[Python-Dev] PEP 493: Redistributor guidance for Python 2.7 HTTPS

[Nick Coghlan]

Hi folks,

As previously discussed on python-ideas, Red Hat has been looking at
ways to provide a smoother migration path for system administrators to
get to a point where system Python installations are verifying HTTPS
by default.

While we're not proposing that these changes be implemented upstream
(given that the change in default behaviour was already implemented
for 2.7.9), we *would* like to pursue an explicit upstream
recommendation regarding how deviations from the upstream behaviour
should be handled. Otherwise we run the risk of different
redistributors pursuing mutually incompatible approaches to the
migration, which would be a rather unfortunate outcome.

The proposal in PEP 493 offers two recommendations:

* one option that leaves the default behaviour alone, but provides an
easy environment variable based way to revert to the legacy behaviour
on a per-application basis (based on a design suggested by MAL)
* a second option designed for backporting the changes to versions
that advertise themselves as older than 2.7.9 that permits opting in
to the new behaviour on a system wide basis (based on a design
suggested by Robert Kuska)

The main change from the last version discussed on python-ideas is
that in both cases there's now a required attribute that
redistributors are expected to add to the SSL module to signal the
presence of the feature (and to provide some useful information about
its implementation).

I think this is mature enough now for me to request pronouncement (if
Guido's interested in doing that himself), or volunteers to be
BDFL-Delegate (if Guido's would prefer someone else handle it).

Regards,
Nick.

======================
PEP: 493
Title: HTTPS verification recommendations for Python 2.7 redistributors
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan at gmail.com>,
        Robert Kuska <rkuska at redhat.com>,
        Marc-André Lemburg <mal at lemburg.com>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 10-May-2015
Post-History: 06-Jul-2015


Abstract
========

PEP 476 updated Python's default handling of HTTPS certificates to be
appropriate for communication over the public internet. 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 PEP provides recommendations to downstream redistributors wishing to
provide a smoother migration experience when helping their users to manage
this change in Python's default behaviour.

*Note that this PEP is not currently accepted, so it is a *proposed*
recommendation, rather than an active one.*


Rationale
=========

PEP 476 changed Python's default behaviour to better match the needs and
expectations of developers operating over the public internet, a category
which appears to include most new Python developers. It is the position of
the authors of this PEP that this was a correct decision.

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.

The long term answer for such environments is to update their internal
certificate management to at least match the standards set by the public
internet, but in the meantime, it is desirable to offer these administrators
a way to continue receiving maintenance updates to the Python 2.7 series,
without having to gate that on upgrades to their certificate management
infrastructure.

PEP 476 did attempt to address this question, by covering how to revert the
new 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 of interest to the
authors 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 two alternative approaches that redistributors
may take when addressing these problems. Redistributors may choose to implement
one, both, or neither of these approaches based on their assessment of the
needs of their particular userbase.

These designs are being proposed as a recommendation for redistributors, rather
than as new upstream features, as they are needed purely to support legacy
environments migrating from older versions of Python 2.7. Neither approach
is being proposed as an upstream Python 2.7 feature, nor as a feature in any
version of Python 3 (whether published directly by the Python Software
Foundation or by a redistributor).


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

As these recommendations are intended to cover backports to earlier Python
versions, the Python version number cannot be used as a reliable means for
detecting them. Instead, the recommendations are defined 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 marker attributes are prefixed with an underscore to indicate the
implementation dependent nature of these capabilities - not all Python
distributions will offer them, only those that are providing a multi-stage
migration process from the legacy HTTPS handling to the new default behaviour.


Recommendation for an environment variable based security downgrade
===================================================================

Some redistributors may wish to provide a per-application option to disable
certificate verification in selected applications that run on or embed CPython
without needing to modify the application itself.

In these cases, a configuration mechanism is needed that provides:

* an opt-out model that allows certificate verification to be selectively
  turned off for particular applications after upgrading to a version of
  Python that verifies certificates by default
* the ability for all users to configure this setting on a per-application
  basis, rather than on a per-system, or per-Python-installation basis

This approach may be used for any redistributor provided version of Python 2.7,
including those that advertise themselves as providing Python 2.7.9 or later.


Required marker attribute
-------------------------

The required marker attribute on the ``ssl`` module when implementing this
recommendation is::

    _https_verify_envvar = 'PYTHONHTTPSVERIFY'

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.


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

The recommended approach to providing a per-application configuration setting
for HTTPS certificate verification that doesn't require modifications to the
application itself is to:

* modify the ``ssl`` module 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():
        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 an unmodified version of CPython 2.7.9 or later, 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 vulnerable configuration used in CPython 2.7.8 and earlier releases.
However, such an attack requires the ability to modify the execution
environment of a Python process prior to the import of the ``ssl`` module,
and any attacker with such access would already be able to modify the
behaviour of the underlying OpenSSL implementation.


Recommendation for backporting to earlier Python versions
=========================================================

Some redistributors, most notably Linux distributions, may choose to backport
the PEP 476 HTTPS verification changes to modified Python versions based on
earlier Python 2 maintenance releases. In these cases, a configuration
mechanism is needed 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
  Python 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

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 validate HTTPS
certificates by default.


Required marker attribute
-------------------------

The required marker attribute on the ``ssl`` module when implementing this
recommendation 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
-------------------------

This approach is currently only defined for \*nix system Python installations.

The recommended configuration file name 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``.

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 the 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 system 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 override the default upstream 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.


Combining the recommendations
=============================

If a redistributor chooses to implement both recommendations, then the
environment variable should take precedence over the system-wide configuration
setting. This allows the setting to be changed for a given user, virtual
environment or application, regardless of the system-wide default behaviour.

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

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

::

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

    def _get_https_context_factory():
        # Check for am environmental override of the default behaviour
        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.


-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia