.. index:: single: security
:app:`Pyramid` provides an optional declarative authorization system that can prevent a :term:`view` from being invoked based on an :term:`authorization policy`. Before a view is invoked, the authorization system can use the credentials in the :term:`request` along with the :term:`context` resource to determine if access will be allowed. Here's how it works at a high level:
- A :term:`request` is generated when a user visits the application.
- Based on the request, a :term:`context` resource is located through :term:`resource location`. A context is located differently depending on whether the application uses :term:`traversal` or :term:`URL dispatch`, but a context is ultimately found in either case. See the :ref:`urldispatch_chapter` chapter for more information.
- A :term:`view callable` is located by :term:`view lookup` using the context as well as other attributes of the request.
- If an :term:`authentication policy` is in effect, it is passed the request; it returns some number of :term:`principal` identifiers.
- If an :term:`authorization policy` is in effect and the :term:`view configuration` associated with the view callable that was found has a :term:`permission` associated with it, the authorization policy is passed the :term:`context`, some number of :term:`principal` identifiers returned by the authentication policy, and the :term:`permission` associated with the view; it will allow or deny access.
- If the authorization policy allows access, the view callable is invoked.
- If the authorization policy denies access, the view callable is not invoked; instead the :term:`forbidden view` is invoked.
Security in :app:`Pyramid`, unlike many systems, cleanly and explicitly separates authentication and authorization. Authentication is merely the mechanism by which credentials provided in the :term:`request` are resolved to one or more :term:`principal` identifiers. These identifiers represent the users and groups in effect during the request. Authorization then determines access based on the :term:`principal` identifiers, the :term:`view callable` being invoked, and the :term:`context` resource.
Authorization is enabled by modifying your application to include an :term:`authentication policy` and :term:`authorization policy`. :app:`Pyramid` comes with a variety of implementations of these policies. To provide maximal flexibility, :app:`Pyramid` also allows you to create custom authentication policies and authorization policies.
.. index:: single: authorization policy
By default, :app:`Pyramid` enables no authorization policy. All views are accessible by completely anonymous users. In order to begin protecting views from execution based on security settings, you need to enable an authorization policy.
Passing an authorization_policy
argument to the constructor of the
:class:`~pyramid.config.Configurator` class enables an
authorization policy.
You must also enable an :term:`authentication policy` in order to
enable the authorization policy. This is because authorization, in
general, depends upon authentication. Use the
authentication_policy
argument to the
:class:`~pyramid.config.Configurator` class during
application setup to specify an authentication policy.
For example:
from pyramid.config import Configurator
from pyramid.authentication import AuthTktAuthenticationPolicy
from pyramid.authorization import ACLAuthorizationPolicy
authentication_policy = AuthTktAuthenticationPolicy('seekrit')
authorization_policy = ACLAuthorizationPolicy()
config = Configurator(authentication_policy=authentication_policy,
authorization_policy=authorization_policy)
Note
the authentication_policy
and authorization_policy
arguments may also be passed to the Configurator as :term:`dotted
Python name` values, each representing the dotted name path to a
suitable implementation global defined at Python module scope.
The above configuration enables a policy which compares the value of an "auth ticket" cookie passed in the request's environment which contains a reference to a single :term:`principal` against the principals present in any :term:`ACL` found in the resource tree when attempting to call some :term:`view`.
While it is possible to mix and match different authentication and authorization policies, it is an error to pass an authentication policy without the authorization policy or vice versa to a :term:`Configurator` constructor.
See also the :mod:`pyramid.authorization` and :mod:`pyramid.authentication` modules for alternate implementations of authorization and authentication policies.
.. index:: single: permissions single: protecting views
To protect a :term:`view callable` from invocation based on a user's security settings when a particular type of resource becomes the :term:`context`, you must pass a :term:`permission` to :term:`view configuration`. Permissions are usually just strings, and they have no required composition: you can name permissions whatever you like.
