Security

Introduction

A typical web application needs to be securely managed. Different types of users need different kinds of access to the components that make up an application. To this end, Zope includes a comprehensive set of security features. This chapter’s goal is to shed light on Zope security in the context of Zope Product development. For a more fundamental overview of Zope security, you may wish to refer to the Zope Book, Chapter 6, Users and Security. Before diving into this chapter, you should have a basic understanding of how to build Zope Products as well as an understanding of how the Zope object publisher works. These topics are covered in Chapter 2 and Chapter 3, respectively.

Security Architecture

The Zope security architecture is built around a security policy, which you can think of as the “access control philosophy” of Zope. This policy arbitrates the decisions Zope makes about whether to allow or deny access to any particular object defined within the system.

How The Security Policy Relates to Zope’s Publishing Machinery

When access to Zope is performed via HTTP, WebDAV, or FTP, Zope’s publishing machinery consults the security policy in order to determine whether to allow or deny access to a visitor for a particular object. For example, when a user visits the root ‘index_html’ object of your site via HTTP, the security policy is consulted by ‘ZPublisher’ to determine whether the user has permission to view the ‘index_html’ object itself. For more information on this topic, see Chapter 3, “Object Publishing”.

The Zope security policy is consulted when an object is accessed the publishing machinery, for example when a web request is submitted.

How The Security Policy Relates to Restricted Code

Restricted code is generally any sort of logic that may be edited remotely (through the Web, FTP, via WebDAV or by other means). DTML Methods, SQLMethods, Python Scripts and Perl Scripts are examples of restricted code.

When restricted code runs, any access to objects integrated with Zope security is arbitrated by the security policy. For example if you write a bit of restricted code with a line that attempts to manipulate an object you don’t have sufficient permission to use, the security policy will deny you access to the object. This generally is accomplished by raising an ‘Unauthorized’ exception, which is a Python string exception caught by a User Folder which signifies that Zope should attempt to get user credentials before obeying the request. The particular code used to attempt to obtain the credentials is determined by the User Folder “closest” (folder-wise) to the object being accessed.

The Zope security policy is consulted when an object is accessed by restricted code.

‘Unauthorized’ Exceptions and Through-The-Web Code

The security policy infrastructure will raise an ‘Unauthorized’ exception automatically when access to an object is denied. When an ‘Unauthorized’ exception is raised within Zope, it is handled in a sane way by Zope, generally by having the User Folder prompt the user for login information. Using this functionality, it’s possible to protect Zope objects through access control, only prompting the user for authentication when it is necessary to perform an action which requires privilege.

An example of this behavior can be witnessed within the Zope Management interface itself. The management interface prompts you to log in when visiting, for example, the ‘/manage’ method of any Zope object. This is due to the fact that an anonymous user does not generally possess the proper credentials to use the management interface. If you’re using Zope in the default configuration with the default User Folder, it prompts you to provide login information via an HTTP basic authentication dialog.

How The Security Policy Relates To Unrestricted Code

There are also types of unrestricted code in Zope, where the logic is not constrained by the security policy. Examples of unrestricted code are the methods of Python classes that implement the objects in Python file-based add-on components. Another example of unrestricted code can be found in External Method objects, which are defined in files on the filesystem. These sorts of code are allowed to run “unrestricted” because access to the file system is required to define such logic. Zope assumes that code defined on the filesystem is “trusted”, while code defined “through the web” is not. All filesystem-based code in Zope is unrestricted code.

We’ll see later that while the security policy does not constrain what your unrestricted code does, it can and should be used to control the ability to call your unrestricted code from within a restricted-code environment.

The Zope security policy is not consulted when unrestricted code is run.

Details Of The Default Zope Security Policy

In short, the default Zope security policy ensures the following:

• access to an object which does not have any associated security information is always denied.
• if an object is associated with a permission, access is granted or denied based on the user’s roles. If a user has a role which has been granted the permission in question, access is granted. If the user does not possess a role that has been granted the permission in question, access is denied.
• if the object has a security assertion declaring it public , then access will be granted.
• if the object has a security assertion declaring it private, then access will be denied.
• accesses to objects that have names beginning with the underscore character ‘_’ are always denied.

As we delve further into Zope security within this chapter, we’ll see exactly what it means to associate security information with an object.

Overview Of Using Zope Security Within Your Product

Of course, now that we know what the Zope security policy is, we need to know how our Product can make use of it. Zope developers leverage the Zope security policy primarily by making security declarations related to methods and objects within their Products. Using security assertions, developers may deny or allow all types of access to a particular object or method unilaterally, or they may protect access to Zope objects more granularly by using permissions to grant or deny access based on the roles of the requesting user to the same objects or methods.

For a more fundamental overview of Zope users, roles, and permissions, see the section titled “Authorization and Managing Security” in the Security Chapter of the Zope Book.

