# Security

Warning

Do not publish any Pyro objects to remote machines unless you’ve read and understood everything that is discussed in this chapter. This is also true when publishing Pyro objects with different credentials to other processes on the same machine. Why? In short: using Pyro has several security risks. Pyro has a few countermeasures to deal with them. Understanding the risks, the countermeasures, and their limits, is very important to avoid creating systems that are very easy to compromise by malicious entities.

## Network interface binding

By default Pyro binds every server on localhost, to avoid exposing things on a public network or over the internet by mistake. If you want to expose your Pyro objects to anything other than localhost, you have to explicitly tell Pyro the network interface address it should use. This means it is a conscious effort to expose Pyro objects to other machines.

It is possible to tell Pyro the interface address via an environment variable or global config item (`HOST`). In some situations - or if you’re paranoid - it is advisable to override this setting in your server program by setting the config item from within your own code, instead of depending on an externally configured setting.

## Running Pyro servers with different credentials/user id

The following is not a Pyro specific problem, but is important nonetheless: If you want to run your Pyro server as a different user id or with different credentials as regular users, be very careful what kind of Pyro objects you expose like this!

Treat this situation as if you’re exposing your server on the internet (even when it’s only running on localhost). Keep in mind that it is still possible that a random user on the same machine connects to the local server. You may need additional security measures to prevent random users from calling your Pyro objects.

## Secure communication via SSL/TLS

Pyro itself doesn’t encrypt the data it sends over the network. This means if you use the default configuration, you must never transfer sensitive data on untrusted networks (especially user data, passwords, and such) because eavesdropping is possible.

You can run Pyro over a secure network (VPN, ssl/ssh tunnel) where the encryption is taken care of externally. It is also possible however to enable SSL/TLS in Pyro itself, so that all communication is secured via this industry standard that provides encryption, authentication, and anti-tampering (message integrity).

Using SSL/TLS

Enable it by setting the `SSL` config item to True, and configure the other SSL config items as required. You’ll need to specify the cert files to use, private keys, and passwords if any. By default, the SSL mode only has a cert on the server (which is similar to visiting a https url in your browser). This means your clients can be sure that they are connecting to the expected server, but the server has no way to know what clients are connecting. You can solve this using SSL and custom certificate verification. You can do this in your client (checks the server’s cert) but you can also tell your clients to use certs as well and check these in your server. This makes it 2-way-SSL or mutual authentication. For more details see here …by using 2-way-SSL and certificate verificiation. The SSL config items are in Overview of Config Items.

For example code on how to set up a 2-way-SSL Pyro client and server, with cert verification, see the ssl example .

## Dotted names (object traversal)

Using “dotted names” to traverse attributes on Pyro proxies (like `proxy.aaa.bbb.ccc()`) is not possible. because that is a security vulnerability (for similar reasons as described here https://legacy.python.org/news/security/PSF-2005-001/ ).

If you require access to a nested attribute, you’ll have to explicitly add a method or attribute on the proxy itself to access it directly.

## Environment variables overriding config items

Almost all config items can be overwritten by an environment variable. If you can’t trust the environment in which your script is running, it may be a good idea to reset the config items to their default builtin values, without using any environment variables. See Configuring Pyro for the proper way to do this.

## Preventing arbitrary connections

### …by using 2-way-SSL and certificate verificiation

When using SSL, you should also do some custom certificate verification, such as checking the serial number and commonName. This way your code is not only certain that the communication is encrypted, but also that it is talking to the intended party and nobody else (middleman). The server hostname and cert expiration dates are checked automatically, but other attributes you have to verify yourself.

This is fairly easy to do: you can use Connection handshake for this. You can then get the peer certificate using `Pyro5.socketutil.SocketConnection.getpeercert()`.

If you configure a client cert as well as a server cert, you can/should also do verification of client certificates in your server. This is a good way to be absolutely certain that you only allow clients that you know and trust, because you can check the required unique certificate attributes.

Having certs on both client and server is called 2-way-SSL or mutual authentication.

It’s a bit too involved to fully describe here but it not much harder than the basic SSL configuration described earlier. You just have to make sure you supply a client certificate and that the server requires a client certificate (and verifies some properties of it). The ssl example shows how to do all this.