[Python-Dev] PEP 466: Network security enhancements for Python 2.7.7

Nick Coghlan ncoghlan at gmail.com
Thu Apr 17 00:04:12 CEST 2014 

I've reworded the PEP to make it clear it is now just about
backporting a specific set of enhancements to 2.7.7, as well as
switching to updating to new OpenSSL feature releases in the binary
installers.

The idea of an open ended backport policy is now listed as a rejected
variant. I believe that change addresses Guido's main remaining
concern, so I think this version is ready for pronouncement.

Regards,
Nick.

========================================
PEP: 466
Title: Network Security Enhancements for Python 2.7.7
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan at gmail.com>,
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 23-Mar-2014
Post-History: 23-Mar-2014, 24-Mar-2014, 25-Mar-2014, 26-Mar-2014, 16-Apr-2014


Abstract
========

Most CPython tracker issues are classified as errors in behaviour or
proposed enhancements. Most patches to fix behavioural errors are
applied to all active maintenance branches.  Enhancement patches are
restricted to the default branch that becomes the next Python version.

This cadence works reasonably well during Python's normal 18-24 month
feature release cycle, which is still applicable to the Python 3 series.
However, the age of the standard library in Python 2 has now reached a point
where it is sufficiently far behind the state of the art in network security
protocols for it to be causing real problems in use cases where upgrading to
Python 3 in the near term may not be feasible.

In recognition of the additional practical considerations that have arisen
during the 4+ year maintenance cycle for Python 2.7, this PEP allows a
critical set of network security related features to be backported from
Python 3.4 to the upcoming Python 2.7.7 maintenance release.

While this PEP does not make any changes to the core development team's
handling of security-fix-only branches that are no longer in active
maintenance, it *does* recommend that commercial redistributors providing
extended support periods for the Python standard library either backport
these features to their supported versions, or else explicitly disclaim
support for the use of older versions in roles that involve connecting
directly to the public internet.


New security related features in Python 2.7.7
=============================================

Under this proposal, the following features will be backported from Python
3.4 to the upcoming Python 2.7.7 maintenance release:

* in the ``os`` module:

  * persistent file descriptor for ``os.urandom()``.

* in the ``hmac`` module:

  * constant time comparison function (``hmac.compare_digest()``).

* in the ``hashlib`` module:

  * password hashing function (``hashlib.pbkdf2_hmac()``).
  * details of hash algorithm availability (``hashlib.algorithms_guaranteed``
    and ``hashlib.algorithms_available``).

* in the ``ssl`` module:

  * this module is almost entirely synchronised with its Python 3
    counterpart, bringing TLSv1.x settings, SSLContext manipulation, Server
    Name Indication, access to platform certificate stores, standard
    library support for peer hostname validation and more to the Python 2
    series.
  * the only ``ssl`` module features *not* backported under this policy are
    the ``ssl.RAND_*`` functions that provide access to OpenSSL's random
    number generation capabilities - use ``os.urandom()`` instead.

As a general change in maintenance policy, permission is also granted to
upgrade to newer feature releases of OpenSSL when preparing the binary
installers for new maintenance releases of Python 2.7.

This PEP does NOT propose a general exception for backporting new features
to Python 2.7 - every new feature proposed for backporting will still need
to be justified independently. In particular, it will need to be explained
why relying on and independently updated backport on the Python Package Index
instead is not an acceptable solution.


Backwards compatibility considerations
======================================

As in the Python 3 series, the backported ``ssl.create_default_context()``
API is granted a backwards compatibility exemption that permits the
protocol, options, cipher and other settings of the created SSL context to
be updated in maintenance releases to use higher default security settings.
This allows them to appropriately balance compatibility and security at the
time of the maintenance release, rather than at the time of the original
feature release.

This PEP does *not* grant any other exemptions to the usual backwards
compatibility policy for maintenance releases. Instead, by explicitly
encouraging the use of feature based checks, it is designed to make it easier
to write more secure cross-version compatible Python software, while still
limiting the risk of breaking currently working software when upgrading to
a new Python 2.7 maintenance release.

In all cases where this proposal allows new features to be backported to
the Python 2.7 release series, it is possible to write cross-version
compatible code that operates by "feature detection" (for example, checking
for particular attributes in a module), without needing to explicitly check
the Python version.