Security Declarations In Zope Products

Zope security declarations allow developers to make security assertions about a Product-defined object and its methods. Security declarations come in three basic forms. These are:

• public – allow anybody to access the protected object or method
• private – deny anyone access to the protected object or method
• protected – protect access to the object or method via a permission

We’ll see how to actually “spell” these security assertions a little later in this chapter. In the meantime, just know that security declarations are fundamental to Zope Product security, and they can be used to protect access to an object by associating it with a permission. We will refer to security declarations as “declarations” and “assertions” interchangeably within this chapter.

Permissions In Zope Products

A permission is the smallest unit of access to an object in Zope, roughly equivalent to the atomic permissions on files seen in Windows NT or UNIX: R (Read), W(Write), X(Execute), etc. However, unlike these types of mnemonic permissions shared by all sorts of different file types in an operating system product, in Zope, a permission usually describes a fine-grained logical operation which takes place upon an object, such as “View Management Screens” or “Add Properties”.

Zope administrators associate these permissions with roles, which they grant to Zope users. Thus, declaring a protection assertion on a method of “View management screens” ensures that only users who possess roles which have been granted the “View management screens” permission are able to perform the action that the method defines.

It is important to note that Zope’s security architecture dictates that roles and users remain the domain of administrators, while permissions remain the domain of developers. Developers of Products should not attempt to define roles or users, although they may (and usually must) define permissions. Most importantly, a Zope administrator who makes use of your product should have the “last word” as regards which roles are granted which permissions, allowing her to protect her site in a manner that fits her business goals.

Permission names are strings, and these strings are currently arbitrary. There is no permission hierarchy, or list of “approved permissions”. Developers are encouraged to reuse Zope core permissions (e.g. “View”, “Access contents information”) when appropriate, or they may create their own as the need arises. It is generally wise to reuse existing Zope permission names unless you specifically need to define your own. For a list of existing Zope core permissions, see Appendix A, “Zope Core Permissions”.

Permissions are often tied to method declarations in Zope. Any number of method declarations may share the same permission. It’s useful to declare the same permission on a set of methods which can logically be grouped together. For example, two methods which return management forms for the object can be provided with the same permission, “View management screens”. Likewise, two entirely different objects can share a permission name to denote that the operation that’s being protected is fundamentally similar. For instance, most Product-defined objects reuse the Zope “View” permission, because most Zope objects need to be viewed in a web browser. If you create an addable Zope class named ‘MyObject’, it doesn’t make much sense to create a permission “View MyObject”, because the generic “View” permission may be reused for this action.

There is an exception to the “developers should not try to define roles” rule inasmuch as Zope allows developers to assign “default roles” to a permission. This is primarily for the convenience of the Zope administrator, as default roles for a permission cause the Zope security machinery to provide a permission to a role by default when instances of a Product class are encountered during security operations. For example, if your Product defines a permission “Add Poll Objects”, this permission may be associated with a set of default roles, perhaps “Manager”. Default roles in Products should not be used against roles other than “Manager”, “Anonymous”, “Owner”, and “Authenticated” (the four default Zope roles), as other roles are not guaranteed to exist in every Zope installation.

Using security assertions in Zope is roughly analogous to assigning permission bit settings and ownership information to files in a UNIX or Windows filesystem. Protecting objects via permissions allows developers and administrators to secure Zope objects independently of statements made in application code.

Implementing Security In Python Products

Security Assertions

You may make several kinds of security assertions at the Python level. You do this to declare accessibility of methods and subobjects of your classes. Three of the most common assertions that you’ll want to make on your objects are:

• this object is public (always accessible)
• this object is private (not accessible by restricted code or by URL traversal)
• this object is protected by a specific permission

There are a few other kinds of security assertions that are much less frequently used but may be needed in some cases:

• asserting that access to subobjects that do not have explicit security information should be allowed rather than denied.
• asserting what sort of protection should be used when determining access to an object itself rather than a particular method of the object

It is important to understand that security assertions made in your Product code do not limit the ability of the code that the assertion protects. Assertions only protect access to this code. The code which constitutes the body of a protected, private, or public method of a class defined in a Zope disk-based Product runs completely unrestricted, and is not subject to security constraints of any kind within Zope. An exception to this rule occurs when disk-based-Product code calls a “through the web” method such as a Python Script or a DTML Method. In this case, the security constraints imposed by these objects respective to the current request are obeyed.

When Should I Use Security Assertions?

If you are building an object that will be used from DTML or other restricted code, or that will be accessible directly through the web (or other remote protocols such as FTP or WebDAV) then you need to define security information for your object.

Making Security Assertions

As a Python developer, you make security assertions in your Python classes using ‘SecurityInfo’ objects. A ‘SecurityInfo’ object provides the interface for making security assertions about an object in Zope.

The convention of placing security declarations inside Python code may at first seem a little strange if you’re used to “plain old Python” which has no notion at all of security declarations. But because Zope provides the ability to make these security assertions at such a low level, the feature is ubiquitous throughout Zope, making it easy to make these declarations once in your code, usable site-wide without much effort.

Class Security Assertions

The most common kind of ‘SecurityInfo’ you will use as a component developer is the ‘ClassSecurityInfo’ object. You use ‘ClassSecurityInfo’ objects to make security assertions about methods on your classes.