For example, the following view declaration protects the view named
add_entry.html
when the context resource is of type Blog
with the
add
permission using the :meth:`pyramid.config.Configurator.add_view`
API:
# config is an instance of pyramid.config.Configurator
config.add_view('mypackage.views.blog_entry_add_view',
name='add_entry.html',
context='mypackage.resources.Blog',
permission='add')
The equivalent view registration including the add
permission name
may be performed via the @view_config
decorator:
from pyramid.view import view_config
from resources import Blog
@view_config(context=Blog, name='add_entry.html', permission='add')
def blog_entry_add_view(request):
""" Add blog entry code goes here """
pass
As a result of any of these various view configuration statements, if an
authorization policy is in place when the view callable is found during
normal application operations, the requesting user will need to possess the
add
permission against the :term:`context` resource in order to be able
to invoke the blog_entry_add_view
view. If he does not, the
:term:`Forbidden view` will be invoked.
If a permission is not supplied to a view configuration, the registered view will always be executable by entirely anonymous users: any authorization policy in effect is ignored.
In support of making it easier to configure applications which are
"secure by default", :app:`Pyramid` allows you to configure a
default permission. If supplied, the default permission is used as
the permission string to all view registrations which don't otherwise
name a permission
argument.
These APIs are in support of configuring a default permission for an application:
- The
default_permission
constructor argument to the :mod:`~pyramid.config.Configurator` constructor. - The :meth:`pyramid.config.Configurator.set_default_permission` method.
When a default permission is registered:
- If a view configuration names an explicit
permission
, the default permission is ignored for that view registration, and the view-configuration-named permission is used. - If a view configuration names an explicit permission as the string
__no_permission_required__
, the default permission is ignored, and the view is registered without a permission (making it available to all callers regardless of their credentials).
Warning
When you register a default permission, all views (even :term:`exception
view` views) are protected by a permission. For all views which are truly
meant to be anonymously accessible, you will need to associate the view's
configuration with the __no_permission_required__
permission.
.. index:: single: ACL single: access control list
When the default :app:`Pyramid` :term:`authorization policy` determines
whether a user possesses a particular permission with respect to a resource,
it examines the :term:`ACL` associated with the resource. An ACL is
associated with a resource by adding an __acl__
attribute to the resource
object. This attribute can be defined on the resource instance if you need
instance-level security, or it can be defined on the resource class if you
just need type-level security.
For example, an ACL might be attached to the resource for a blog via its class:
from pyramid.security import Everyone
from pyramid.security import Allow
class Blog(object):
__acl__ = [
(Allow, Everyone, 'view'),
(Allow, 'group:editors', 'add'),
(Allow, 'group:editors', 'edit'),
]
Or, if your resources are persistent, an ACL might be specified via the
__acl__
attribute of an instance of a resource:
from pyramid.security import Everyone
from pyramid.security import Allow
class Blog(object):
pass
blog = Blog()
blog.__acl__ = [
(Allow, Everyone, 'view'),
(Allow, 'group:editors', 'add'),
(Allow, 'group:editors', 'edit'),
]
Whether an ACL is attached to a resource's class or an instance of the resource itself, the effect is the same. It is useful to decorate individual resource instances with an ACL (as opposed to just decorating their class) in applications such as "CMS" systems where fine-grained access is required on an object-by-object basis.
.. index:: single: ACE single: access control entry
Here's an example ACL:
from pyramid.security import Everyone
from pyramid.security import Allow
__acl__ = [
(Allow, Everyone, 'view'),
(Allow, 'group:editors', 'add'),
(Allow, 'group:editors', 'edit'),
]
The example ACL indicates that the
:data:`pyramid.security.Everyone` principal -- a special
system-defined principal indicating, literally, everyone -- is allowed
to view the blog, the group:editors
principal is allowed to add to
and edit the blog.
Each element of an ACL is an :term:`ACE` or access control entry.
For example, in the above code block, there are three ACEs: (Allow,
Everyone, 'view')
, (Allow, 'group:editors', 'add')
, and
(Allow, 'group:editors', 'edit')
.
The first element of any ACE is either :data:`pyramid.security.Allow`, or :data:`pyramid.security.Deny`, representing the action to take when the ACE matches. The second element is a :term:`principal`. The third argument is a permission or sequence of permission names.
A principal is usually a user id, however it also may be a group id if your
authentication system provides group information and the effective
:term:`authentication policy` policy is written to respect group information.
For example, the
:class:`pyramid.authentication.RepozeWho1AuthenicationPolicy` respects group
information if you configure it with a callback
.
