python-peps/pep-0493.txt

PEP: 493
Title: HTTPS verification migration tools for Python 2.7
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>,
        Robert Kuska <rkuska@redhat.com>,
        Marc-André Lemburg <mal@lemburg.com>
BDFL-Delegate: Barry Warsaw
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 10-May-2015
Python-Version: 2.7.12
Post-History: 06-Jul-2015, 11-Nov-2015, 24-Nov-2015, 24-Feb-2016
Resolution: https://mail.python.org/pipermail/python-dev/2016-March/143450.html


Abstract
========

PEP 476 updated Python's default handling of HTTPS certificates in client
modules to align with certificate handling in web browsers, by validating
that the certificates received belonged to the server the client was attempting
to contact. The Python 2.7 long term maintenance series was judged to be in
scope for this change, with the new behaviour introduced in the Python 2.7.9
maintenance release.

This has created a non-trivial barrier to adoption for affected Python 2.7
maintenance releases, so this PEP proposes additional Python 2.7 specific
features that allow system administrators and other users to more easily
decouple the decision to verify server certificates in HTTPS client modules
from the decision to update to newer Python 2.7 maintenance releases.


Rationale
=========

PEP 476 changed Python's default behaviour to align with expectations
established by web browsers in regards to the semantics of HTTPS URLs:
starting with Python 2.7.9 and 3.4.3, HTTPS clients in the standard library
validate server certificates by default.

However, it is also the case that this change *does* cause problems for
infrastructure administrators operating private intranets that rely on
self-signed certificates, or otherwise encounter problems with the new default
certificate verification settings.

To manage these kinds of situations, web browsers provide users with "click
through" warnings that allow the user to add the server's certificate to the
browser's certificate store. Network client tools like ``curl`` and ``wget``
offer options to switch off certificate checking entirely (by way of
``curl --insecure`` and ``wget --no-check-certificate``, respectively).

At a different layer of the technology stack, Linux security modules like
`SELinux` and `AppArmor`, while enabled by default by distribution vendors,
offer relatively straightforward mechanisms for turning them off.

At the moment, no such convenient mechanisms exist to disable Python's
default certificate checking for a whole process.

PEP 476 did attempt to address this question, by covering how to revert to the
old settings process wide by monkeypatching the ``ssl`` module to restore the
old behaviour. Unfortunately, the ``sitecustomize.py`` based technique proposed
to allow system administrators to disable the feature by default in their
Standard Operating Environment definition has been determined to be
insufficient in at least some cases. The specific case that led to the
initial creation of this PEP is the one where a Linux distributor aims to
provide their users with a
`smoother migration path <https://bugzilla.redhat.com/show_bug.cgi?id=1173041>`__
than the standard one provided by consuming upstream CPython 2.7 releases
directly, but other potential challenges have also been pointed out with
updating embedded Python runtimes and other user level installations of Python.

Rather than allowing a plethora of mutually incompatibile migration techniques
to bloom, this PEP proposes an additional feature to be added to Python 2.7.12
to make it easier to revert a process to the past behaviour of skipping
certificate validation in HTTPS client modules. It also provides additional
recommendations to redistributors backporting these features to versions of
Python prior to Python 2.7.9.

Alternatives
------------

In the absence of clear upstream guidance and recommendations, commercial
redistributors will still make their own design decisions in the interests of
their customers. The main approaches available are:

* Continuing to rebase on new Python 2.7.x releases, while providing no
  additional assistance beyond the mechanisms defined in PEP 476 in migrating
  from unchecked to checked hostnames in standard library HTTPS clients
* Gating availability of the changes in default handling of HTTPS connections
  on upgrading from Python 2 to Python 3
* For Linux distribution vendors, gating availability of the changes in default
  handling of HTTPS connections on upgrading to a new operating system version
* Implementing one or both of the backport suggestions described in this PEP,
  regardless of the formal status of the PEP


Scope Limitations
=================

These changes are being proposed purely as tools for helping to manage the
transition to the new default certificate handling behaviour in the context
of Python 2.7. They are not being proposed as new features for Python 3, as
it is expected that the vast majority of client applications affected by this
problem without the ability to update the application itself will be Python 2
applications.