Classes that need security assertions are any classes that define methods that can be called “through the web”. This means any methods that can be called directly with URL traversal, from DTML Methods, or from Python-based Script objects.

Declaring Class Security

When writing the classes in your product, you create a ‘ClassSecurityInfo’ instance within each class that needs to play with the security model. You then use the ‘ClassSecurityInfo’ object to make assertions about your class, its subobjects and its methods.

The ‘ClassSecurityInfo’ class is defined in the ‘AccessControl’ package of the Zope framework. To declare class security information create a ‘ClassSecurityInfo’ class attribute named ‘security’. The name ‘security’ is used for consistency and for the benefit of new component authors, who often learn from looking at other people’s code. You do not have to use the name ‘security’ for the security infrastructure to recognize your assertion information, but it is recommended as a convention. For example:

from AccessControl import ClassSecurityInfo

class Mailbox(ObjectManager):
  """A mailbox object that contains mail message objects."""

  # Create a SecurityInfo for this class. We will use this
  # in the rest of our class definition to make security
  # assertions.
  security = ClassSecurityInfo()

  # Here is an example of a security assertion. We are
  # declaring that access to messageCount is public.
  security.declarePublic('messageCount')

  def messageCount(self):
    """Return a count of messages."""
    return len(self._messages)

Note that in the example above we called the ‘declarePublic’ method of the ‘ClassSecurityInfo’ instance to declare that access to the ‘messageCount’ method be public. To make security assertions for your object, you just call the appropriate methods of the ‘ClassSecurityInfo’ object, passing the appropriate information for the assertion you are making.

‘ClassSecurityInfo’ approach has a number of benefits. A major benefit is that it is very explicit, it allows your security assertions to appear in your code near the objects they protect, which makes it easier to assess the state of protection of your code at a glance. The ‘ClassSecurityInfo’ interface also allows you as a component developer to ignore the implementation details in the security infrastructure and protects you from future changes in those implementation details.

Let’s expand on the example above and see how to make the most common security assertions using the ‘SecurityInfo’ interface.

To assert that a method is public (anyone may call it) you may call the ‘declarePublic’ method of the ‘SecurityInfo’ object, passing the name of the method or subobject that you are making the assertion on:

security.declarePublic(methodName)

To assert that a method is private you call the ‘declarePrivate’ method of the ‘SecurityInfo’ object, passing the name of the method or subobject that you are making the assertion on:

security.declarePrivate(methodName)

To assert that a method or subobject is protected by a particular permission, you call the ‘declareProtected’ method of the ‘SecurityInfo’ object, passing a permission name and the name of a method to be protected by that permission:

security.declareProtected(permissionName, methodName)

If you have lots of methods you want to protect under the same permission, you can pass as many methodNames ase you want:

security.declareProtected(permissionName, methodName1,
methodName2, methodName3, ...)

Passing multiple names like this works for all of the ‘declare’ security methods (‘declarePublic’, ‘declarePrivate’, and ‘declareProtected’).

Deciding To Use ‘declareProtected’ vs. ‘declarePublic’ or ‘declarePrivate’

If the method you’re making the security declaration against is innocuous, and you’re confident that its execution will not disclose private information nor make inappropriate changes to system state, you should declare the method public.

If a method should never be run under any circumstances via traversal or via through-the-web code, the method should be declared private. This is the default if a method has no security assertion, so you needn’t explicitly protect unprotected methods unless you’ve used ‘setDefaultAccess’ to set the object’s default access policy to ‘allow’ (detailed in Other Assertions, below).

If the method should only be executable by a certain class of users, you should declare the method protected.

A Class Security Example

Let’s look at an expanded version of our ‘Mailbox’ example that makes use of each of these types of security assertions:

from AccessControl import ClassSecurityInfo
import Globals

class Mailbox(ObjectManager):
  """A mailbox object."""

# Create a SecurityInfo for this class
  security = ClassSecurityInfo()

  security.declareProtected('View management screens', 'manage')
  manage=HTMLFile('mailbox_manage', globals())

  security.declarePublic('messageCount')
  def messageCount(self):
    """Return a count of messages."""
    return len(self._messages)

  # protect 'listMessages' with the 'View Mailbox' permission
  security.declareProtected('View Mailbox', 'listMessages')

  def listMessages(self):
    """Return a sequence of message objects."""
    return self._messages[:]

  security.declarePrivate('getMessages')
  def getMessages(self):
    self._messages=GoGetEm()
    return self._messages

# call this to initialize framework classes, which
# does the right thing with the security assertions.
Globals.InitializeClass(Mailbox)

Note the last line in the example. In order for security assertions to be correctly applied to your class, you must call the global class initializer (‘Globals.InitializeClass’) for all classes that have security information. This is very important - the global initializer does the “dirty work” required to ensure that your object is protected correctly based on the security assertions that you have made. If you don’t run it on the classes that you’ve protected with security assertions, the security assertions will not be effective.

Deciding Permission Names For Protected Methods

When possible, you should make use of an existing Zope permission within a ‘declareProtected’ assertion. A list of the permissions which are available in a default Zope installation is available within Appendix A. When it’s not possible to reuse an existing permission, you should choose a permission name which is a verb or a verb phrase.

