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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:

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:

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:


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:

"__lt__", "__le__", "__eq__", "__gt__", "__ge__", "__ne__"
boolean value
class introspection
interface introspection
"__providedBy__", "__implements__"
low-level string representation

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:

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.