It would likely be desirable for a future version of Python 3 to allow the
default certificate handling for secure protocols to be configurable on a
per-protocol basis, but that question is beyond the scope of this PEP.


Requirements for capability detection
=====================================

As the proposals in this PEP aim to facilitate backports to earlier Python
versions, the Python version number cannot be used as a reliable means for
detecting them. Instead, they are designed to allow the presence
or absence of the feature to be determined using the following technique::

    python -c "import ssl; ssl.<_relevant_attribute>"

This will fail with `AttributeError` (and hence a non-zero return code) if the
relevant capability is not available.

The feature detection attributes defined by this PEP are:

* ``ssl._https_verify_certificates``: runtime configuration API
* ``ssl._https_verify_envvar``: environment based configuration
* ``ssl._cert_verification_config``: file based configuration (PEP 476 opt-in)

The marker attributes are prefixed with an underscore to indicate the
implementation dependent and security sensitive nature of these capabilities.


Feature: Configuration API
==========================

This change is proposed for inclusion in CPython 2.7.12 and later CPython 2.7.x
releases. It consists of a new ``ssl._https_verify_certificates()`` to specify
the default handling of HTTPS certificates in standard library client libraries.

It is not proposed to forward port this change to Python 3, so Python 3
applications that need to support skipping certificate verification will still
need to define their own suitable security context.

Feature detection
-----------------

The marker attribute on the ``ssl`` module related to this feature is the
``ssl._https_verify_certificates`` function itself.

Specification
-------------

The ``ssl._https_verify_certificates`` function will work as follows::

    def _https_verify_certificates(enable=True):
        """Verify server HTTPS certificates by default?"""
        global _create_default_https_context
        if enable:
            _create_default_https_context = create_default_context
        else:
            _create_default_https_context = _create_unverified_context

If called without arguments, or with ``enable`` set to a true value, then
standard library client modules will subsequently verify HTTPS certificates by default, otherwise they will skip verification.

If called with ``enable`` set to a false value, then standard library client
modules will subsequently skip verifying HTTPS certificates by default.

Security Considerations
-----------------------

The inclusion of this feature will allow security sensitive applications to
include the following forward-compatible snippet in their code::

    if hasattr(ssl, "_https_verify_certificates"):
        ssl._https_verify_certificates()

Some developers may also choose to opt out of certificate checking using
``ssl._https_verify_certificates(enable=False)``. This doesn't introduce any
major new security concerns, as monkeypatching the affected internal APIs was
already possible.


Feature: environment based configuration
========================================

This change is proposed for inclusion in CPython 2.7.12 and later CPython 2.7.x
releases. It consists of a new ``PYTHONHTTPSVERIFY`` environment variable that
can be set to ``'0'`` to disable the default verification without modifying the
application source code (which may not even be available in cases of
bytecode-only application distribution)

It is not proposed to forward port this change to Python 3, so Python 3
applications that need to support skipping certificate verification will still
need to define their own suitable security context.

Feature detection
-----------------

The marker attribute on the ``ssl`` module related to this feature is:

* the ``ssl._https_verify_envvar`` attribute, giving the name of environment
  variable affecting the default behaviour

This not only makes it straightforward to detect the presence (or absence) of
the capability, it also makes it possible to programmatically determine the
relevant environment variable name.