Object Assertions

Often you will also want to make a security assertion on the object itself. This is important for cases where your objects may be accessed in a restricted environment such as DTML. Consider the example DTML code:

<dtml-var expr="some_method(someObject)">

Here we are trying to call ‘some_method’, passing the object ‘someObject’. When this is evaluated in the restricted DTML environment, the security policy will attempt to validate access to both ‘some_method’ and ‘someObject’. We’ve seen how to make assertions on methods - but in the case of ‘someObject’ we are not trying to access any particular method, but rather the object itself (to pass it to ‘some_method’). Because the security machinery will try to validate access to ‘someObject’, we need a way to let the security machinery know how to handle access to the object itself in addition to protecting its methods.

To make security assertions that apply to the object itself you call methods on the ‘SecurityInfo’ object that are analogous to the three that we have already seen:

security.declareObjectPublic()

security.declareObjectPrivate()

security.declareObjectProtected(permissionName)

The meaning of these methods is the same as for the method variety, except that the assertion is made on the object itself.

An Object Assertion Example

Here is the updated ‘Mailbox’ example, with the addition of a security assertion that protects access to the object itself with the ‘View Mailbox’ permission:

from AccessControl import ClassSecurityInfo
import Globals

class Mailbox(ObjectManager):
  """A mailbox object."""

  # Create a SecurityInfo for this class
  security = ClassSecurityInfo()

  # Set security for the object itself
  security.declareObjectProtected('View Mailbox')

  security.declareProtected('View management screens', 'manage')
  manage=HTMLFile('mailbox_manage', globals())

  security.declarePublic('messageCount')
  def messageCount(self):
    """Return a count of messages."""
    return len(self._messages)

  # protect 'listMessages' with the 'View Mailbox' permission
  security.declareProtected('View Mailbox', 'listMessages')

  def listMessages(self):
    """Return a sequence of message objects."""
    return self._messages[:]

  security.declarePrivate('getMessages')
  def getMessages(self):
    self._messages=GoGetEm()
    return self._messages

# call this to initialize framework classes, which
# does the right thing with the security assertions.
Globals.InitializeClass(Mailbox)

Other Assertions

The SecurityInfo interface also supports the less common security assertions noted earlier in this document.

To assert that access to subobjects that do not have explicit security information should be allowed rather than denied by the security policy, use:

security.setDefaultAccess("allow")

This assertion should be used with caution. It will effectively change the access policy to “allow-by-default” for all attributes in your object instance (not just class attributes) that are not protected by explicit assertions. By default, the Zope security policy flatly denies access to attributes and methods which are not mentioned within a security assertion. Setting the default access of an object to “allow” effectively reverses this policy, allowing access to all attributes and methods which are not explicitly protected by a security assertion.

‘setDefaultAccess’ applies to attributes that are simple Python types as well as methods without explicit protection. This is important because some mutable Python types (lists, dicts) can then be modified by restricted code. Setting default access to “allow” also affects attributes that may be defined by the base classes of your class, which can lead to security holes if you are not sure that the attributes of your base classes are safe to access.

Setting the default access to “allow” should only be done if you are sure that all of the attributes of your object are safe to access, since the current architecture does not support using explicit security assertions on non-method attributes.

What Happens When You Make A Mistake Making ‘SecurityInfo’ Declarations?

It’s possible that you will make a mistake when making ‘SecurityInfo’ declarations. For example, it is not legal to declare two conflicting permissions on a method:

class Foo(SimpleItem):
    security = ClassSecurityInfo()

    meta_type='Foo'

    security.declareProtected('View foos', 'index_html')
    def index_html(self):
        """ make index_html web-publishable """
        return "<html><body>hi!</body></html>"

security.declareProtected(‘View’, ‘index_html’) # whoops, declared a conflicting permission on index_html!

When you make a mistake like this, the security machinery will accept the first declaration made in the code and will write an error to the Zope debug log upon encountering the second and following conflicting declarations during class initialization. It’s similarly illegal to declare a method both private and public, or to declare a method both private and protected, or to declare a method both public and protected. A similar error will be raised in all of these cases.

Note that Zope will not warn you if you misspell the name of a method in a declareProtected, declarePublic, or declarePrivate assertion. For instance, you try to protect the ‘index_html’ method with the ‘View’ permission and make a mistake, spelling the name ‘index_html’ as ‘inde_html’, like so:

security.declareProtected('View', 'inde_html')
# whoops, declared a permission assertion for 'inde_html'
# when I really wanted it to be 'index_html'!
def index_html(self):
    """ make index_html web-publishable """
    return "<html><body>hi!</body></html>"

You’ll need to track down these kinds of problems yourself.

Setting Default Roles For Permissions

When defining operations that are protected by permissions, one thing you commonly want to do is to arrange for certain roles to be associated with a particular permission by default for instances of your object.

For example, say you are creating a News Item object. You want ‘Anonymous’ users to have the ability to view news items by default; you don’t want the site manager to have to explicitly change the security settings for each News Item just to give the ‘Anonymous” role ‘View’ permission.