It is then up to library and framework code to provide an appropriate warning
and fallback behaviour if a desired feature is found to be missing. While
some especially security sensitive software MAY fail outright if a desired
security feature is unavailable, most software SHOULD instead emit a warning
and continue operating using a slightly degraded security configuration.

The backported APIs allow library and application code to perform the
following actions after detecting the presence of a relevant
network security related feature:

* explicitly opt in to more secure settings (to allow the use of enhanced
  security features in older maintenance releases of Python with less
  secure default behaviour)
* explicitly opt in to less secure settings (to allow the use of newer Python
  feature releases in lower security environments)
* determine the default setting for the feature (this MAY require explicit
  Python version checks to determine the Python feature release, but DOES
  NOT require checking for a specific maintenance release)

Security related changes to other modules (such as higher level networking
libraries and data format processing libraries) will continue to be made
available as backports and new modules on the Python Package Index, as
independent distribution remains the preferred approach to handling
software that must continue to evolve to handle changing development
requirements independently of the Python 2 standard library. Refer to
the `Motivation and Rationale`_ section for a review of the characteristics
that make the secure networking infrastructure worthy of special
consideration.


OpenSSL compatibility
---------------------

Under this proposal, OpenSSL may be upgraded to more recent feature releases
in Python 2.7 maintenance releases. On Linux and most other POSIX systems,
the specific version of OpenSSL used already varies, as CPython dynamically
links to the system provided OpenSSL library by default.

For the Windows binary installers, the ``_ssl`` and ``_hashlib`` modules are
statically linked with OpenSSL and the associated symbols are not exported.
Marc-Andre Lemburg indicates that updating to newer OpenSSL releases in the
``egenix-pyopenssl`` binaries has not resulted in any reported compatibility
issues [3]_

The Mac OS X binary installers historically followed the same policy as
other POSIX installations and dynamically linked to the Apple provided
OpenSSL libraries. However, Apple has now ceased updating these
cross-platform libraries, instead requiring that even cross-platform
developers adopt Mac OS X specific interfaces to access up to date security
infrastructure on their platform. Accordingly, and independently of this
PEP, the Mac OS X binary installers were already going to be switched to
statically linker newer versions of OpenSSL [4]_


Other Considerations
====================

Maintainability
---------------

A number of developers, including Alex Gaynor and Donald Stufft, have
expressed interest in carrying out the feature backports covered by this
policy, and assisting with any additional maintenance burdens that arise
in the Python 2 series as a result.

Steve Dower and Brian Curtin have offered to help with the creation of the
Windows installers, allowing Martin von Löwis the opportunity to step back
from the task of maintaining the 2.7 Windows installer.

This PEP is primarily about establishing the consensus needed to allow them
to carry out this work. For other core developers, this policy change
shouldn't impose any additional effort beyond potentially reviewing the
resulting patches for those developers specifically interested in the
affected modules.


Security releases
-----------------

This PEP does not propose any changes to the handling of security
releases - those will continue to be source only releases that
include only critical security fixes.

However, the recommendations for library and application developers are
deliberately designed to accommodate commercial redistributors that choose
to apply these changes to additional Python release series that are either
in security fix only mode, or have been declared "end of life" by the core
development team.

Whether or not redistributors choose to exercise that option will be up
to the individual redistributor.


Integration testing
-------------------

Third party integration testing services should offer users the ability
to test against multiple Python 2.7 maintenance releases (at least 2.7.6
and 2.7.7+), to ensure that libraries, frameworks and applications can still
test their handling of the legacy security infrastructure correctly (either
failing or degrading gracefully, depending on the security sensitivity of
the software), even after the features covered in this proposal have been
backported to the Python 2.7 series.


Handling lower security environments with low risk tolerance
------------------------------------------------------------

For better or for worse (mostly worse), there are some environments where
the risk of latent security defects is more tolerated than even a slightly
increased risk of regressions in maintenance releases. This proposal largely
excludes these environments from consideration where the modules covered by
the exemption are concerned - this approach is entirely inappropriate for
software connected to the public internet, and defence in depth security
principles suggest that it is not appropriate for most private networks
either.