Each ACE in an ACL is processed by an authorization policy in the order dictated by the ACL. So if you have an ACL like this:
from pyramid.security import Everyone
from pyramid.security import Allow
from pyramid.security import Deny
__acl__ = [
(Allow, Everyone, 'view'),
(Deny, Everyone, 'view'),
]
The default authorization policy will allow everyone the view permission, even though later in the ACL you have an ACE that denies everyone the view permission. On the other hand, if you have an ACL like this:
from pyramid.security import Everyone
from pyramid.security import Allow
from pyramid.security import Deny
__acl__ = [
(Deny, Everyone, 'view'),
(Allow, Everyone, 'view'),
]
The authorization policy will deny everyone the view permission, even though later in the ACL is an ACE that allows everyone.
The third argument in an ACE can also be a sequence of permission
names instead of a single permission name. So instead of creating
multiple ACEs representing a number of different permission grants to
a single group:editors
group, we can collapse this into a single
ACE, as below.
from pyramid.security import Everyone
from pyramid.security import Allow
__acl__ = [
(Allow, Everyone, 'view'),
(Allow, 'group:editors', ('add', 'edit')),
]
.. index:: single: principal single: principal names
Special principal names exist in the :mod:`pyramid.security` module. They can be imported for use in your own code to populate ACLs, e.g. :data:`pyramid.security.Everyone`.
:data:`pyramid.security.Everyone`
Literally, everyone, no matter what. This object is actually a
string "under the hood" (system.Everyone
). Every user "is" the
principal named Everyone during every request, even if a security
policy is not in use.
:data:`pyramid.security.Authenticated`
Any user with credentials as determined by the current security
policy. You might think of it as any user that is "logged in".
This object is actually a string "under the hood"
(system.Authenticated
).
.. index:: single: permission names single: special permission names
Special permission names exist in the :mod:`pyramid.security` module. These can be imported for use in ACLs.
:data:`pyramid.security.ALL_PERMISSIONS`
An object representing, literally, all permissions. Useful in an ACL like so:(Allow, 'fred', ALL_PERMISSIONS)
. TheALL_PERMISSIONS
object is actually a stand-in object that has a__contains__
method that always returnsTrue
, which, for all known authorization policies, has the effect of indicating that a given principal "has" any permission asked for by the system.
.. index:: single: special ACE single: ACE (special)
A convenience :term:`ACE` is defined representing a deny to everyone of all
permissions in :data:`pyramid.security.DENY_ALL`. This ACE is often used as
the last ACE of an ACL to explicitly cause inheriting authorization
policies to "stop looking up the traversal tree" (effectively breaking any
inheritance). For example, an ACL which allows only fred
the view
permission for a particular resource despite what inherited ACLs may say when
the default authorization policy is in effect might look like so:
from pyramid.security import Allow
from pyramid.security import DENY_ALL
__acl__ = [ (Allow, 'fred', 'view'), DENY_ALL ]
"Under the hood", the :data:`pyramid.security.DENY_ALL` ACE equals the following:
from pyramid.security import ALL_PERMISSIONS
__acl__ = [ (Deny, Everyone, ALL_PERMISSIONS) ]
.. index:: single: ACL inheritance pair: location-aware; security
While the default :term:`authorization policy` is in place, if a resource object does not have an ACL when it is the context, its parent is consulted for an ACL. If that object does not have an ACL, its parent is consulted for an ACL, ad infinitum, until we've reached the root and there are no more parents left.
In order to allow the security machinery to perform ACL inheritance, resource
objects must provide location-awareness. Providing location-awareness
means two things: the root object in the resource tree must have a
__name__
attribute and a __parent__
attribute.
class Blog(object):
__name__ = ''
__parent__ = None
An object with a __parent__
attribute and a __name__
attribute
is said to be location-aware. Location-aware objects define an
__parent__
attribute which points at their parent object. The
root object's __parent__
is None
.
See :ref:`location_module` for documentations of functions which use location-awareness. See also :ref:`location_aware`.
.. index:: single: forbidden view
When :app:`Pyramid` denies a view invocation due to an
authorization denial, the special forbidden
view is invoked. "Out
of the box", this forbidden view is very plain. See
:ref:`changing_the_forbidden_view` within :ref:`hooks_chapter` for
instructions on how to create a custom forbidden view and arrange for
it to be called when view authorization is denied.