What you want as a programmer is a way to specify that certain roles should have certain permissions by default on instances of your object, so that your objects have sensible and useful security settings at the time they are created. Site managers can always change those settings if they need to, but you can make life easier for the site manager by setting up defaults that cover the common case by default.

As we saw earlier, the ‘SecurityInfo’ interface provided a way to associate methods with permissions. It also provides a way to associate a permission with a set of default roles that should have that permission on instances of your object.

To associate a permission with one or more roles, use the following:

security.setPermissionDefault(permissionName, rolesList)

The permissionName argument should be the name of a permission that you have used in your object and rolesList should be a sequence (tuple or list) of role names that should be associated with permissionName by default on instances of your object.

Note that it is not always necessary to use this method. All permissions for which you did not set defaults using ‘setPermissionDefault’ are assumed to have a single default role of ‘Manager’. Notable exceptions to this rule include ‘View’ and ‘Access contents information’, which always have the default roles ‘Manager’ and ‘Anonymous’.

The ‘setPermissionDefault’ method of the ‘SecurityInfo’ object should be called only once for any given permission name.

An Example of Associating Default Roles With Permissions

Here is our ‘Mailbox’ example, updated to associate the ‘View Mailbox’ permission with the roles ‘Manager’ and ‘Mailbox Owner’ by default:

from AccessControl import ClassSecurityInfo
import Globals

class Mailbox(ObjectManager):
  """A mailbox object."""

  # Create a SecurityInfo for this class
  security = ClassSecurityInfo()

  # Set security for the object itself
  security.declareObjectProtected('View Mailbox')

  security.declareProtected('View management screens', 'manage')
  manage=DTMLFile('mailbox_manage', globals())

  security.declarePublic('messageCount')
  def messageCount(self):
    """Return a count of messages."""
    return len(self._messages)

  security.declareProtected('View Mailbox', 'listMessages')
  def listMessages(self):
    """Return a sequence of message objects."""
    return self._messages[:]

  security.setPermissionDefault('View Mailbox', ('Manager', 'Mailbox Owner'))

# call this to initialize framework classes, which
# does the right thing with the security assertions.
Globals.InitializeClass(Mailbox)

What Happens When You Make A Mistake Declaring Default Roles?

It’s possible that you will make a mistake when making default roles declarations. For example, it is not legal to declare two conflicting default roles for a permission:

class Foo(SimpleItem):
    security = ClassSecurityInfo()

    meta_type='Foo'

    security.declareProtected('View foos', 'index_html')
    def index_html(self):
        """ """
        return "<html><body>hi!</body></html>"

    security.setPermissionDefault('View foos', ('Manager',))

    security.setPermissionDefault('View foos', ('Anonymous',))
    # whoops, conflicting permission defaults!

When you make a mistake like this, the security machinery will accept the first declaration made in the code and will write an error to the Zope debug log about the second and following conflicting declarations upon class initialization.

What Can (And Cannot) Be Protected By Class Security Info?

It is important to note what can and cannot be protected using the ‘ClassSecurityInfo’ interface. First, the security policy relies on Acquisition to aggregate access control information, so any class that needs to work in the security policy must have either ‘Acquisition.Implicit’ or ‘Acquisition.Explicit’ in its base class hierarchy.

The current security policy supports protection of methods and protection of subobjects that are instances. It does not currently support protection of simple attributes of basic Python types (strings, ints, lists, dictionaries). For instance:

from AccessControl import ClassSecurityInfo
import Globals

# We subclass ObjectManager, which has Acquisition in its
# base class hierarchy, so we can use SecurityInfo.

class MyClass(ObjectManager):
  """example class"""

  # Create a SecurityInfo for this class
  security = ClassSecurityInfo()

  # Set security for the object itself
  security.declareObjectProtected('View')

  # This is ok, because subObject is an instance
  security.declareProtected('View management screens', 'subObject')
  subObject=MySubObject()

  # This is ok, because sayHello is a method
  security.declarePublic('sayHello')
  def sayHello(self):
    """Return a greeting."""
    return "hello!"

  # This will not work, because foobar is not a method
  # or an instance - it is a standard Python type
  security.declarePublic('foobar')
  foobar='some string'

Keep this in mind when designing your classes. If you need simple attributes of your objects to be accessible (say via DTML), then you need to use the ‘setDefaultAccess’ method of ‘SecurityInfo’ in your class to allow this (see the note above about the security implications of this). In general, it is always best to expose the functionality of your objects through methods rather than exposing attributes directly.

Note also that the actual ‘ClassSecurityInfo’ instance you use to make security assertions is implemented such that it is never accessible from restricted code or through the Web (no action on the part of the programmer is required to protect it).

Inheritance And Class Security Declarations

Python inheritance can prove confusing in the face of security declarations.

If a base class which has already been run through “InitializeClass” is inherited by a superclass, nothing special needs to be done to protect the base class’ methods within the superclass unless you wish to modify the declarations made in the base class. The security declarations “filter down” into the superclass.

