9. Security

• Imporant Security Warning
• Authenticating using Connection Validators
• Customizing authentication using custom Validator
• Name Server security plugins
• Mobile objects and Code Validators
• Firewalls
• The pickle trojan security problem, and XML pickling
• SSL (secure socket layer) support

Imporant Security Warning

Authenticating using Connection Validators

The default validator already supports passphrase protection as authentication validation. This means that a client that wants to connect to your Pyro server needs to supply a valid authentication passphrase, or the connection is denied. The check takes place automatically (it is performed by the default connection validator), at connect time. The following items are important:

• Unless you tell Pyro otherwise, no authentication check is done. Clients can connect anonymously, unless you deny access based on max. connections or client host address.
• No passphrases are stored by Pyro, not on disk, not in memory. Pyro uses secure hmac-md5 hashes to compare passphrases.
• Server sends an authentication challenge to the client that must be used to generate the actual auth ident (digest of passphrase+challenge). Because the challenge changes for each connection attempt, and thus the required auth ident changes, this scheme is safe against eavesdroppers (auth idents are not reusable).
• Once a client is accepted and connected, no further checks are done.
• The same passphrases are valid for all objects connected to a certain Pyro Daemon.
• Clients can use a different passphrase for each object.
• There is no config item to specify the passphrase (because this would be very insecure). Code your own passphrase requester/supplier.

Look at the "denyhosts", "authenticate" and "user_passwd_auth" examples to see how you can use the connection validators.

How to use the default connection Validator

To enable passphrase authentication, you must tell the Pyro Daemon a list of accepted passphrases. Do this by calling the setAllowedIdentifications(ids) method of the daemon, where ids is a list of passphrases (strings). If you use None for this, the authentication is disabled again. Note that the ID list is a shared resource and that you will have to use thread locking if you change it from different threads. To specify for your client what passphrase to use for a specific object, call the proxy._setIdentification(id) method of the Pyro proxy, where id is your passphrase (string). Use Null to disable authentication again. Call the method right after you obtained the proxy using getProxyForURI or whatever.

If a connection is denied, Pyro will raise a ConnectionDeniedError, otherwise the connection is granted and your client proxy can invoke any methods it likes, untill disconnected.

The default SSL connection Validator

Customizing authentication using custom Validator

Client side, on the Proxy:

_setNewConnectionValidator(validator)

Server side, on the Daemon:

setNewConnectionValidator(validator)

Client side, on the Proxy:

_setIdentification(ident)
Note: the ident that you provide doesn't have to be a single string (or passphrase). It can be any Python object you want, for instance a login/password tuple! It is passed unchanged into the connection validator (see below) that creates a protocol token from it.

Server side, on the Daemon:

setAllowedIdentifications(idents)

Below you see the meaning of the different methods that are used in the connection validator class (and that you can override in your custom validator):

class MyCustomValidator(Pyro.protocol.DefaultConnValidator):

...has to be inherited...

def __init__(self):

Pyro.protocol.DefaultConnValidator.__init__(self) ...required...

def acceptHost(self,daemon,connection):

...called first, to check the client's origin. Arguments are the current Pyro Daemon, and the connection object (Pyro.protocol.TCPConnection object). The client's socket address is in connection.addr. You can check the client's IP address for instance, to see if it is in a trusted range. The default implementation of this method checks if the number of active connections has not reached the limit. (Pyro.config.PYRO_MAXCONNECTIONS) See table below for return codes

def acceptIdentification(self, daemon, connection, token, challenge):

...called to verify the client's identification token (check if the client supplied a correct authentication passphrase). The arguments are: daemon and connection same as above, client's token object (that was created by createAuthToken below), server challenge object that was sent to the client. The default implementation uses createAuthToken to create a secure hash of the auth id plus the challenge to compare that to the client's token. Effectively, it checks if the client-supplied hash is among the accepted passphrases of the daemon (hash of passphrase+challenge) -- if any are specified, otherwise it is just accepted. See table below for return codes. NOTE: you can use the connection argument to store the authentication token (which might be a username). A Pyro object may access this again by getting the connection object self.getLocalStorage().caller and getting the authentication token from there.

def createAuthToken(self, authid, challenge, peeraddr, URI, daemon):

...called from both client (proxy) and server (daemon) to create a token that is used to validate the connection. The arguments are: identification string (comes from mungeIdent below), challenge from server, socket address of the other party, Pyro URI of the object that is to be accessed, current Pyro Daemon. When in the client (proxy), daemon is always None. When in the server (daemon), URI is always None. The default implementation returns a secure hmac-md5 hash of the ident string and the challenge.

def createAuthChallenge(self, tcpserver, conn):

...called in the server (daemon) when a new connection comes in. It must return a challenge string that is to be sent to the client, to be used in creating the authentication token. By default it returns a secure hash of server IP, process ID, timestamp and a random value. Currently it is required that the challenge string is exactly 16 bytes long! (a md5 hash is 16 bytes).

def mungeIdent(self, ident):

utility method to change a clear-text ident string into something that isn't easily recognised. By default it returns the secure hash of the ident string. This is used to store the authentication strings more securely (setAllowedIdentifications). The ident object that is passed is actually free to be what you want, for instance you could use obj._setIdentification( ("login", "password") ) to use a login/password tuple. You have to use a custom connection validator to handle this, of course.

def setAllowedIdentifications(self, ids):

To tell the Daemon what identification strings are valid (the allowed secure passphrases).

