Wednesday, 5 September 2018

Why do we install as root?

A couple of common questions I hear from customers (usually long-time users of a particular database from Redwood) via our guys in the field is “why do we install our software as root?” And “why do we run services as postgres?”. The simple, TLDR; answer is “for security”. For a detailed explanation, read on…

A basic principle when securing a software installation is “install with maximum privilege requirements and run with minimal”. In practice this equates to having software being installed and binaries/executables etc. owned by the root user, whilst the services themselves are actually run under a minimally privileged (and ideally dedicated) service user account, typically postgres in a PostgreSQL installation. Data files, and any other files that need to be modified by the software in normal operation are also owned by the service user account.

Let’s look at the running software first. Postgres (which will in fact refuse to run as root), is a server process which is often running on a network port that is accessible from other nodes on the network. Of course, we should limit access as much as possible to only those nodes that need access using both a firewall (even simple iptables rules will work), and Postgres’ pg_hba.conf access control file, but even with those measures in place, it’s possible that a determined attacker (let’s call him Zero Cool) can still gain access to the port the database server is running on.

Once our arch-nemesis Zero Cool has access to the database server port, he needs a way to escalate his attack. This may involve exploiting an unknown security issue in Postgres itself (as with any software, we hope there are none but we’re kidding ourselves if we think it’s totally secure), or it may be that he’s used other techniques such as social engineering to learn a users credentials.

If Zero gains “regular” access to Postgres, then he will be subject to any security measures (access control lists, RLS policies etc) that limit the scope of what the user account he’s used can access/delete/update/whatever. If the user account has superuser privileges or access to un-trusted procedural languages, or if Zero gained access using a lower-level exploit that allows him to execute arbitrary code in other ways, then he will be able to wreak chaos at a lower level in the system, such as overwriting files on disk.

However - and this is the important bit - assuming there are no exploits in the Operating System that he can leverage to gain further privileges, his chaos will be restricted to things that the service account under which Postgres is running can do. In a well secured system where an unprivileged account like postgres is used, that will be limited to damage to the Postgres data files and other files (or processes etc) that user can modify or control. If Postgres were running under a privileged account like root, Zero would have pwned (in script-kiddie parlance) the entire system at this point!

Now consider the case where the Postgres software files were also owned by the postgres user. Zero would not only be able to affect files and processes owned by the service account, but would also be able to modify the software itself, allowing him the opportunity to add backdoors for future access or other malware such as spyware etc. In the case of software that is started as root (even that which later drops those privileges or switches to another user account for normal operation), this could be exploited to gain even easier privileged access at a later time.

This is why we want our software to be installed and owned by a high privilege user such as root and run as a low privileged user such as postgres. Doing so ensures that even if Zero manages to crack his way into Postgres and potentially access or modify data, he cannot modify the software or other aspects of the host system and thus has a much harder time further escalating his attack.

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