On the other hand, if a base class hasn’t been run through the global class initializer (‘InitializeClass’), you need to proxy its security declarations in the superclass if you wish to access any of its methods within through-the-web code or via URL traversal.

In other words, security declarations that you make using ‘ClassSecurityInfo’ objects effect instances of the class upon which you make the declaration. You only need to make security declarations for the methods and subobjects that your class actually defines. If your class inherits from other classes, the methods of the base classes are protected by the security declarations made in the base classes themselves. The only time you would need to make a security declaration about an object defined by a base class is if you needed to redefine the security information in a base class for instances of your own class. An example below redefines a security assertion in a subclass:

from AccessControl import ClassSecurityInfo
import Globals

class MailboxBase(ObjectManager):
  """A mailbox base class."""

  # Create a SecurityInfo for this class
  security = ClassSecurityInfo()

  security.declareProtected('View Mailbox', 'listMessages')
  def listMessages(self):
    """Return a sequence of message objects."""
    return self._messages[:]

  security.setPermissionDefault('View Mailbox', ('Manager', 'Mailbox Owner'))

Globals.InitializeClass(MailboxBase)

class MyMailbox(MailboxBase):
  """A mailbox subclass, where we want the security for
    listMessages to be public instead of protected (as
    defined in the base class)."""

  # Create a SecurityInfo for this class
  security = ClassSecurityInfo()

  security.declarePublic('listMessages')

Globals.InitializeClass(MyMailbox)

Class Security Assertions In Non-Product Code (External Methods/Python Scripts)

Objects that are returned from Python Scripts or External Methods need to have assertions declared for themselves before they can be used in restricted code. For example, assume you have an External Method that returns instances of a custom ‘Book’ class. If you want to call this External Method from DTML, and you’d like your DTML to be able to use the returned ‘Book’ instances, you will need to ensure that your class supports Acquisition, and you’ll need to make security assertions on the ‘Book’ class and initialize it with the global class initializer (just as you would with a class defined in a Product). For example:

# an external method that returns Book instances

from AccessControl import ClassSecurityInfo
from Acquisition import Implicit
import Globals

class Book(Implicit):

  def __init__(self, title):
    self._title=title

  # Create a SecurityInfo for this class
  security = ClassSecurityInfo()
  security.declareObjectPublic()

  security.declarePublic('getTitle')
  def getTitle(self):
    return self._title

Globals.InitializeClass(Book)

# The actual external method
def GetBooks(self):
  books=[]
  books.append(Book('King Lear').__of__(self))
  books.append(Book('Romeo and Juliet').__of__(self))
  books.append(Book('The Tempest').__of__(self))
  return books

Note that we wrap the book instances by way of their __of__ methods to obtain a security context before returning them.

Note that this particular example is slightly dangerous. You need to be careful that classes defined in external methods not be made persistent, as this can cause Zope object database inconsistencies. In terms of this example, this would mean that you would need to be careful to not attach the Book object returned from the ‘GetBook’ method to a persistent object within the ZODB. See Chapter 4, “ZODB Persistent Components” for more information. Thus it’s generally a good idea to define the Book class in a Product if you want books to be persistent. It’s also less confusing to have all of your security declarations in Products.

However, one benefit of the ‘SecurityInfo’ approach is that it is relatively easy to subclass and add security info to classes that you did not write. For example, in an External Method, you may want to return instances of ‘Book’ although ‘Book’ is defined in another module out of your direct control. You can still use ‘SecurityInfo’ to define security information for the class by using:

# an external method that returns Book instances

from AccessControl import ClassSecurityInfo
from Acquisition import Implicit
import bookstuff
import Globals

class Book(Implicit, bookstuff.Book):
  security = ClassSecurityInfo()
  security.declareObjectPublic()
  security.declarePublic('getTitle')

Globals.InitializeClass(Book)

# The actual external method
def GetBooks(self):
  books=[]
  books.append(Book('King Lear'))
  books.append(Book('Romeo and Juliet'))
  books.append(Book('The Tempest'))
  return books

Module Security Assertions

Another kind of ‘SecurityInfo’ object you will use as a component developer is the ‘ModuleSecurityInfo’ object.

‘ModuleSecurityInfo’ objects do for objects defined in modules what ‘ClassSecurityInfo’ objects do for methods defined in classes. They allow module-level objects (generally functions) to be protected by security assertions. This is most useful when attempting to allow through-the-web code to ‘import’ objects defined in a Python module.

One major difference between ‘ModuleSecurityInfo’ objects and ClassSecurityInfo objects is that ‘ModuleSecurityInfo’ objects cannot be declared ‘protected’ by a permission. Instead, ModuleSecurityInfo objects may only declare that an object is ‘public’ or ‘private’. This is due to the fact that modules are essentially “placeless”, global things, while permission protection depends heavily on “place” within Zope.