(the following only if you subclass from Pyro.protocol.BasicSSLValidator (for SSL connections):
def checkCertificate(self, cert):

...checks the SSL certificate. The client's SLL certificate is passed as an argument. Note: this method is called from the acceptHost method, so you must leave that one as-is or call the BasicSSLValidator base class implementation of that method if you override it. See table below for return codes

Pyro will raise the appropriate ConnectionDeniedError on the client when you deny a new connection. On the server, you'll have to log the reason in the Pyro logfile yourself, if desired. When you accept a connection, the daemon will log an entry for you.

Name Server security plugins

• A validator for incoming broadcast server requests (such as 'what is your location' and 'shutdown'). You can now decide what or when such commands are accepted or denied, for instance, based on client IP address.
• A new connection validator for the NS server itself (which is a regular Pyro object, and this is the general validator mentioned earlier). With this you can implement very coarse access control to the NS, for instance, deny certain clients based on their IP address.

• BCGuard() function that returns a BC request validator object, or None.
• NSGuard() function that returns a NS new conn validator object, or None.
• A class that implements a BC request validator. This class must inherit from Pyro.naming.BCReqValidator. You must override the two methods that check for each command if it is allowed or if it is refused. These are acceptLocationCmd(self) and acceptShutdownCmd(self), and they return 0 or 1 (accept or deny). You can access self.addr to have the client's address (ip,port). You can call self.reply('message') to send a message back to the client. This may be polite, to let it know why you refused the command.
• A class that implements a NS new conn validator. See the documentation on connection validators, above, to see how you must implement this.

Mobile objects and Code Validators

This codeValidator is a function (or callable object) that takes three arguments: the name of the module, the code itself, and the address of the client (usually a (IP,port) tuple). It should return 0 or 1, for 'deny' and 'accept'. Pyro.core.ObjBase, the base class of all Pyro objects, has a setCodeValidator(v) method that you must call with your custom validator function (or callable object). You can set a different validator for each Pyro object that your server has.

The codeValidator is used for both directions; it checks if code is allowed from clients into the server, but also if code is allowed to be sent from the server to clients. In the first case, all three parameters have a value as mentioned above. In the second case (code from server to client), only the name has a value, the other two are None. For example, the code validator shown below is taken from the "agent2" example. It checks if incoming code is from the "agent.ShoppingAgent" module, and outgoing code is from the "objects" package:

def codeValidator(name,module,address):
        if module and address:
                return name=='agent.ShoppingAgent'              # client uploads to us
        else:
                return name.startswith("objects.")              # client downloads from us
   .
   .
   .
mall.setCodeValidator(codeValidator)

Notice that a client doesn't have a code validator. If you're using 2-way mobile code (you've enabled PYRO_MOBILE_CODE on the client), you will silently receive everything you need from the server. This is because the clients usually trust the server... otherwise they wouldn't be calling it, would they?

Firewalls

The pickle trojan security problem, and XML pickling

Security warning: possible trojan attack

Using the (safe) marshal module is no option for Pyro because we lose the ability to serialize user defined objects. But, if you accept a performance penalty of an order of a magnitude, and more required bandwith (2-4 times more), you can choose to use the safe XML pickling. To enable this, set the PYRO_XML_PICKLE config item to the appropriate value. You need to have the appropriate library installed otherwise Pyro won't start. The server will answer in XML pickled messages also, regardless of the server's PYRO_XML_PICKLE setting. So make sure that the correct XML packages are installed on both ends of the communication. If the server is configured to use PYRO_XML_PICKLE, it will only accept XML pickled requests! This means that if you set this option, your server is safe against pickling attacks.

Please note that at least since Python 2.2 a few pickle security flaws appear to have been removed, and the obvious trojan exploit with pickle no longer works on Python 2.2+. But still, do you trust pickle? ;-) Use PYRO_XML_PICKLE if you want to be safe.

If you decide to use the 'gnosis' XML Pickler, there is an additional config item to think about: PYRO_GNOSIS_PARANOIA. It sets the 'paranoia' level that will be used for the Gnosis XML pickler. Higher=more secure. The default setting (0) prevents automatic imports of modules during unpickling, because this is potentially unsafe. However, it creates problems when you are sending arbitrary user-defined types across the wire. The receiving side may not be able to fully reconstruct the data types that were sent. You could explicitly import the needed modules on the receiving side, or you could consider to set this config item to -1, which enables automatic imports of user defined modules in the Gnosis pickler. Note that setting it to a higer value than 0 breaks Pyro altogether because the pickler will operate in a too strict way. The only sensible values at this time are 0 and -1. When you want to use mobile code with Gnosis XML pickler, you need to set this to -1 as well. Note that you have to use the same Gnosis XML library version everywhere. You can't mix older versions with newer versions.

SSL (secure socket layer) support

• M2Crypto
• OpenSSL (needed by M2Crypto)

To start using SSL, you need to tell your Pyro daemon that it must use SSL instead of regular sockets. Do that by passing a prtcol parameter when you create a daemon, as follows:

daemon = Pyro.core.Daemon(prtcol='PYROSSL')

There are a few config items to tweak how Pyro uses SSL, where it can find your cert files, key files, and so on. Have a look in the Configuration chapter to see what they are (they all have 'SSL' in their name)

Note: it is very likely that you have to set PYRO_DNS_URI to True, because Pyro uses IP addresses by default while SSL certificates will usually contain hostnames. If you don't set this config item the SSL certificate will be rejected because the name won't match.

Please note that there is a known bug in M2Crypto that makes it impossible to use socket timeouts (socket.setdefaulttimeout) when using SSL.