Specification
-------------

Rather than always defaulting to the use of ``ssl.create_default_context``,
the ``ssl`` module will be modified to:

* read the ``PYTHONHTTPSVERIFY`` environment variable when the module is first
  imported into a Python process
* set the ``ssl._create_default_https_context`` function to be an alias for
  ``ssl._create_unverified_context`` if this environment variable is present
  and set to ``'0'``
* otherwise, set the ``ssl._create_default_https_context`` function to be an
  alias for ``ssl.create_default_context`` as usual

Example implementation
----------------------

::

    _https_verify_envvar = 'PYTHONHTTPSVERIFY'

    def _get_https_context_factory():
        if not sys.flags.ignore_environment:
            config_setting = os.environ.get(_https_verify_envvar)
            if config_setting == '0':
                return _create_unverified_context
        return create_default_context

    _create_default_https_context = _get_https_context_factory()

Security Considerations
-----------------------

Relative to the behaviour in Python 3.4.3+ and Python 2.7.9->2.7.11, this
approach does introduce a new downgrade attack against the default security
settings that potentially allows a sufficiently determined attacker to revert
Python to the default behaviour used in CPython 2.7.8 and earlier releases.

This slight increase in the available attack surface is a key reason why:

* security sensitive applications should still define their own SSL context
* the migration features described in this PEP are not being added to Python 3

However, it's also worth keeping in mind that carrying out such an attack
requires the ability to modify the execution environment of a Python process
prior to the import of the ``ssl`` module. In combination with the ability
to write to any part of the filesystem (such as ``/tmp``), any attacker with
such access would already be able to modify the behaviour of the underlying
OpenSSL implementation, the dynamic library loader, and other potentially
security sensitive components.

Interaction with Python virtual environments
--------------------------------------------

The default setting is read directly from the process environment, and hence
works the same way regardless of whether or not the interpreter is being run
inside an activated Python virtual environment.


Reference Implementation
========================

A patch for Python 2.7 implementing the above two features is attached to
the `relevant tracker issue <http://bugs.python.org/issue23857>`__.


Backporting this PEP to earlier Python versions
===============================================

If this PEP is accepted, then commercial Python redistributors may choose to
backport the per-process configuration mechanisms defined in this PEP to base
versions older than Python 2.7.9, *without* also backporting PEP 476's change
to the default behaviour of the overall Python installation.

Such a backport would differ from the mechanism proposed in this PEP solely in
the default behaviour when ``PYTHONHTTPSVERIFY`` was not set at all: it would
continue to default to skipping certificate validation.

In this case, if the ``PYTHONHTTPSVERIFY`` environment variable is defined, and
set to anything *other* than ``'0'``, then HTTPS certificate verification
should be enabled.

Feature detection
-----------------

There's no specific attribute indicating that this situation applies. Rather,
it is indicated by the ``ssl._https_verify_certificates`` and
``ssl._https_verify_envvar`` attributes being present in a Python version that
is nominally older than Python 2.7.12.

Specification
-------------

Implementing this backport involves backporting the changes in PEP 466, 476 and
this PEP, with the following change to the handling of the
``PYTHONHTTPSVERIFY`` environment variable in the ``ssl`` module:

* read the ``PYTHONHTTPSVERIFY`` environment variable when the module is first
  imported into a Python process
* set the ``ssl._create_default_https_context`` function to be an alias for
  ``ssl.create_default_context`` if this environment variable is present
  and set to any value other than ``'0'``
* otherwise, set the ``ssl._create_default_https_context`` function to be an
  alias for ``ssl._create_unverified_context``

Example implementation
----------------------

::

    _https_verify_envvar = 'PYTHONHTTPSVERIFY'

    def _get_https_context_factory():
        if not sys.flags.ignore_environment:
            config_setting = os.environ.get(_https_verify_envvar)
            if config_setting != '0':
                return create_default_context
        return _create_unverified_context

    _create_default_https_context = _get_https_context_factory()

    def _disable_https_default_verification():
        """Skip verification of HTTPS certificates by default"""
        global _create_default_https_context
        _create_default_https_context = _create_unverified_context

Security Considerations
-----------------------

This change would be a strict security upgrade for any Python version that
currently defaults to skipping certificate validation in standard library
HTTPS clients. The technical trade-offs to be taken into account relate largely
to the magnitude of the PEP 466 backport also required rather than to anything
security related.

