Sandbox

The Jinja sandbox can be used to render untrusted templates. Access to attributes, method calls, operators, mutating data structures, and string formatting can be intercepted and prohibited.

.. code-block:: pycon

>>> from jinja2.sandbox import SandboxedEnvironment
>>> env = SandboxedEnvironment()
>>> func = lambda: "Hello, Sandbox!"
>>> env.from_string("{{ func() }}").render(func=func)
'Hello, Sandbox!'
>>> env.from_string("{{ func.__code__.co_code }}").render(func=func)
Traceback (most recent call last):
  ...
SecurityError: access to attribute '__code__' of 'function' object is unsafe.

A sandboxed environment can be useful, for example, to allow users of an internal reporting system to create custom emails. You would document what data is available in the templates, then the user would write a template using that information. Your code would generate the report data and pass it to the user's sandboxed template to render.

Security Considerations

The sandbox alone is not a solution for perfect security. Keep these things in mind when using the sandbox.

Templates can still raise errors when compiled or rendered. Your code should attempt to catch errors instead of crashing.

It is possible to construct a relatively small template that renders to a very large amount of output, which could correspond to a high use of CPU or memory. You should run your application with limits on resources such as CPU and memory to mitigate this.

Jinja only renders text, it does not understand, for example, JavaScript code. Depending on how the rendered template will be used, you may need to do other postprocessing to restrict the output.

Pass only the data that is relevant to the template. Avoid passing global data, or objects with methods that have side effects. By default the sandbox prevents private and internal attribute access. You can override :meth:~SandboxedEnvironment.is_safe_attribute to further restrict attributes access. Decorate methods with :func:unsafe to prevent calling them from templates when passing objects as data. Use :class:ImmutableSandboxedEnvironment to prevent modifying lists and dictionaries.

API

.. module:: jinja2.sandbox

.. autoclass:: SandboxedEnvironment([options]) :members: is_safe_attribute, is_safe_callable, default_binop_table, default_unop_table, intercepted_binops, intercepted_unops, call_binop, call_unop

.. autoclass:: ImmutableSandboxedEnvironment([options])

.. autoexception:: SecurityError

.. autofunction:: unsafe

.. autofunction:: is_internal_attribute

.. autofunction:: modifies_known_mutable

Operator Intercepting

For performance, Jinja outputs operators directly when compiling. This means it's not possible to intercept operator behavior by overriding :meth:SandboxEnvironment.call <Environment.call> by default, because operator special methods are handled by the Python interpreter, and might not correspond with exactly one method depending on the operator's use.

The sandbox can instruct the compiler to output a function to intercept certain operators instead. Override :attr:SandboxedEnvironment.intercepted_binops and :attr:SandboxedEnvironment.intercepted_unops with the operator symbols you want to intercept. The compiler will replace the symbols with calls to :meth:SandboxedEnvironment.call_binop and :meth:SandboxedEnvironment.call_unop instead. The default implementation of those methods will use :attr:SandboxedEnvironment.binop_table and :attr:SandboxedEnvironment.unop_table to translate operator symbols into :mod:operator functions.

For example, the power (**) operator can be disabled:

.. code-block:: python

from jinja2.sandbox import SandboxedEnvironment

class MyEnvironment(SandboxedEnvironment):
    intercepted_binops = frozenset(["**"])

    def call_binop(self, context, operator, left, right):
        if operator == "**":
            return self.undefined("The power (**) operator is unavailable.")

        return super().call_binop(self, context, operator, left, right)