How secure is IP address filtering?

Question

I'm planning to deploy an internal app that has sensitive data. I suggested that we put it on a machine that isn't exposed to the general internet, just our internal network. The I.T. department rejected this suggestion, saying it's not worth it to set aside a whole machine for one application. (The app has its own domain in case that's relevant, but I was told they can't block requests based on the URL.)

Inside the app I programmed it to only respect requests if they come from an internal I.P. address, otherwise it just shows a page saying "you can't look at this." Our internal addresses all have a distinct pattern, so I'm checking the request I.P. against a regex.

But I'm nervous about this strategy. It feels kind of janky to me. Is this reasonably secure?

Answer 1

IP filtering is better than nothing, but it's got two problems:

1. IP addresses can be spoofed.

2. If an internal machine is compromised (that includes a client workstation, e.g. via installation of a Trojan), then the attacker can use that as a jump host or proxy to attack your system.

If this is really sensitive data, it doesn't necessarily need a dedicated machine (though that is best practice), but you should at least authenticate your users somehow, and don't run less sensitive (and more easily attacked) apps on the same machine.

And if it is truly sensitive, get a security professional in to review what you're doing.

edit: incidentally, if you can, ditch the regex and use something like tcpwrappers or the firewalling features within the OS if it has any. Or, if you can have a different IP address for your app, use the firewall to block external access. (And if you have no firewall then you may as well give up and email your data to the attackers :-)

Answer 2

I would rather go with SSL and some certificates, or a simple username / password protection instead of IP filtering.

Answer 3

It depends exactly HOW secure you really need it to be.

I am assuming your server is externally hosted and not connected via a VPN. Therefore, you are checking that the requesting addresses for your HTTPS (you are using HTTPS, aren't you??) site are within your own organisation's networks.

Using a regex to match IP addresses sounds iffy, can't you just use a network/netmask like everyone else?

How secure does it really need to be? IP address spoofing is not easy, spoofed packets cannot be used to establish a HTTPS connection, unless they also manipulate upstream routers to enable the return packets to be redirected to the attacker.

If you need it to be really secure, just get your IT department to install a VPN and route over private IP address space. Set up your IP address restrictions for those private addresses. IP address restrictions where the routing is via a host-based VPN are still secure even if someone compromises an upstream default gateway.

Answer 4

If your application is checking the IP Address, then it is extremely vulnerable. At that point you don't have any protection at the router which is where IP filtering really needs to be. Your application is probably checking HTTP header information for the sending IP address and this is extremely easy to spoof. If you lock the IP address down at the router, that is a different story and will buy you some real security about who can access the site from where.

If are you are doing is accessing the application internally, then SSL won't buy you much unless you are trying to secure the information from parties internal to the organization, or you require client certificates. This is assuming you won't ever access the site from an external connection (VPNs don't count, because you are tunneling into the internal network and are technically part of it at that point). It won't hurt either and isn't that hard to setup, just don't think that it is going to be the solution to all your problems.

Answer 5

If it's limited by IP address, then although they can spoof the IP address, they won't be able to get the reply. Of course, if it's exposed to the internet, it can still get hit by attacks other than against the app.

Answer 6

My first thought on the ressource issue would be to ask if it wouldn't be possible to work some magic with a virtual machine?

Other than that - if the IP addresses you check up against are either IPs you KNOW belongs to computers that are supposed to access the application or in the local IP range, then I cannot see how it could not be secure enough (I am actually using a similiar approach atm on a project, although it is not incredibly important that the site is kept "hidden").

Answer 7

Just because all your internal IPs match a given regex, that doesn't mean that all IPs that match a given regex are internal. So, your regex is a point of possible security failure.

I don't know what technology you used to build your site, but if it's Windows/ASP.net, you can check the permissions of the requesting machine based on its Windows credentials when the request is made.

Answer 8

Like all security, it's useless on it's own. If you do have to put it on a public-facing webserver, use IP whitelisting, with basic username/password auth, with SSL, with a decent monitoring setup, with an up-to-date server application.

That said, what is the point in having the server be publicly accessible, then restrict it to only internal IP addresses? It seems like it's basically reinventing what NAT gives you for free, and with an internal-only server, plus you have to worry about web-server exploits and the likes.

You don't seem to gain anything by having it externally accessible, and there are many benefits to having having it be internal-only..

Answer 9

Your security is only as strong as your weakest link. In the grand scheme of things, spoofing an IP is child's play. Use SSL and require client certs.

Answer 10

It became useful first to distinguish among different kinds of IP vpn based on the administrative relationships, not the technology, interconnecting the nodes. Once the relationships were defined, different technologies could be used, depending on requirements such as security and quality of service.

Answer 11

Maybe this will help ? I've been looking for the same answer, and found this stackoverflow as well as this idea from Red Hat Linux Ent. I'll try it soon. I hope it helps.

iptables -A FORWARD -s 192.168.1.0/24 -i eth0 -j DROP

Where 0/24 is the LAN range you want to protect. The idea is to block "internet" facing (Forward) devices from being able to spoof the local IP network.

Ref: http://www.centos.org/docs/4/html/rhel-sg-en-4/s1-firewall-ipt-rule.html

Answer 12

A proper firewall can protect against IP spoofing, and it's not as easy as say spoofing your caller ID, so the argument /not/ to use IP filtering because of the spoofing danger is a bit antiquated. Security is best applied in layers, so you're not relying on only one mechanism. That's why we have WAF systems, username+password, layer 3 firewalls, layer 7 firewalls, encryption, MFA, SIEM and a host of other security measures, each which adds protection (with increasing cost).

If this is a web application you're talking about (which was not clear from your question), the solution is fairly simple without the cost of advanced security systems. Whether using IIS, Apache, etc. you have the ability to restrict connections to your app to a specific target URL as well as source IP address - no changes to your app necessary - on a per-application basis. Preventing IP-based web browsing of your app, coupled with IP source restrictions, should give you significant protection against casual browsing/attacks. If this is not a web app, you'll have to be more specific so people know whether OS-based security (as has been proposed by others) is your only option or not.

Answer 13

IP Whitelisting is, as others have mentioned, vulnerable to IP spoofing and Man-in-the-Middle attacks. On an MITM, consider that some switch or router has been compromised and will see the "replies". It can either monitor or even alter them.

Consider also vulnerabilities with SSL encryption. Depending on the effort, this can be foiled in a MITM as well, as well as the well-known gaffs with the reuse of the primes, etc.

Depending on the sensitivity of your data, I would not settle for SSL, but would go with StrongSWAN or OpenVPN for more security. If handled properly, these will be a lot less vulnerable to a MITM.

Reliance on whitelisting alone (even with SSL) I would consider "low grade", but may be sufficient for your needs. Just be keenly aware of the implications and do not fall into the trap of a "false sense of security".