Interaction with Python virtual environments
--------------------------------------------

The default setting is read directly from the process environment, and hence
works the same way regardless of whether or not the interpreter is being run
inside an activated Python virtual environment.


Backporting PEP 476 to earlier Python versions
==============================================

The backporting approach described above leaves the default HTTPS certificate
verification behaviour of a Python 2.7 installation unmodified: verifying
certificates still needs to be opted into on a per-connection or per-process
basis.

To allow the default behaviour of the entire installation to be modified
without breaking backwards compatibility, Red Hat designed a configuration
mechanism for the system Python 2.7 installation in Red Hat Enterprise Linux
7.2+ that provides:

* an opt-in model that allows the decision to enable HTTPS certificate
  verification to be made independently of the decision to upgrade to the
  operating system version where the feature was first backported
* the ability for system administrators to set the default behaviour of Python
  applications and scripts run directly in the system Python installation
* the ability for the redistributor to consider changing the default behaviour
  of *new* installations at some point in the future without impacting existing
  installations that have been explicitly configured to skip verifying HTTPS
  certificates by default

As it only affects backports to earlier releases of Python 2.7, this change is
not proposed for inclusion in upstream CPython, but rather is offered as
a recommendation to other redistributors that choose to offer a similar feature
to their users.

This PEP doesn't take a position on whether or not this particular change is a
good idea - rather, it suggests that *if* a redistributor chooses to go down
the path of making the default behaviour configurable in a version of Python
older than Python 2.7.9, then maintaining a consistent approach across
redistributors would be beneficial for users.

However, this approach SHOULD NOT be used for any Python installation that
advertises itself as providing Python 2.7.9 or later, as most Python users
will have the reasonable expectation that all such environments will verify
HTTPS certificates by default.


Feature detection
-----------------

The marker attribute on the ``ssl`` module related to this feature is::

    _cert_verification_config = '<path to configuration file>'

This not only makes it straightforward to detect the presence (or absence) of
the capability, it also makes it possible to programmatically determine the
relevant configuration file name.


Recommended modifications to the Python standard library
--------------------------------------------------------

The recommended approach to backporting the PEP 476 modifications to an earlier
point release is to implement the following changes relative to the default
PEP 476 behaviour implemented in Python 2.7.9+:

* modify the ``ssl`` module to read a system wide configuration file when the
  module is first imported into a Python process
* define a platform default behaviour (either verifying or not verifying HTTPS
  certificates) to be used if this configuration file is not present
* support selection between the following three modes of operation:

  * ensure HTTPS certificate verification is enabled
  * ensure HTTPS certificate verification is disabled
  * delegate the decision to the redistributor providing this Python version

* set the ``ssl._create_default_https_context`` function to be an alias for
  either ``ssl.create_default_context`` or ``ssl._create_unverified_context``
  based on the given configuration setting.


Recommended file location
-------------------------

As the PEP authors are not aware of any vendors providing long-term support
releases targeting Windows, Mac OS X or \*BSD systems, this approach is
currently only specifically defined for Linux system Python installations.

The recommended configuration file name on Linux systems is
``/etc/python/cert-verification.cfg``.

The ``.cfg`` filename extension is recommended for consistency with the
``pyvenv.cfg`` used by the ``venv`` module in Python 3's standard library.


Recommended file format
-----------------------

The configuration file should use a ConfigParser ini-style format with a
single section named ``[https]`` containing one required setting ``verify``.

The suggested section name is taken from the "https" URL schema passed to
affected client APIs.

Permitted values for ``verify`` are:

* ``enable``: ensure HTTPS certificate verification is enabled by default
* ``disable``: ensure HTTPS certificate verification is disabled by default
* ``platform_default``: delegate the decision to the redistributor providing
  this particular Python version

If the ``[https]`` section or the ``verify`` setting are missing, or if the
``verify`` setting is set to an unknown value, it should be treated as if the
configuration file is not present.


