Overview

Introduction

The Security framework provides a generic mechanism to implement security policies on Python objects. This introduction provides a tutorial of the framework explaining concepts, design, and going through sample usage from the perspective of a Python programmer using the framework outside of Zope.

Definitions

Principal

A generalization of a concept of a user.

Permission

A kind of access, i.e. permission to READ vs. permission to WRITE. Fundamentally the whole security framework is organized around checking permissions on objects.

Purpose

The security framework’s primary purpose is to guard and check access to Python objects. It does this by providing mechanisms for explicit and implicit security checks on attribute access for objects. Attribute names are mapped onto permission names when checking access and the implementation of the security check is defined by the security policy, which receives the object, the permission name, and an interaction.

Interactions are objects that represent the use of the system by one or more principals. An interaction contains a list of participations, which represents the way a single principal participates in the interaction. An HTTP request is one example of a participation.

Its important to keep in mind that the policy provided is just a default, and it can be substituted with one which doesn’t care about principals or interactions at all.

Framework Components

Low Level Components

These components provide the infrastructure for guarding attribute access and providing hooks into the higher level security framework.

Checkers

A checker is associated with an object kind, and provides the hooks that map attribute checks onto permissions deferring to the security manager (which in turn defers to the policy) to perform the check.

Additionally, checkers provide for creating proxies of objects associated with the checker.

There are several implementation variants of checkers, such as checkers that grant access based on attribute names.

Proxies

Wrappers around Python objects that implicitly guard access to their wrapped contents by delegating to their associated checker. Proxies are also viral in nature, in that values returned by proxies are also proxied.

High Level Components

Security Management

Provides accessors for setting up interactions and the global security policy.

Interaction

Stores transient information on the list of participations.

Participation

Stores information about a principal participating in the interaction.

Security Policy

Provides a single method that accepts the object, the permission, and the interaction of the access being checked and is used to implement the application logic for the security framework.

Narrative (agent sandbox)

As an example we take a look at constructing a multi-agent distributed system, and then adding a security layer using the Zope security model onto it.

Scenario

Our agent simulation consists of autonomous agents that live in various agent homes/sandboxes and perform actions that access services available at their current home. Agents carry around authentication tokens which signify their level of access within any given home. Additionally agents attempt to migrate from home to home randomly.

The agent simulation was constructed separately from any security aspects. Now we want to define and integrate a security model into the simulation. The full code for the simulation and the security model is available separately; we present only relevant code snippets here for illustration as we go through the implementation process.

For the agent simulation we want to add a security model such that we group agents into two authentication groups, “norse legends”, including the principals thor, odin, and loki, and “greek men”, including prometheus, archimedes, and thucydides.

We associate permissions with access to services and homes. We differentiate the homes such that certain authentication groups only have access to services or the home itself based on the local settings of the home in which they reside.

We define the homes/sandboxes

  • origin - all agents start here, and have access to all services here.
  • valhalla - only agents in the authentication group ‘norse legend’ can reside here.
  • jail - all agents can come here, but only ‘norse legend’s can leave or access services.

Process

Loosely we define a process for implementing this security model

  • mapping permissions onto actions
  • mapping authentication tokens onto permissions
  • implementing checkers and security policies that use our authentication tokens and permissions.
  • binding checkers to our simulation classes
  • inserting the hooks into the original simulation code to add proxy wrappers to automatically check security.
  • inserting hooks into the original simulation to register the agents as the active principal in an interaction.

Defining a Permission Model

We define the following permissions:

NotAllowed = 'Not Allowed'
Public = Checker.CheckerPublic
TransportAgent = 'Transport Agent'
AccessServices = 'Access Services'
AccessAgents = 'Access Agents'
AccessTimeService = 'Access Time Services'
AccessAgentService = 'Access Agent Service'
AccessHomeService = 'Access Home Service'

and create a dictionary database mapping homes to authentication groups which are linked to associated permissions.

Defining and Binding Checkers

Checkers are the foundational unit for the security framework. They define what attributes can be accessed or set on a given instance. They can be used implicitly via Proxy objects, to guard all attribute access automatically or explicitly to check a given access for an operation.

Checker construction expects two functions or dictionaries, one is used to map attribute names to permissions for attribute access and another to do the same for setting attributes.

We use the following checker factory function:

def PermissionMapChecker(permissions_map={},
                         setattr_permission_func=NoSetAttr):
    res = {}
    for k,v in permissions_map.items():
        for iv in v:
            res[iv]=k
    return checker.Checker(res.get, setattr_permission_func)

time_service_checker = PermissionMapChecker(
                               # permission : [methods]
                               {'AccessTimeService':['getTime']}
                               )

with the NoSetAttr function defined as a lambda which always return the permission NotAllowed.

