[Python-Dev] Request for pronouncement on PEP 493 (HTTPS verification backport guidance)

[Nick Coghlan]

On 17 November 2015 at 09:07, Guido van Rossum <guido at python.org> wrote:
> So I dropped the ball on this too -- I was going to either have a look
> or tell you quickly to find a BDFL-delegate, but ended up doing
> neither. Skimming it now I really don't think I'm the right person to
> review this. Maybe you could ask Alex Gaynor?

If Alex is interested, that could definitely work. I've also asked
Christian Heimes if he might be interested in handling it, as he
worked on several of the SSL module changes that were backported, and
he also works on FreeIPA [1] now, which will potentially be affected
by the changes to RHEL & CentOS 7.

Regards,
Nick.

[1] FreeIPA is an open source Identity Management & Authentication
system: http://www.freeipa.org/

>
> On Tue, Nov 10, 2015 at 4:47 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> Hi folks,
>>
>> I have a confession to make - I dropped the ball on the HTTPS
>> verification backport proposals in PEP 493, and let the upstream and
>> downstream approval processes get out of sequence.
>>
>> As a result, the RHEL 7.2 beta released back in September incorporates
>> the HTTPS verification feature backport based on the current PEP 493
>> draft, even though that hasn't formally been pronounced as an Active
>> recommendation by python-dev yet.
>>
>> Accordingly, I'm belatedly submitting it for pronouncement now:
>> https://www.python.org/dev/peps/pep-0493/
>>
>> There's currently no BDFL-Delegate assigned, so if Guido doesn't want
>> to handle it, we'll need to address that question first.
>>
>> Our last discussion back in July seemed to show that folks either
>> didn't care about the question (because they're using unmodified
>> upstream versions so the PEP didn't affect them), or else thought the
>> approach described in the PEP was reasonable, so I'm hoping the
>> consequences of my mistake won't be too severe.
>>
>> Regards,
>> Nick.
>>
>> P.S. I'm aware that this looks like presenting a fait accompli at a
>> point where it's too late to realistically say "No", but the truth is
>> that preparation for the Python in Education miniconf at PyCon
>> Australia ramped up immediately after the July discussion, and then I
>> personally got confused as to the scope of what was being included in
>> 7.2 (I mistakenly thought it was just PEP 466 for now, with 476+493
>> being deferred to a later release, but it's actually the whole package
>> of 466+476+493). That's my fault for trying to keep track of too many
>> things at once (and thus failing at some of them), not anyone else's.
>>
>> ================================
>>
>> 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.
>>
>>
>> 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
>> _______________________________________________
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>
>
>
> --
> --Guido van Rossum (python.org/~guido)



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