Downstream redistributors may still choose to cater to such environments,
but they will need to handle the process of downgrading the security
related modules and doing the associated regression testing themselves.
The main CPython continuous integration infrastructure will not cover this
scenario.


Motivation and Rationale
========================

The creation of this PEP was prompted primarily by the aging SSL support in
the Python 2 series. As of March 2014, the Python 2.7 SSL module is
approaching four years of age, and the SSL support in the still popular
Python 2.6 release had its feature set locked six years ago.

These are simply too old to provide a foundation that can be recommended
in good conscience for secure networking software that operates over the
public internet, especially in an era where it is becoming quite clearly
evident that advanced persistent security threats are even more widespread
and more indiscriminate in their targeting than had previously been
understood. While they represented reasonable security infrastructure in
their time, the state of the art has moved on, and we need to investigate
mechanisms for effectively providing more up to date network security
infrastructure for users that, for whatever reason, are not currently in
a position to migrate to Python 3.

While the use of the system OpenSSL installation addresses many of these
concerns on Linux platforms, it doesn't address all of them (in particular,
it is still difficult for sotware to explicitly require some higher level
security settings). The standard library support can be bypassed by using a
third party library like PyOpenSSL or Pycurl, but this still results in a
security problem, as these can be difficult dependencies to deploy, and many
users will remain unaware that they might want them. Rather than explaining
to potentially naive users how to obtain and use these libraries, it seems
better to just fix the included batteries.

In the case of the binary installers for Windows and Mac OS X that are
published on python.org, the version of OpenSSL used is entirely within
the control of the Python core development team, but is currently limited
to OpenSSL maintenance releases for the version initially shipped with the
corresponding Python feature release.

With increased popularity comes increased responsibility, and this proposal
aims to acknowledge the fact that Python's popularity and adoption is at a
sufficiently high level that some of our design and policy decisions have
significant implications beyond the Python development community.

As one example, the Python 2 ``ssl`` module does not support the Server
Name Indication standard. While it is possible to obtain SNI support
by using the third party ``requests`` client library, actually doing so
currently requires using not only ``requests`` and its embedded dependencies,
but also half a dozen or more additional libraries. The lack of support
in the Python 2 series thus serves as an impediment to making effective
use of SNI on servers, as Python 2 clients will frequently fail to handle
it correctly.

Another more critical example is the lack of SSL hostname matching in the
Python 2 standard library - it is currently necessary to rely on a third
party library, such as ``requests`` or ``backports.ssl_match_hostname`` to
obtain that functionality in Python 2.

The Python 2 series also remains more vulnerable to remote timing attacks
on security sensitive comparisons than the Python 3 series, as it lacks a
standard library equivalent to the timing attack resistant
``hmac.compare_digest()`` function. While appropriate secure comparison
functions can be implemented in third party extensions, many users don't
even consider the issue and use ordinary equality comparisons instead
- while a standard library solution doesn't automatically fix that problem,
it *does* make the barrier to resolution much lower once the problem is
pointed out.

Python 2.7 represents the only long term maintenance release the core
development team has provided, and it is natural that there will be things
that worked over a historically shorter maintenance lifespan that don't work
over this longer support period. In the specific case of the problem
described in this PEP, the simplest available solution is to acknowledge
that long term maintenance of network security related modules *requires*
the ability to add new features, even while retaining backwards compatibility
for existing interfaces.

For those familiar with it, it is worth comparing the approach described in
this PEP with Red Hat's handling of its long term open source support
commitments: it isn't the RHEL 6.0 release itself that receives 10 years
worth of support, but the overall RHEL 6 *series*. The individual RHEL 6.x
point releases within the series then receive a wide variety of new
features, including security enhancements, all while meeting strict
backwards compatibility guarantees for existing software. The proposal
covered in this PEP brings our approach to long term maintenance more into
line with this precedent - we retain our strict backwards compatibility
requirements, but make an exception to the restriction against adding new
features.

To date, downstream redistributors have respected our upstream policy of
"no new features in Python maintenance releases". This PEP explicitly
accepts that a more nuanced policy is appropriate in the case of network
security related features, and the specific change it describes is
deliberately designed such that it is potentially suitable for Red Hat
Enterprise Linux and its downstream derivatives.