Example implementation
----------------------

::

    _cert_verification_config = '/etc/python/cert-verification.cfg'

    def _get_https_context_factory():
        # Check for a system-wide override of the default behaviour
        context_factories = {
            'enable': create_default_context,
            'disable': _create_unverified_context,
            'platform_default': _create_unverified_context, # For now :)
        }
        import ConfigParser
        config = ConfigParser.RawConfigParser()
        config.read(_cert_verification_config)
        try:
            verify_mode = config.get('https', 'verify')
        except (ConfigParser.NoSectionError, ConfigParser.NoOptionError):
            verify_mode = 'platform_default'
        default_factory = context_factories.get('platform_default')
        return context_factories.get(verify_mode, default_factory)

    _create_default_https_context = _get_https_context_factory()


Security Considerations
-----------------------

The specific recommendations for this backporting case are designed to work for
privileged, security sensitive processes, even those being run in the following
locked down configuration:

* run from a locked down administrator controlled directory rather than a normal
  user directory (preventing ``sys.path[0]`` based privilege escalation attacks)
* run using the ``-E`` switch (preventing ``PYTHON*`` environment variable based
  privilege escalation attacks)
* run using the ``-s`` switch (preventing user site directory based privilege
  escalation attacks)
* run using the ``-S`` switch (preventing ``sitecustomize`` based privilege
  escalation attacks)

The intent is that the *only* reason HTTPS verification should be getting
turned off installation wide when using this approach is because:

* an end user is running a redistributor provided version of CPython rather
  than running upstream CPython directly
* that redistributor has decided to provide a smoother migration path to
  verifying HTTPS certificates by default than that being provided by the
  upstream project
* either the redistributor or the local infrastructure administrator has
  determined that it is appropriate to retain the default pre-2.7.9 behaviour
  (at least for the time being)

Using an administrator controlled configuration file rather than an environment
variable has the essential feature of providing a smoother migration path, even
for applications being run with the ``-E`` switch.

Interaction with Python virtual environments
--------------------------------------------

This setting is scoped by the interpreter installation and affects all Python
processes using that interpreter, regardless of whether or not the interpreter
is being run inside an activated Python virtual environment.

Origins of this recommendation
------------------------------

This recommendation is based on the backporting approach adopted for Red Hat
Enterprise Linux 7.2, as published in the original July 2015 draft of this PEP
and described in detail in `this KnowledgeBase article
<https://access.redhat.com/articles/2039753>`__. Red Hat's patches implementing
this backport for Python 2.7.5 can be found in the `CentOS git repository
<https://git.centos.org/commit/rpms!python.git/refs!heads!c7>`__.


Recommendation for combined feature backports
=============================================

If a redistributor chooses to backport the environment variable based
configuration setting from this PEP to a modified Python version that also
implements the configuration file based PEP 476 backport, then the environment
variable should take precedence over the system-wide configuration setting.
This allows the setting to be changed for a given user or application,
regardless of the installation-wide default behaviour.

Example implementation
----------------------

::

    _https_verify_envvar = 'PYTHONHTTPSVERIFY'
    _cert_verification_config = '/etc/python/cert-verification.cfg'

    def _get_https_context_factory():
        # Check for an environmental override of the default behaviour
        if not sys.flags.ignore_environment:
            config_setting = os.environ.get(_https_verify_envvar)
            if config_setting is not None:
                if config_setting == '0':
                    return _create_unverified_context
                return create_default_context

        # Check for a system-wide override of the default behaviour
        context_factories = {
            'enable': create_default_context,
            'disable': _create_unverified_context,
            'platform_default': _create_unverified_context, # For now :)
        }
        import ConfigParser
        config = ConfigParser.RawConfigParser()
        config.read(_cert_verification_config)
        try:
            verify_mode = config.get('https', 'verify')
        except (ConfigParser.NoSectionError, ConfigParser.NoOptionError):
            verify_mode = 'platform_default'
        default_factory = context_factories.get('platform_default')
        return context_factories.get(verify_mode, default_factory)

    _create_default_https_context = _get_https_context_factory()


Copyright
=========

This document has been placed into the public domain.


..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8