.. index:: single: debugging authorization failures
If your application in your judgment is allowing or denying view
access inappropriately, start your application under a shell using the
PYRAMID_DEBUG_AUTHORIZATION
environment variable set to 1
. For
example:
$ PYRAMID_DEBUG_AUTHORIZATION=1 bin/paster serve myproject.ini
When any authorization takes place during a top-level view rendering, a message will be logged to the console (to stderr) about what ACE in which ACL permitted or denied the authorization based on authentication information.
This behavior can also be turned on in the application .ini
file
by setting the debug_authorization
key to true
within the
application's configuration section, e.g.:
[app:main]
use = egg:MyProject#app
debug_authorization = true
With this debug flag turned on, the response sent to the browser will also contain security debugging information in its body.
The :func:`pyramid.security.has_permission` API is used to check
security within view functions imperatively. It returns instances of
objects that are effectively booleans. But these objects are not raw
True
or False
objects, and have information attached to them
about why the permission was allowed or denied. The object will be
one of :data:`pyramid.security.ACLAllowed`,
:data:`pyramid.security.ACLDenied`,
:data:`pyramid.security.Allowed`, or
:data:`pyramid.security.Denied`, as documented in
:ref:`security_module`. At the very minimum these objects will have a
msg
attribute, which is a string indicating why the permission was
denied or allowed. Introspecting this information in the debugger or
via print statements when a call to
:func:`~pyramid.security.has_permission` fails is often useful.
.. index:: single: authentication policy (creating)
:app:`Pyramid` ships with a number of useful out-of-the-box security policies (see :mod:`pyramid.authentication`). However, creating your own authentication policy is often necessary when you want to control the "horizontal and vertical" of how your users authenticate. Doing so is a matter of creating an instance of something that implements the following interface:
class IAuthenticationPolicy(object):
""" An object representing a Pyramid authentication policy. """
def authenticated_userid(self, request):
""" Return the authenticated userid or ``None`` if no
authenticated userid can be found. This method of the policy
should ensure that a record exists in whatever persistent store is
used related to the user (the user should not have been deleted);
if a record associated with the current id does not exist in a
persistent store, it should return ``None``."""
def unauthenticated_userid(self, request):
""" Return the *unauthenticated* userid. This method performs the
same duty as ``authenticated_userid`` but is permitted to return the
userid based only on data present in the request; it needn't (and
shouldn't) check any persistent store to ensure that the user record
related to the request userid exists."""
def effective_principals(self, request):
""" Return a sequence representing the effective principals
including the userid and any groups belonged to by the current
user, including 'system' groups such as
``pyramid.security.Everyone`` and
``pyramid.security.Authenticated``. """
def remember(self, request, principal, **kw):
""" Return a set of headers suitable for 'remembering' the
principal named ``principal`` when set in a response. An
individual authentication policy and its consumers can decide
on the composition and meaning of **kw. """
def forget(self, request):
""" Return a set of headers suitable for 'forgetting' the
current user on subsequent requests. """
After you do so, you can pass an instance of such a class into the
:class:`~pyramid.config.Configurator` class at configuration
time as authentication_policy
to use it.
.. index:: single: authorization policy (creating)
An authorization policy is a policy that allows or denies access after a user has been authenticated. By default, :app:`Pyramid` will use the :class:`pyramid.authorization.ACLAuthorizationPolicy` if an authentication policy is activated and an authorization policy isn't otherwise specified.
In some cases, it's useful to be able to use a different authorization policy than the default :class:`~pyramid.authorization.ACLAuthorizationPolicy`. For example, it might be desirable to construct an alternate authorization policy which allows the application to use an authorization mechanism that does not involve :term:`ACL` objects.
:app:`Pyramid` ships with only a single default authorization policy, so you'll need to create your own if you'd like to use a different one. Creating and using your own authorization policy is a matter of creating an instance of an object that implements the following interface:
class IAuthorizationPolicy(object):
""" An object representing a Pyramid authorization policy. """
def permits(self, context, principals, permission):
""" Return ``True`` if any of the ``principals`` is allowed the
``permission`` in the current ``context``, else return ``False``
"""
def principals_allowed_by_permission(self, context, permission):
""" Return a set of principal identifiers allowed by the
``permission`` in ``context``. This behavior is optional; if you
choose to not implement it you should define this method as
something which raises a ``NotImplementedError``. This method
will only be called when the
``pyramid.security.principals_allowed_by_permission`` API is
used."""
After you do so, you can pass an instance of such a class into the
:class:`~pyramid.config.Configurator` class at configuration
time as authorization_policy
to use it.