Declaring Module Security

In order to use a filesystem Python module from restricted code such as Python Scripts, the module must have Zope security declarations associated with functions within it. There are a number of ways to make these declarations:

• By embedding the security declarations in the target module. A module that is written specifically for Zope may do so, whereas a module not specifically written for Zope may not be able to do so.
• By creating a wrapper module and embedding security declarations within it. In many cases it is difficult, impossible, or simply undesirable to edit the target module. If the number of objects in the module that you want to protect or make public is small, you may wish to simply create a wrapper module. The wrapper module imports objects from the wrapped module and provides security declarations for them.
• By placing security declarations in a filesystem Product. Filesystem Python code, such as the ‘__init__.py’ of a Product, can make security declarations on behalf of an external module. This is also known as an “external” module security info declaration.

The ‘ModuleSecurityInfo’ class is defined in the ‘AccessControl’ package of the Zope framework.

Using ModuleSecurityInfo Objects

Instances of ‘ModuleSecurityInfo’ are used in two different situations. In embedded declarations, inside the module they affect. And in external declarations, made on behalf of a module which may never be imported.

Embedded ModuleSecurityInfo Declarations

An embedded ModuleSecurityInfo declaration causes an object in its module to be importable by through-the-web code.

Here’s an example of an embedded declaration:

from AccessControl import ModuleSecurityInfo
modulesecurity = ModuleSecurityInfo()
modulesecurity.declarePublic('foo')

def foo():
    return "hello"
    # foo

modulesecurity.apply(globals())

When making embedded ModuleSecurityInfo declarations, you should instantiate a ModuleSecurityInfo object and assign it to a name. It’s wise to use the recommended name ‘modulesecurity’ for consistency’s sake. You may then use the modulesecurity object’s ‘declarePublic’ method to declare functions inside of the current module as public. Finally, appending the last line (“modulesecurity.apply(globals())”) is an important step. It’s necessary in order to poke the security machinery into action. The above example declares the ‘foo’ function public.

The name ‘modulesecurity’ is used for consistency and for the benefit of new component authors, who often learn from looking at other people’s code. You do not have to use the name ‘modulesecurity’ for the security infrastructure to recognize your assertion information, but it is recommended as a convention.

External ModuleSecurityInfo Declarations

By creating a ModuleSecurityInfo instance with a module name argument, you can make declarations on behalf of a module without having to edit or import the module.

Here’s an example of an external declaration:

from AccessControl import ModuleSecurityInfo
# protect the 'foo' function within (yet-to-be-imported) 'foomodule'
ModuleSecurityInfo('foomodule').declarePublic('foo')

This declaration will cause the following code to work within PythonScripts:

from foomodule import foo

When making external ModuleSecurityInfo declarations, you needn’t use the “modulesecurity.apply(globals())” idiom demonstrated in the embedded declaration section above. As a result, you needn’t assign the ModuleSecurityInfo object to the name ‘modulesecurity’.

Providing Access To A Module Contained In A Package

Note that if you want to provide access to a module inside of a package which lives in your PYTHONPATH, you’ll need to provide security declarations for all of the the packages and sub-packages along the path used to access the module.

For example, assume you have a function foo, which lives inside a module named ‘module’, which lives inside a package named ‘package2’, which lives inside a package named ‘package1’ You might declare the ‘foo’ function public via this chain of declarations:

ModuleSecurityInfo('package1').declarePublic('package2')
ModuleSecurityInfo('package1.package2').declarePublic('module')
ModuleSecurityInfo('package1.package2.module').declarePublic('foo')

Note that in the code above we took the following steps:

• make a ModuleSecurityInfo object for ‘package1’
• call the declarePublic method of the ‘package1’ ModuleSecurityInfo object, specifying ‘package2’ as what we’re declaring public. This allows through the web code to “see” package2 inside package1.
• make a ModuleSecurityInfo object for ‘package1.package2’.
• call the declarePublic method of the ‘package1.package2’ ModuleSecurityInfo object, specifying ‘module’ as what we’re declaring public. This allows through the web code to “see” ‘package1.package2.module’.
• declare ‘foo’ public inside the ModuleSecurityInfo for ‘package1.package2.module’.

Through-the-web code may now perform an import ala: ‘import package1.package2.module.foo’

Beware that Zope is buggy from 2.3 to 2.5.0b3. If you make module security declarations in more than one Product, only one of the Products’ security assertions will actually take effect. This is repaired in Zope 2.5.0 and beyond.

Many people who use Zope will be concerned with using ModuleSecurityInfo to make declarations on modules which live within Zope’s Products directory. This is just an example of declaring module security on a module within a package. Here is an example of using ModuleSecurityInfo to make security declarations on behalf of the ‘CatalogError’ class in the ‘ZCatalog.py’ module. This could be placed, for instance, within the any Product’s ‘__init__.py’ module:

from AccessControl import ModuleSecurityInfo
ModuleSecurityInfo('Products').declarePublic('Catalog')
ModuleSecurityInfo('Products.Catalog').declarePublic('CatalogError')

Declaring Module Security On Modules Implemented In C

Certain modules, such as the standard Python ‘sha’ module, provide extension types instead of classes, as the ‘sha’ module is implemented in C. Security declarations typically cannot be added to extension types, so the only way to use this sort of module is to write a Python wrapper class, or use External Methods.

