8. Security
8.1. 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 “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. This is covered in Object Publishing.
8.2. 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.
8.2.1. How The Security Policy Relates to Zope’s Publishing Machinery
When access to Zope is performed via HTTP or WebDAV, 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.
On top of that, the publisher also defines other rules to determine which objects can be published. The most important of these is that objects which are published must have a docstring.
For more information on this topic, see the chapter on Object Publishing.
8.2.2. How The Security Policy Relates to Restricted Code
Restricted code is generally any sort of logic that may be edited remotely (through the Web, via WebDAV or by other means). DTML Methods, SQLMethods, Python Scripts and Page Templates 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 access to the object. This
is accomplished by raising an Unauthorized exception, which is a
Python exception caught by the publisher and handed to a user folder,
which will then attempt to get user credentials before continuing with
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.
8.2.4. 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 filesystem-based add-on components. Another example of unrestricted code can be found in External Method objects (must be installed separately), 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.
8.2.5. 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.
access to an object whose name begins with the underscore character
_is always denied.if the object has a security assertion declaring it private, then access will be denied.
if the object has a security assertion declaring it public , then access will be granted.
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.
objects can only be published if they have a doc string. This restriction exists outside the security policy itself.
As we delve further into Zope security within this chapter, we’ll see exactly what it means to associate security information with an object.
8.2.6. 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, Roles and Permissions” in the Security Chapter of the Zope Book.
8.2.7. 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 with a permission
We’ll see how to actually define 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.
8.3. 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”) where 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.
8.4. Implementing Security In Python Products
8.4.1. 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 filesystem-based Product runs completely unrestricted, and is not subject to security constraints of any kind within Zope. An exception to this rule occurs when filesystem-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.
8.4.2. When Should I Use Security Assertions?
If you are building an object that will be used from a Page Template or other restricted code, or that will be accessible directly through the web (or other remote protocols such as WebDAV) then you need to define security information for your object.
8.4.3. 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.
8.5. 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 Page templates, DTML Methods, or from Python Script objects.
8.5.1. 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.
The ClassSecurityInfo object can be used to declare access in two ways,
as a function decorator or by calling the required method explicitly.
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 using a decorator.
# We are declaring that access to messageCount is public.
@security.public
def messageCount(self):
"""Return a count of messages."""
return len(self._messages)
In the example above we decorated the messageCount method with the
decorator method security.public 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.
The 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
use the public decorator:
@security.public
def myMethod(self):
...
To assert that a method is private you may use the private
decorator:
@security.private
def myMethod(self):
...
To assert that a method or subobject is protected by a particular
permission, you use the protected decorator, passing a permission name:
@security.protected(permissionName)
def myMethod(self):
...
If you have lots of methods you want to protect under the same
permission, you can pass as many methodNames ase you want to a call to
the declareProtected method:
security.declareProtected(permissionName, methodName1,
methodName2, methodName3, ...)
Passing multiple names like this works for all of the non-decorator
declare security methods (declarePublic, declarePrivate, and
declareProtected).
8.5.2. Deciding To Use Protected vs. Public or Private
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.
8.5.3. 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
from AccessControl.class_init import InitializeClass
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.public
def messageCount(self):
"""Return a count of messages."""
return len(self._messages)
# protect 'listMessages' with the 'View Mailbox' permission
@security.protected('View Mailbox')
def listMessages(self):
"""Return a sequence of message objects."""
return self._messages[:]
@security.private
def getMessages(self):
self._messages=GoGetEm()
return self._messages
# call this to initialize framework classes, which
# does the right thing with the security assertions.
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 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.
8.5.4. Deciding Permission Names For Protected Methods
When possible, you should make use of an existing Zope permission
within protected/declareProtected assertions. A list of the
permissions which are available in a default Zope installation is available
within Appendix A: Zope Core Permissions. When it’s not possible to reuse an existing
permission, you should choose a permission name which is a verb or a
verb phrase.
8.5.5. 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 a Page Template or a Python Script. Consider the example Page Template code:
<span tal:content="python: some_method(someObject)">Result</span>
Here we are trying to call some_method, passing the object
someObject. When this is evaluated in the restricted
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.
