @array = @_;
    my %count;
    map { $count{$_}++ } @array;
	
	#print them out:
	
    map {print "$_ = ${count{$_}}\n"} sort keys(%count);

}

And while we duck from @Merlyn’s criticisms (although we love his criticism), the basic usage is:

perl [Whatever you named it].pl messages
or perl [Whatever you named it].pl messages.1

Results look like:

184.73.53.22 = 13586
64.76.45.100 = 9895
78.46.87.14 = 9960

Or “no failed registrations.” if you have no failed attempts.

Vulnerability Assessment and Scans

Vulnerability scanning and assessment identifies security risks within your network. Team Forrest highly recommends proactive, routine scanning to help assess, react, and improve your network security.

Utilizing a variety of techniques, applications, and tools, Team Forrest remotely examines your network over the public Internet. identified weaknesses and vulnerabilities are assessed for risk and detailed, with recommendations, to the customer.

What is a Vulnerability Scan?

A vulnerability scan assesses computer systems, networks, and applications for weaknesses. Vulnerability Scans are recommended (and may be required) for any business conducting e-commerce, hosting a server with a publicly accessible IP Address, or allowing remote access to company assets. Team Forrest recommends a comprehensive scan, including:

  1. Checking for vulnerabilities of remote systems
  2. Checking for misconfiguration of remote systems, software, and services
  3. Checking commonly used passwords
  4. Checking Denial of Service sensitivity
  5. Checking for Web Vulnerability (such as SQL Injection)

How does a Vulnerability Assessment Work?

Team Forrest performs the scan remotely, accessing your network over the Public Internet. There is nothing for you to do and no software will need to be installed. Our servers will simply assess your network remotely.

Once the scan completes, Team Forrest provides a detailed assessment including identified risks and vulnerabilities, as well as their severity level. Team Forrest also provides recommendations and assisting in correcting any identified flaws or vulnerabilities.

For more information on a Team Forrest Vulnerability Scan / Assessment, please call 888-295-0025 or contact us for details.

Firefox 3.6.2 Corrects Vulnerability

If you’re running Firefox 3.6, Mozilla strongly recommends you update to version 3.6.2. The new version corrects a critical security hole allowing an attacker to crash your browser and/or run arbitrary code on your machine.

For more information, check out the post at VoIP Tech Chat.

Zero-day Flaw in Firefox 3.5

Update On 7/16/2009, Firefox released version 3.5.1 to address the issue. Read Update Below!

Mozilla.com released details today on a critical JavaScript vulnerability in the latest version of the popular Firefox 3.5 Web Browser. The vulnerability allows execution of code on the client (or target) system simply by visiting a website.

No patch is currently available for the flaw and several organizations (such as Scurnia, The Sans Institute, and the United States Computer Emergency Response Team) are recommending the complete disabling of JavaScript in Firefox (see below). Additionally, The Sans Institute is recommending the use of the NoScript Firefox plugin (that enables javascript only from white-listed sites).

Additional information:

How to Disable the Javascript Engine in Firefox:

  1. Enter about:config in the browser’s location bar.
  2. Type jit in the Filter box at the top of the config editor.
  3. Double-click the line containing javascript.options.jit.content setting the value to false.

Mozilla advises that disabling the JIT will result in decreased JavaScript performance and is only recommended as a temporary security measure.  Once users have been received the security update containing the fix for this issue, they should restore the JIT repeating the process above and setting the javascript.options.jit.content value to true.

Update — 7/16/2009

Firefox has introduced version 3.5.1 to address the security risk, as posted on their developer blog:

Firefox 3.5.1 update is now available for download

As part of the Mozilla Corporation’s ongoing security and stability process, Firefox 3.5.1 is now available for Windows, Mac, and Linux users as a free download from www.firefox.com.

We strongly recommend that all Firefox 3.5 users upgrade to this latest release. If you already have Firefox 3.5, you will receive an automated update notification within 24 to 48 hours. This update can also be applied manually by selecting “Check for Updates…” from the Help menu.

For a list of changes and more information, please see the Firefox 3.5.1 release notes.

Please note: If you’re still using Firefox 2.0.0.x, this version is no longer supported and contains known security vulnerabilities. Please upgrade to Firefox 3.5 by downloading Firefox 3.5.1 from www.firefox.com.

This entry was posted by beltzner on Thursday, July 16th, 2009 at 6:34 pm.