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Grok security: more Grokkerdam sprint topics

In my previous article on the Grokkerdam sprint, I discussed various topics people may want to work on during the sprint. This blog entry is dedicated to some more topics, all security-related.

Easier authentication

Zope 3, and thus Grok, offers a flexible infrastructure for authentication of users and groups. It's actually very nice to have this flexibility: just a couple of weeks ago I could quite quickly write a bit of code that authenticates a user against a relational database table, for instance.

Unfortunately, this flexibility also has the usual price: it takes some time and effort to figure out how it works. It would be nice if Grok had some default authentication setup that was just there. It should let you manage users (in the ZODB) and let them log in. Then you can move on with the more important parts of your application, and you don't have to spend time building a user interface for user management when the basics of your application doesn't even work yet. The flexibility you need should still be there later, of course.

Some experiments towards improving the default authentication story for Grok have resulted in the LoginDemo and PlainLoginDemo demo applications, which are certainly worth studying.

Easier authorization

Authentication is one thing, authorization another. Zope offers a powerful security infrastructure. The model is based on permissions, and principals. For "principal" just read "user" (it can also be a group). The lowest level security check in Zope is just whether a principal has a permission on a certain object.

On top of this, a security policy is built. The security policy is pluggable, meaning you can write application-specific security politicies if you want. The default security policy of Zope 3 and Grok lets you assign permissions locally, that is, on a per-object basis, and offers other features, such as roles. It's also pluggable - to find out what permissions a principal has on an object it uses an adapter (IPrincipalPermissionMap), and you can plug in special adapters for your own objects, which is very handy sometimes.

Assigning roles and permissions is not particularly hard, and it's not very hard to check whether someone has a permission either. I think we could already improve the authorization story a lot if we simply documented best practice patterns on how one uses the default security policy in Grok. This would make a good sprint task.

Skin-based security

Grok offers a skin concept. You can put your views for an object in a namespace called a skin and then trigger the use of this namespace in the URL (++skin++foo), or in code. A skin presents all views available in a layer. Views can be associated with a layer, causing these views to be in those skins that use this layer. Layers are interfaces, and this means a layer can inherit from another one, or be composed from multiple smaller layers.

Skins allow you to build multiple UIs for your application. For a CMS application you could for instance have one (default) skin to present your public website, and another skin that lets you edit your content.

All this is already there in Grok. One thing we could get better at is skin-based security. It would nice if you can exactly control what views your web application exposes to the world. The default views that some Zope 3 package supply for various objects may not at all be what you want to expose to the world. In many cases, for security reasons, you only want to show the views that you defined in your application's code, and not expose any other URLs in your applications.

Skins may be able to help with this. If a skin used a layer that didn't inherit from any other layer (not even the default one), and you then tell your application to only let users access content in that skin, you are precisely controlling what views a user is able to accesss. At the sprint, it would be good if we could figure out how to make this straightforward with Grok. We need to experiment with this approach, and write some tests that check whether our assumptions are correct. We also need to make sure Grok doesn't break when we block all other views - some are probably essential for the operation of the application.

Model-based security

Zope 3 offers an extensive infrastructure for model-based security out of the box. You can specify in ZCML what permissions are needed to access which methods and attributes of an object. Zope then takes care at a low level that code running for a particular principal (triggered by the current web request) cannot access, or even see, those methods or attributes. It does this by wrapping objects in otherwise-transparent security proxies.

All this sounds like a great way to accomplish security by default in your application. You have a fine grained way to allow the user to access various methods and attributes. This should help you catch security problems in your application. You cannot actually call a method before you give it a permission, and in fact you might get an AttributeError if you try to access an attribute when you don't have permission to. You first need to write some ZCML assigning permissions to be able to do that.

Unfortunately all this is also a great way to make development less agile than it should be. It can slow you down quite a bit if you have to start writing security declarations right away, at the start of development. You then have to maintain them throughout the evolution of your application. Early on in development many developer typically don't really know yet what permissions would go with what methods. I don't even know what methods there are yet. Or what classes there are.

Since I also don't know exactly what permissions I'd want in my application yet, I'd be tempted to just declare everything public, so I can see something in my web browser. There goes the idea of security by default - if I forget to give something the right permission later on, everything in my application will appear to work correctly, except that people might have access to things they have no business to. The only thing I ended up doing is typing up a lot of ZCML security declarations just to make my application work. It's also not very nice to have to tell a beginner about all this when they just want to present "Hello world" on the screen.

Because you sometimes really want to be able to call a method and don't have time to figure out why the security proxies aren't letting you this time, or even if you know, don't have time to refactor the application so it is right and proper with security, people sometimes remove proxies manually in order to do this. Security by default once again actually may end up making you less secure.

Security-proxies spread around, a bit like acquisition wrappers in Zope 2. This probably sounds rather worrying to those who have worked with Zope 2 acquisition. With Zope 2 implicit acquisition, your objects suddenly have attributes that are not really theirs. With Zope 3 security proxies, your objects appear to lack attributes that the user you are logged in as doesn't have the permission for. Your object also lacks those attributes that you haven't actually declared the permission for yet. The declarations are off in some ZCML file, so it's not immediately obvious what has a permission and what is lacking. Also similar to acquisition is that, at least with the default security policy, an object without a parent connection is forbidden to be used at all. All this can cause quite a bit of frustration during development.

Finally, after all this, security proxies don't atually wrap everything: some common operations actually result in content-objects that aren't proxies. If you use a catalog index to look up objects you actually get unproxied objects back. Suddenly, no security checks are taking place at all. Just when you were used to them.

With Grok, we decided to get rid of security proxies entirely, much simplifying the life of a developer. Security proxies make for bad usability for developers. Instead, Grok offers security per view, not per model. You can say which permission a user needs to have in order to see a view. The full pluggable security model of Zope 3 still works, but if you aren't satisfied with the automatic view-level checks you will have to check permissions by hand in your code. This is much simpler to work with than Zope 3's security proxy model, and it's still likely more powerful than what just about any other web framework offers out of the box.

After all this negativity about security proxies, I might surprise you and put security proxies back on the table. Model-based security certainly has benefits - views can be left entirely public, but since they can't access any models that that the user doesn't have access to, your application remains safe. Some of the drawbacks of Zope 3's approach also have to do with ZCML - your security declarations are off in some other file away from your code. If we brought the security declarations closer to the code itself again, things will likely be clearer. Maybe we can also come up with ways to make the addition of model-based security more incremental in an application. Even if maintaining this stuff too early on during development is a major pain, it might be very valuable to have the ability to do model-based security later in the evolution of the application. At the sprint, perhaps we can think of ways to tame model-based security and add it to Grok.


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