To bind the checkers to the simulation classes we register our checkers with the security model’s global checker registry:

import sandbox_simulation
from zope.security.checker import defineChecker
defineChecker(sandbox_simulation.TimeService, time_service_checker)

Defining a Security Policy

We implement our security policy such that it checks the current agent’s authentication token against the given permission in the home of the object being accessed:

@implementer(ISecurityPolicy)
class SimulationSecurityPolicy:

    createInteraction = staticmethod(simpleinteraction.createInteraction)

    def checkPermission(self, permission, object, interaction):

        home = object.getHome()
        db = getattr(SimulationSecurityDatabase, home.getId(), None)

        if db is None:
            return False

        allowed = db.get('any', ())
        if permission in allowed or ALL in allowed:
            return True

        if interaction is None:
            return False
        if not interaction.participations:
            return False
        for participation in interaction.participations:
            token = participation.principal.getAuthenticationToken()
            allowed = db.get(token, ())
            if permission not in allowed:
                return False

        return True

There are no specific requirements for the interaction class, so we can just use zope.security.simpleinteraction.Interaction.

Since an interaction can have more than one principal, we check that all of them are given the necessary permission. This is not really necessary since we only create interactions with a single active principal.

There is some additional code present to allow for shortcuts in defining the permission database when defining permissions for all auth groups and all permissions.

Integration

At this point we have implemented our security model, and we need to integrate it with our simulation model. We do so in three separate steps.

First we make it such that agents only access homes that are wrapped in a security proxy. By doing this all access to homes and services (proxies have proxied return values for their methods) is implicitly guarded by our security policy.

The second step is that we want to associate the active agent with the security context so the security policy will know which agent’s authentication token to validate against.

The third step is to set our security policy as the default policy for the Zope security framework. It is possible to create custom security policies at a finer grained than global, but such is left as an exercise for the reader.

Interaction Access

The default implementation of the interaction management interfaces defines interactions on a per thread basis with a function for an accessor. This model is not appropriate for all systems, as it restricts one to a single active interaction per thread at any given moment. Reimplementing the interaction access methods though is easily doable and is noted here for completeness.

Perspectives

It’s important to keep in mind that there is a lot more that is possible using the security framework than what’s been presented here. All of the interactions are interface based, such that if you need to re-implement the semantics to suite your application a new implementation of the interface will be sufficient. Additional possibilities range from restricted interpreters and dynamic loading of untrusted code to non Zope web application security systems. Insert imagination here ;-).

Zope Perspective

A Zope3 programmer will never commonly need to interact with the low level security framework. Zope3 defines a second security package over top the low level framework and authentication sources and checkers are handled via zcml registration. Still those developing Zope3 will hopefully find this useful as an introduction into the underpinnings of the security framework.

Code

The complete code for this example is available.

  • sandbox.py - the agent framework
  • sandbox_security.py - the security implementation and binding to the agent framework.

Authors

  • Kapil Thangavelu <hazmat at objectrealms.net>
  • Guido Wesdorp <guido at infrae.com>
  • Marius Gedminas <marius at pov.lt>

Untrusted interpreters

Untrusted programs are executed by untrusted interpreters. Untrusted interpreters make use of security proxies to prevent un-mediated access to assets. An untrusted interpreter defines an environment for running untrusted programs. All objects within the environment are either:

  • “safe” objects created internally by the environment or created in the course of executing the untrusted program, or
  • “basic” objects
  • security-proxied non-basic objects

The environment includes proxied functions for accessing objects outside of the environment. These proxied functions provide the only way to access information outside the environment. Because these functions are proxied, as described below, any access to objects outside the environment is mediated by the target security functions.

Safe objects are objects whose operations, except for attribute retrieval, and methods access only information stored within the objects or passed as arguments. Safe objects contained within the interpreter environment can contain only information that is already in the environment or computed directly from information that is included in the environment. For this reason, safe objects created within the environment cannot be used to directly access information outside the environment.

Safe objects have some attributes that could (very) indirectly be used to access assets. For this reason, an untrusted interpreter always proxies the results of attribute accesses on a safe objects.

Basic objects are safe objects that are used to represent elemental data values such as strings and numbers. Basic objects require a lower level of protection than non-basic objects, as will be described detail in a later section.

Security proxies mediate all object operations. Any operation access is checked to see whether a subject is authorized to perform the operation. All operation results other than basic objects are, in turn, security proxied. Security proxies will be described in greater detail in a later section. Any operation on a security proxy that results in a non-basic object is also security proxied.

All external resources needed to perform an operation are security proxied.