Rejected alternative: just advise developers to migrate to Python 3
-------------------------------------------------------------------

This alternative represents the status quo. Unfortunately, it has proven
to be unworkable in practice, as the backwards compatibility implications
mean that this is a non-trivial migration process for large applications
and integration projects. While the tools for migration have evolved to
a point where it is possible to migrate even large applications
opportunistically and incrementally (rather than all at once) by updating
code to run in the large common subset of Python 2 and Python 3, using the
most recent technology often isn't a priority in commercial environments.

Previously, this was considered an acceptable harm, as while it was an
unfortunate problem for the affected developers to have to face, it was
seen as an issue between them and their management chain to make the case
for infrastructure modernisation, and this case would become naturally
more compelling as the Python 3 series evolved.

However, now that we're fully aware of the impact the limitations of the
Python 2 standard library may be having on the evolution of internet
security standards, I no longer believe that it is reasonable to expect
platform and application developers to resolve all of the latent defects
in an application's Unicode correctness solely in order to gain access to
the network security enhancements already available in Python 3.

While Ubuntu (and to some extent Debian as well) are committed to porting all
default system services and scripts to Python 3, and to removing Python 2
from its default distribution images (but not from its archives), this is
a mammoth task and won't be completed for the Ubuntu 14.04 LTS release
(at least for the desktop image - it may be achieved for the mobile and
server images).

Fedora has even more work to do to migrate, and it will take a non-trivial
amount of time to migrate the relevant infrastructure components. While
Red Hat are also actively working to make it easier for users to use more
recent versions of Python on our stable platforms, it's going to take time
for those efforts to start having an impact on end users' choice of version,
and any such changes also don't benefit the core platform infrastructure
that runs in the integrated system Python by necessity.

The OpenStack migration to Python 3 is also still in its infancy, and even
though that's a project with an extensive and relatively robust automated
test suite, it's still large enough that it is going to take quite some time
to migrate fully to a Python 2/3 compatible code base.

And that's just three of the highest profile open source projects that
make heavy use of Python. Given the likely existence of large amounts of
legacy code that lacks the kind of automated regression test suite needed
to help support a migration from Python 2 to Python 3, there are likely to
be many cases where reimplementation (perhaps even in Python 3) proves
easier than migration. The key point of this PEP is that those situations
affect more people than just the developers and users of the affected
application: the existence of clients and servers with outdated network
security infrastructure becomes something that developers of secure
networked services need to take into account as part of their security
design, and that's a problem that inhibits the adoption of better security
standards.

As Terry Reedy noted, if we try to persist with the status quo, the likely
outcome is that commercial redistributors will attempt to do something
like this on behalf of their customers *anyway*, but in a potentially
inconsistent and ad hoc manner. By drawing the scope definition process
into the upstream project we are in a better position to influence the
approach taken to address the situation and to help ensure some consistency
across redistributors.

The problem is real, so *something* needs to change, and this PEP describes
my preferred approach to addressing the situation.


Rejected alternative: create and release Python 2.8
---------------------------------------------------

With sufficient corporate support, it likely *would* be possible to create
and release Python 2.8 (it's highly unlikely such a project would garner
enough interest to be achievable with only volunteers). However, this
wouldn't actually solve the problem, as the aim is to provide a *relatively
low impact* way to incorporate enhanced security features into integrated
products and deployments that make use of Python 2.

Upgrading to a new Python feature release would mean both more work for the
core development team, as well as a more disruptive update that most
potential end users would likely just skip entirely.

Attempting to create a Python 2.8 release would also bring in suggestions
to backport many additional features from Python 3 (such as ``tracemalloc``
and the improved coroutine support), making the migration from Python 2.7
to this hypothetical 2.8 release even riskier and more disruptive.

This is not a recommended approach, as it would involve substantial
additional work for a result that is actually less effective in achieving
the original aim (which is to eliminate the current widespread use of the
aging network security infrastructure in the Python 2 series).

Furthermore, while I can't make any commitments to actually addressing
this issue on Red Hat platforms, I *can* categorically rule out the idea
of a Python 2.8 being of any use to me in even attempting to get it
addressed.


