Zero day exploits, best explained here, will be coming out daily for the month of January, it seems, due to a security research firm in Russia. No matter what you think about their methods, this does highlight a fact that is sometimes forgotten, every running service presents the potential for an exploit. But without those services a computer is just an overpriced electric heater. So how do we protect ourselves against the unknown and unpatched? By being very careful about what our servers are running, only allowing access to the minimum number of resources required to get the job done, and having a plan for when your monitoring reports the service is down.
Since linux distributions are varied in their installs I won't go through each but most of the "friendly" distributions start, by default, a variety of services that may not be required but could potentially have exploits. While most of these don't have a network component, combined with other exploits they could help open the server to attack. For example, Red Hat starts processes to monitor the software raid and logical volume manager even if you aren't using them. It also starts processes for handling bluetooth devices, HP printers, and command line mouse support, even if you don't have them. None of these should cause you any concern but if you don't need them they don't need to run at all.
Most Apache HTTP server installs suffer from the same desire for usability, many modules are made available to the server by default. For example, you probably aren't using LDAP authentication or WebDAV as part of your server but the modules for them are preloaded on most default installs. Identifying the modules that are required for your web site or application to run and then disabling the ones that are not will reduce your apache httpd footprint and therefore reduce your exposure.
MySQL server doesn't have the modular nature of our prior two examples but there are some steps that you can take to reduce your exposure. First off, after doing the install and setting the root password, remove the test user and database. These have no known exploits but aren't needed. Second, ensure that your users are bound to a host instead of a wild card address, this makes sure that connections are only authorized from known hosts. Finally, if you are running mysql on the same host as your webserver and this is the only server that needs to access it, configure it to only listen on localhost ( There is no place like 127.0.0.1 ), this ensures that remote hosts cannot connect to your database even if your firewall fails.
While I did focus on some of the more simple things that can be done to a LAMP server, this should give you an idea of what kind of changes can be made that won't effect your service but will reduce your exposure footprint. Remember that before you make any changes you should do a backup and make copies of the files you are editing. We will see what this month brings as far as unpublished exploits and should also take this time to remember that not all exploits are published or patched, or even discovered yet.
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