8.5.6. 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
from AccessControl.class_init import InitializeClass
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.public
def messageCount(self):
"""Return a count of messages."""
return len(self._messages)
# protect 'listMessages' with the 'View Mailbox' permission
@security.protected('View Mailbox')
def listMessages(self):
"""Return a sequence of message objects."""
return self._messages[:]
@security.private
def getMessages(self):
self._messages=GoGetEm()
return self._messages
# call this to initialize framework classes, which
# does the right thing with the security assertions.
InitializeClass(Mailbox)
8.5.7. 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 like list or dict
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.
8.5.8. 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.protected('View foos')
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 call. 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.
8.5.9. 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.
8.5.10. 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
from AccessControl.class_init import InitializeClass
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.public
def messageCount(self):
"""Return a count of messages."""
return len(self._messages)
@security.protected('View Mailbox')
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.
InitializeClass(Mailbox)
8.5.11. 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.protected('View foos')
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.
8.5.12. 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 like
string, int, list or dict. For instance:
from AccessControl import ClassSecurityInfo
from OFS.ObjectManager import ObjectManager
# 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.public
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 TAL or 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, so no action on the
part of the programmer is required to protect it.
8.5.13. 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 subclass, nothing special needs to be done to
protect the base class’ methods within the subclass unless you wish
to modify the declarations made in the base class. The security
declarations “filter down” into the subclass.
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 subclass 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
from AccessControl.class_init import InitializeClass
from OFS.ObjectManager import ObjectManager
class MailboxBase(ObjectManager):
"""A mailbox base class."""
# Create a SecurityInfo for this class
security = ClassSecurityInfo()
@security.protected('View Mailbox')
def listMessages(self):
"""Return a sequence of message objects."""
return self._messages[:]
security.setPermissionDefault('View Mailbox',
('Manager', 'Mailbox Owner'))
InitializeClass(MailboxBase)
class MyMailbox(MailboxBase):
"""A mailbox subclass
Here 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')
InitializeClass(MyMailbox)
8.5.14. Class Security Assertions In Non-Product Code (External Methods/Python Scripts)
Note
The examples in this section use so-called “External Methods”,
which require installing the Products.ExternalMethod package.
Use of that package is discouraged in favor of filesystem-based
Product code.
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 a Page Template, and you’d like your
template 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 AccessControl.class_init import InitializeClass
from Acquisition import Implicit
class Book(Implicit):
def __init__(self, title):
self._title = title
# Create a SecurityInfo for this class
security = ClassSecurityInfo()
security.declareObjectPublic()
@security.public
def getTitle(self):
return self._title
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 getBooks
method to a persistent object within the ZODB. See
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 assertions 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 AccessControl.class_init import InitializeClass
from Acquisition import Implicit
import bookstuff
class Book(Implicit, bookstuff.Book):
security = ClassSecurityInfo()
security.declareObjectPublic()
security.declarePublic('getTitle')
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
8.6. 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.
8.6.1. 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__.pyof 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.
8.6.2. 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.
8.6.3. 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.
8.6.4. 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 Python Scripts:
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.
8.6.5. 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
ModuleSecurityInfoobject forpackage1call the
declarePublicmethod of thepackage1ModuleSecurityInfoobject, specifyingpackage2as what we’re declaring public. This allows through the web code to “see”package2insidepackage1.make a
ModuleSecurityInfoobject forpackage1.package2.call the
declarePublicmethod of thepackage1.package2’ModuleSecurityInfoobject, specifyingmoduleas what we’re declaring public. This allows through the web code to “see”package1.package2.module.declare
foopublic inside theModuleSecurityInfoforpackage1.package2.module.
Through-the-web code may now perform an import ala: import
package1.package2.module.foo
8.6.6. 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.
8.6.7. 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.
8.6.8. 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', ...)
8.6.9. Making Permission Assertions On A Constructor
When you develop a Python filesystem-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 the chapter 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 initialize 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.
8.7. 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.ImplicitorAcquisition.Explicitsomewhere 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.
8.8. 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 many of the built-in classes such as
OFS.SimpleItem.SimpleItem or OFS.ObjectManager.ObjectManager
already inherit from RoleManager.
8.9. 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 your 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.