One of the interesting thing that I noticed after shellshock was the amount of probes for vulnerabilities that counted on webapp users to have direct network access. Not only ping to known addresses to just verify the vulnerability, or wget or curl with unique IDs, but even very rough nc or even /dev/tcp connections to give remote shells. The fact that probes are there makes it logical to me to expect that for at least some of the systems these actually worked.
The reason why this piqued my interest is because I realized that most people don’t do the one obvious step to mitigate this kind of problems by removing (or at least limiting) the access to the network of their web apps. So I decided it might be a worth idea to describe a moment why you should think of that. This is in part because I found out last year at LISA that not all sysadmins have enough training in development to immediately pick up how things work, and in part because I know that even if you’re a programmer it might be counterintuitive for you to think that web apps should not have access, well, to the web.
Indeed, if you think of your app in the abstract, it has to have access to the network to serve the response to the users, right? But what happens generally is that you have some division between the web server and the app itself. People who have looked into Java in the early nougthies probably have heard of the term Application Server, which usually is present in form of Apache Tomcat or IBM WebSphere, but here is essentially the same “actor” for Rails app in the form of Passenger, or for PHP with the php-fpm service. These “servers” are effectively self-contained environments for your app, that talk with the web server to receive user requests and serve them responses. This essentially mean that in the basic web interaction, there is no network access needed for the application service.
Things gets a bit more complicated in the Web 2.0 era though: OAuth2 requires your web app to talk, from the backend, with the authentication or data providers. Similarly even my blog needs to talk with some services, to either ping them to tell them that a new post is out, and to check with Akismet for blog comments that might or might not be spam. WordPress plugins that create thumbnails are known to exist and to have a bad history of security and they fetch external content, such as videos from YouTube and Vimeo, or images from Flickr and other hosting websites to process. So there is a good amount of network connectivity needed for web apps too. Which means that rather than just isolating apps from the network, what you need to implement is some sort of filter.
Now, there are plenty of ways to remove access to the network from your webapp: SElinux, GrSec RBAC, AppArmor, … but if you don’t want to set up a complex security system, you can do the trick even with the bare minimum of the Linux kernel, iptables and CONFIG_NETFILTER_XT_MATCH_OWNER. Essentially what this allows you to do is to match (and thus filter) connections based of the originating (or destination) user. This of course only works if you can isolate your webapps on a separate user, which is definitely what you should do, but not necessarily what people are doing. Especially with things like mod_perl or mod_php, separating webapps in users is difficult – they run in-process with the webserver, and negate the split with the application server – but at least php-fpm and Passenger allow for that quite easily. Running as separate users, by the way, has many more advantages than just network filtering, so start doing that now, no matter what.
Now depending on what webapp you have in front of you, you have different ways to achieve a near-perfect setup. In my case I have a few different applications running across my servers. My blog, a WordPress blog of a customer, phpMyAdmin for that database, and finally a webapp for an old customer which is essentially an ERP. These have different requirements so I’ll start from the one that has the lowest.
The ERP app was designed to be as simple as possible: it’s a basic Rails app that uses PostgreSQL to store data. The authentication is done by Apache via HTTP Basic Auth over HTTPS (no plaintext), so there is no OAuth2 or other backend interaction. The only expected connection is to the PostgreSQL server. Pretty similar the requirements for phpMyAdmin: it only has to interface with Apache and with the MySQL service it administers, and the authentication is also done on the HTTP side (also encrypted). For both these apps, your network policy is quite obvious: negate any outside connectivity. This becomes a matter of iptables -A OUTPUT -o eth0 -m owner --uid-owner phpmyadmin -j REJECT — and the same for the other user.
The situation for the other two apps is a bit more complex: my blog wants to at least announce that there are new blog posts, and it needs to reach Akismet; both actions use HTTP and HTTPS. WordPress is a bit more complex because I don’t have much control over it (it has a dedicated server, so I don’t have to care), but I assume it mostly is also HTTP and HTTPS. The obvious idea would be to allow ports 80, 443 and 53 (for resolution). But you can do something better. You can put a proxy on your localhost, and force the webapp to go through it, either as a transparent proxy or by using the environment variable http_proxy to convince the webapp to never connect directly to the web. Unfortunately that is not straight forward to implement as neither Passenger not php-fpm has a clean way to pass environment variables per users.
What I’ve done is for now is to hack the environment.rb file to set ENV['http_proxy'] = 'http://127.0.0.1:3128/' so that Ruby will at least respect it. I’m still out for a solution for PHP unfortunately. In the case of Typo, this actually showed me two things I did not know: when looking at the admin dashboard, it’ll make two main HTTP calls: one to Google Blog Search – which was shut down back in May – and one to Typo’s version file — which is now a 404 page since the move to the Publify name. I’ll be soon shutting down both implementations since I really don’t need it. Indeed the Publify development still seems to go toward the “let’s add all possible new features that other blogging sites have” without considering the actual scalability of the platform. I don’t expect me to go back to it any time soon.