Tag: SIP security

Stop SOME SipVicious attacks from reaching your Asterisk, FreeSwitch, YATE, etc. PBX server

This tip was posted by user “infotek” on the FreePBX site but applies to all software PBX systems that use the iptables firewall. “infotek” wrote:

By default the SipVicious scanner uses the ua : “friendly-scanner”. To block this ua, you can have iptables search the packet for that text.

add the following line to /etc/sysconfig/iptables

-A INPUT -p udp -m udp –dport 5060 -m string –string “friendly-scanner” –algo bm –to 500 -j DROP

Now the thing to keep in mind about this is that it only works if you know the string that will be sent as the user agent, and some hackers using SipVicious may take the trouble to change that default string, but some protection is better than none.  However this same technique can be used to block any attack that constantly sends the same string as the user agent, if you know what that string contains.

For those that use Webmin to manage iptables, here are the settings to use.  This should come BEFORE any other rules applicable to port 5060 – I made it the very first rule on the page “Incoming packets (INPUT) – Only applies to packets addressed to this host“:

Rule comment: Stop SipVicious
Action to take: Drop
Network protocol: Equals UDP
Destination TCP or UDP port: Equals Port(s) 5060
Additional IPtables modules: string
Additional parameters: –string “friendly-scanner” –algo bm –to 500

All other settings on the Webmin “Add Rule” page should be left at the default value (usually <ignored>).

To stop the hackers clever enough to change the default user agent string, consider also using this technique.

If you used the information from the article, “A possible way to thwart SIP hack attempts on your Asterisk (or other) PBX server”, please read this

If you took the advice in the article, A possible way to thwart SIP hack attempts on your Asterisk (or other) PBX server, please be aware that an important note has been added:

IMPORTANT: Be sure to have a separate iptables rule (higher on the list than those above) that allows connections to port 5060 from devices on your local network. Otherwise, you may find that new extensions that you are adding for the first time will not register with your Asterisk server, or that after a system reboot, none of your local extensions will register!

Please take heed of that note, or you may be one power failure or reboot away from discovering that none of your local extensions are able to register with your PBX server.

A possible way to thwart SIP hack attempts on your Asterisk (or other) PBX server

If you’ve had the problem of hackers trying to break into your Asterisk server, you probably know that you can use tools like Fail2ban to at least slow them down.  But why let them know you even have an Asterisk server in the first place?  Maybe you need to leave port 5060 open so that remote users (not on your local network) can connect to the server, but that doesn’t mean that you have to advertise to the bad guys that you might have something of interest.  With that in mind, we direct your attention to this post in the DSLReports VoIP forum:

The Linux netfilter/iptables firewall is capable of stopping these attacks before they even start.

At a bare minimum, this stops 99% of the attacks when added to your iptables ruleset:

-A INPUT -p udp --dport 5060 -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p udp --dport 5060 -m string --string "REGISTER sip:your.pbx.dns.name" --algo bm -j ACCEPT
-A INPUT -p udp --dport 5060 -m string --string "REGISTER sip:" --algo bm -j DROP
-A INPUT -p udp --dport 5060 -m string --string "OPTIONS sip:" --algo bm -j DROP
-A INPUT -p udp --dport 5060 -j ACCEPT

Warning
IMPORTANT: Be sure to have a separate iptables rule (higher on the list than those above) that allows connections to port 5060 from devices on your local network. Otherwise, you may find that new extensions that you are adding for the first time will not register with your Asterisk server, or that after a system reboot, none of your local extensions will register!

To understand how this works, read the original post by DSLReports user espaeth.

For another line of defense against such attacks, see the article Stop SOME SipVicious attacks from reaching your Asterisk, FreeSwitch, YATE, etc. PBX server.