Default Module Security Info Declarations

Through-the-web Python Scripts are by default able to import a small number of Python modules for which there are security declarations. These include ‘string’, ‘math’, and ‘random’. The only way to make other Python modules available for import is to add security declarations to them in the filesystem.

Utility Functions For Allowing Import of Modules By Through The Web Code

Instead of manually providing security declarations for each function in a module, the utility function “allow_class” and “allow_module” have been created to help you declare the entire contents of a class or module as public.

You can handle a module, such as base64, that contains only safe functions by writing ‘allow_module(“module_name”)’. For instance:

from Products.PythonScripts.Utility import allow_module
allow_module("base64")

This statement declares all functions in the ‘base64’ module ( ‘encode’, ‘decode’, ‘encodestring’, and ‘decodestring’ ) as public, and from a script you will now be able to perform an import statement such as “from base64 import encodestring”.

To allow access to only some names in a module, you can eschew the allow_class and allow_module functions for the lessons you learned in the previous section and do the protection “manually”:

from AccessControl import ModuleSecurityInfo
ModuleSecurityInfo('module_name').declarePublic('name1','name2', ...)

Making Permission Assertions On A Constructor

When you develop a Python disk-based product, you will generally be required to make “constructor” methods for the objects which you wish to make accessible via the Zope management interface by users of your Product. These constructors are usually defined within the modules which contain classes which are intended to be turned into Zope instances. For more information on how constructors are used in Zope with security, see Chapter 3 “Zope Products”.

The Zope Product machinery “bootstraps” Product-based classes with proper constructors into the namespace of the Zope management interface “Add” list at Zope startup time. This is done as a consequence of registering a class by way of the Product’s ‘__init__.py’ ‘intialize’ function. If you want to make, for example, the imaginary ‘FooClass’ in your Product available from the “Add” list, you may construct an ‘__init__.py’ file that looks much like this:

from FooProduct import FooClass

def initialize(context):
    """ Initialize classes in the FooProduct module """
    context.registerClass(
        FooProduct.FooClass, # the class object
        permission='Add FooClasses',
        constructors=(FooProduct.manage_addFooClassForm,
                      FooProduct.manage_addFooClass),
        icon='foo.gif'
        )

The line of primary concern to us above is the one which says “permission=’Add FooClasses’”. This is a permission declaration which, thanks to Zope product initialization, restricts the adding of FooClasses to those users who have the ‘Add FooClasses’ permission by way of a role association determined by the system administrator.

If you do not include a ‘permission’ argument to ‘registerClass’, then Zope will create a default permission named ‘Add [meta-type]s’. So, for example, if your object had a meta_type of ‘Animal’, then Zope would create a default permission, ‘Add Animals’. For the most part, it is much better to be explicit then to rely on Zope to take care of security details for you, so be sure to specify a permission for your object.

Designing For Security

“Security is hard.” – Jim Fulton.

When you’re under a deadline, and you “just want it to work”, dealing with security can be difficult. As a component developer, following these basic guidelines will go a long way toward avoiding problems with security integration. They also make a good debugging checklist!

• Ensure that any class that needs to work with security has ‘Acquisition.Implicit’ or ‘Acquisition.Explicit’ somewhere in its base class hierarchy.
• Design the interface to your objects around methods; don’t expect clients to access instance attributes directly.
• Ensure that all methods meant for use by restricted code have been protected with appropriate security assertions.
• Ensure that you called the global class initializer on all classes that need to work with security.

Compatibility

The implementation of the security assertions and ‘SecurityInfo’ interfaces described in this document are available in Zope 2.3 and higher.

Older Zope Products do not use the ‘SecurityInfo’ interfaces for security assertions, because these interfaces didn’t exist at the time. These Zope products will continue to work without modification until further notice.

Using The RoleManager Base Class With Your Zope Product

After your Product is deployed, system managers and other users of your Product often must deal with security settings on instances they make from your classes.

Product classes which inherit Zope’s standard RoleManager base class allow instances of the class to present a security interface. This security interface allows managers and developers of a site to control an instance’s security settings via the Zope management interface.

The user interface is exposed via the Security management view. From this view, a system administrator may secure instances of your Product’s class by associating roles with permissions and by asserting that your object instance contains “local roles”. It also allows them to create “user-defined roles” within the Zope management framework in order to associate these roles with the permissions of your product and with users. This user interface and its usage patterns are explained in more detail within the Zope Book’s security chapter.

If your Product’s class does not inherit from ‘RoleManager’, its methods will still retain the security assertions associated with them, but you will be unable to allow users to associate roles with the permissions you’ve defined respective to instances of your class. Your objects will also not allow local role definitions. Note that objects which inherit from the ‘SimpleItem.SimpleItem’ mixin class already inherit from ‘RoleManager’.

Conclusion

Zope security is based upon roles and permissions. Users have roles. Security policies map permissions to roles. Classes protect methods with permissions. As a developer you main job is to protect your classes by associating methods with permissions. Of course there are many other details such as protecting modules and functions, creating security user interfaces, and initializing security settings.