Let’s consider the trusted interpreter for evaluating URLs. In operation 1 of the example, the interpreter uses a proxied method for getting the system root object. Because the method is proxied, the result of calling the method and the operation is also proxied.

The interpreter has a function for traversing objects. This function is proxied. When traversing an object, the function is passed an object and a name. In operation 2, the function is passed the result of operation 1, which is the proxied root object and the name ‘A’. We may traverse an object by invoking an operation on it. For example, we may use an operation to get a sub-object. Because any operation on a proxied object returns a proxied object or a basic object, the result is either a proxied object or a basic object. Traversal may also look up a component. For example, in operation 1, we might look up a presentation component named “A” for the root object. In this case, the external object is not proxied, but, when it is returned from the traversal function, it is proxied (unless it is a a basic object) because the traversal function is proxied, and the result of calling a proxied function is proxied (unless the result is a basic object). Operation 3 proceeds in the same way.

When we get to operation 4, we use a function for computing the default presentation of the result of operation 3. As with traversal, the result of getting the default presentation is either a proxied object or a basic object because the function for getting the default presentation is proxied.

When we get to the last operation, we have either a proxied object or a basic object. If the result of operation 4 is a basic object, we simply convert it to a string and return it as the result page. If the result of operation 4 is a non-basic object, we invoke a render operation on it and return the result as a string.

Note that an untrusted interpreter may or may not provide protection against excessive resource usage. Different interpreters will provide different levels of service with respect to limitations on resource usage.

If an untrusted interpreter performs an attribute access, the trusted interpreter must proxy the result unless the result is a basic object.

In summary, an untrusted interpreter assures that any access to assets is mediated through security proxies by creating an environment to run untrusted code and making sure that:

  • The only way to access anything from outside of the environment is to call functions that are proxied in the environment.
  • Results of any attribute access in the environment are proxied unless the results are basic objects.

Security proxies

Security proxies are objects that wrap and mediate access to objects.

The Python programming language used by Zope defines a set of specific named low-level operations. In addition to operations, Python objects can have attributes, used to represent data and methods. Attributes are accessed using a dot notation. Applications can, and usually do, define methods to provide extended object behaviors. Methods are accessed as attributes through the low-level operation named “__getattribute__”. The Python code:

a.b()

invokes 2 operations:

  1. Use the low-level __getattribute__ operation with the name “b”.
  2. Use the low-level ‘__call__’ operation on the result of the first operation.

For all operations except the __getattribute__ and __setattribute__ operations, security proxies have a permission value defined by the permission-declaration subsystem. Two special permission values indicate that access is either forbidden (never allowed) or public (always allowed). For all other permission values, the authorization subsystem is used to decide whether the subject has the permission for the proxied object. If the subject has the permission, then access to the operation is allowed. Otherwise, access is denied.

For getting or setting attributes, a proxy has permissions for getting and a permission for setting attribute values for a given attribute name. As described above, these permissions may be one of the two special permission values indicating forbidden or public access, or another permission value that must be checked with the authorization system.

For all objects, Zope defines the following operations to be always public:

comparison
“__lt__”, “__le__”, “__eq__”, “__gt__”, “__ge__”, “__ne__”
hash
“__hash__”
boolean value
“__nonzero__”
class introspection
“__class__”
interface introspection
“__providedBy__”, “__implements__”
adaptation
“__conform__”
low-level string representation
“__repr__”

The result of an operation on a proxied object is a security proxy unless the result is a basic value.

Basic objects

Basic objects are safe immutable objects that contain only immutable subobjects. Examples of basic objects include:

  • Strings,
  • Integers (long and normal),
  • Floating-point objects,
  • Date-time objects,
  • Boolean objects (True and False), and
  • The special (nil) object, None.

Basic objects are safe, so, as described earlier, operations on basic objects, other than attribute access, use only information contained within the objects or information passed to them. For this reason, basic objects cannot be used to access information outside of the untrusted interpreter environment.

The decision not to proxy basic objects is largely an optimization. It allows low-level safe computation to be performed without unnecessary overhead,

Note that a basic object could contain sensitive information, but such a basic object would need to be obtained by making a call on a proxied object. Therefore, the access to the basic object in the first place is mediated by the security functions.

Rationale for mutable safe objects

Some safe objects are not basic. For these objects, we proxy the objects if they originate from outside of the environment. We do this for two reasons:

  1. Non-basic objects from outside the environment need to be proxied to prevent unauthorized access to information.
  2. We need to prevent un-mediated change of information from outside of the environment.

We don’t proxy safe objects created within the environment. This is safe to do because such safe objects can contain and provide access to information already in the environment. Sometimes the interpreter or the interpreted program needs to be able to create simple data containers to hold information computed in the course of the program execution. Several safe container types are provided for this purpose.