Security is a topic that is broad, deep, and in some places complex. Security is nearly always bought with compromises - for example by decreasing performance, usability, or interoperability.
By default Prosody aims to provide a server that is quite secure enough for most people and follows all current security best practices, while not becoming difficult to use or introducing unnecessary interoperability issues. This article is to help you dig further into our default settings, and understand which ones you might want to change to adjust depending on your actual security needs.
There are two primary aspects of security that typically concern an XMPP server administrator: information security and communication security.
Information security is about protecting the data stored on your server from unauthorized access or public leakage. By default, this is the kind of data Prosody might store on behalf of users:
Prosody's data store should be adequately protected. If you are using the default file-based storage, the data directory's filesystem permissions should allow access only by Prosody's user account and trusted users on the system. For ultimate security, also consider encrypting the disk that the files are stored on using your system's facilities for that.
If you have configured Prosody to use an external database for storage, ensure that Prosody's config file containing the database credentials is protected from read access by unauthorized users. Also note that some plugins may still make use of the configured data directory - as of 0.9 this currently includes storage of offline messages, which never get stored in an external database.
Account credentials should be hashed if you can, using for example mod_auth_internal_hashed. But note that enabling this can have unexpected security consequences for users when authenticating - see Plain or hashed password storage? for some discussion on this.
Protecting your messages and data as it travels across the network and internet falls into the realm of communication security. Prosody uses OpenSSL to implement connection encryption using SSL/TLS.
There are various aspects to consider regarding communication security:
The first step is to decide who you need to be able to communicate with. The smaller and less diverse your set of users and contacts is, the more you can lock down your server.
Almost all clients will use SSL/TLS out of the box, and warn users loudly if their connection is not encrypted, or if the certificate cannot be verified. However Prosody has a couple of controls for client-to-server encryption policy on the server side:
Remember that an encrypted connection is only truly secure if your users have securely verified your server's certificate. This might mean getting a certificate by a widely-trusted CA, or securely sending your certificate's fingerprint to users so that they can verify it manually. More discussion on certificates below.
Server-to-server encryption policy has some additional complexity, because Prosody has to verify the certificates of other servers. Sometimes (too often!) servers present invalid, expired, or self-signed/untrusted certificates that prevent Prosody from securely authenticating them. When this happens, Prosody will fall back to DNS-based authentication (XMPP dialback) - but the connection will still be encrypted. Prosody will always use encryption on server-to-server connections when the other server supports it.
Some servers support no encryption at all (most notably this includes all Google-hosted servers, including gmail.com). By default Prosody allows unencrypted server-to-server connections if the other server does not seem to support it - but this can be a security concern. You can prevent this behaviour by setting the
s2s_require_encryption option to
Once you have decided on your encryption policy, you can move on to your authentication policy. Authentication is the process of determining that the other server is really the server it claims to be, and not an attacker (or "man in the middle"). The most secure form of authentication is using certificates - Prosody will attempt to use these by default if your version of LuaSec supports it.
If you wish to always use certificates for authentication, and never fall back to (weaker) DNS authentication, set
Regardless of this default authentication policy, you can selectively require or not require certificate authentication for certain domains by using the lists
s2s_insecure_domains. You can find examples in the default config file, and documentation at Server-to-server security.
The curse of many service admins! Certificates are important, because they allow your users (and other servers on the network) to identify you - and ensure that there is not a "man in the middle" attacker reading your messages and passing them on.
We have an article on obtaining certificates for use with Prosody.
You can check any XMPP server's certificate manually by using a service such as https://check.messaging.one/.
The cipher is the actual algorithm used to encrypt your data as it goes along the wires. There are many many ciphers available, and all of them have strengths and weaknesses - some are fast and weak, some are slow and secure - so there is no such thing as the "best" cipher, and the set of recommended ciphers changes over time as weaknesses are found.
It is not recommended to change Prosody's default cipher list unless you know what you are doing, but if you do wish to do this, see our advanced SSL configuration article.
Forward secrecy ('FS', also called 'Perfect Forward Secrecy' or 'PFS') is a property of some TLS ciphers/handshakes to protect encrypted streams from later decryption if the server's certificate's private key is ever compromised (e.g. stolen or computationally cracked). There are currently two kinds of forward-secrecy handshakes in use today, standard Diffie-Hellman (DH) and Elliptic Curve Diffie-Hellman (ECDH).
For both methods you will need an up-to-date LuaSec (not 0.4.x).
Out of both methods, ECDH is the newest and fastest. Prosody enables support for this by default when your versions of LuaSec and OpenSSL support it.
Standard DH is more widely implemented, mathematically simpler (a good thing in security), and has been around longer than ECDH. However it decreases performance and requires you to generate some parameters before you can use it. It is disabled by default in Prosody, but you can enable it by generating a DH parameters file and setting the 'ssl' dhparam option in your config.