Rejected alternative: distribute the security enhancements via PyPI
-------------------------------------------------------------------

While this initially appears to be an attractive and easier to manage
approach, it actually suffers from several significant problems.

Firstly, this is complex, low level, cross-platform code that integrates
with the underlying operating system across a variety of POSIX platforms
(including Mac OS X) and Windows. The CPython BuildBot fleet is already set
up to handle continuous integration in that context, but most of the
freely available continuous integration services just offer Linux, and
perhaps paid access to Windows. Those services work reasonably well for
software that largely runs on the abstraction layers offered by Python and
other dynamic languages, as well as the more comprehensive abstraction
offered by the JVM, but won't suffice for the kind of code involved here.

The OpenSSL dependency for the network security support also qualifies as
the kind of "complex binary dependency" that isn't yet handled well by the
``pip`` based software distribution ecosystem. Relying on a third party
binary dependency also creates potential compatibility problems for ``pip``
when running on other interpreters like ``PyPy``.

Another practical problem with the idea is the fact that ``pip`` itself
relies on the ``ssl`` support in the standard library (with some additional
support from a bundled copy of ``requests``, which in turn bundles
``backport.ssl_match_hostname``), and hence would require any replacement
module to also be bundled within ``pip``. This wouldn't pose any
insurmountable difficulties (it's just another dependency to vendor), but
it *would* mean yet another copy of OpenSSL to keep up to date.

This approach also has the same flaw as all other "improve security by
renaming things" approaches: they completely miss the users who most need
help, and raise significant barriers against being able to encourage users
to do the right thing when their infrastructure supports it (since
"use this other module" is a much higher impact change than "turn on this
higher security setting"). Deprecating the aging SSL infrastructure in the
standard library in favour of an external module would be even more user
hostile than accepting the slightly increased risk of regressions associated
with upgrading it in place.

Last, but certainly not least, this approach suffers from the same problem
as the idea of doing a Python 2.8 release: likely not solving the actual
problem. Commercial redistributors of Python are set up to redistribute
*Python*, and a pre-existing set of additional packages. Getting new
packages added to the pre-existing set *can* be done, but means approaching
each and every redistributor and asking them to update their
repackaging process accordingly. By contrast, the approach described in
this PEP would require redistributors to deliberately *opt out* of the
security enhancements by deliberately downgrading the provided network
security infrastructure, which most of them are unlikely to do.


Rejected variant: provide a "legacy SSL infrastructure" branch
--------------------------------------------------------------

Earlier versions of this PEP included the concept of a ``2.7-legacy-ssl``
branch that preserved the exact feature set of the Python 2.7.6 network
security infrastructure.

In my opinion, anyone that actually wants this is almost certainly making a
mistake, and if they insist they really do want it in their specific
situation, they're welcome to either make it themselves or arrange for a
downstream redistributor to make it for them.

If they are made publicly available, any such rebuilds should be referred to
as "Python 2.7 with Legacy SSL" to clearly distinguish them from the official
Python 2.7 releases that include more up to date network security
infrastructure.

After the first Python 2.7 maintenance release that implements this PEP, it
would also be appropriate to refer to Python 2.7.6 and earlier releases as
"Python 2.7 with Legacy SSL".


Rejected variant: synchronise particular modules entirely with Python 3
-----------------------------------------------------------------------

Earlier versions of this PEP suggested synchronising the ``hmac``,
``hashlib`` and ``ssl`` modules entirely with their Python 3 counterparts.

This approach proved too vague to build a compelling case for the exception,
and has thus been replaced by the current more explicit proposal.


Rejected variant: open ended backport policy
--------------------------------------------

Earlier versions of this PEP suggested a general policy change related to
future Python 3 enhancements that impact the general security of the
internet.

That approach created unnecessary uncertainty, so it has been simplified to
propose backport a specific concrete set of changes. Future feature
backport proposals can refer back to this PEP as precedent, but it will
still be necessary to make a specific case for each feature addition to
the Python 2.7 long term support release.


Disclosure of Interest
======================

The author of this PEP currently works for Red Hat on test automation tools.
If this proposal is accepted, I will be strongly encouraging Red Hat to take
advantage of the resulting opportunity to help improve the overall security
of the Python ecosystem. However, I do not speak for Red Hat in this matter,
and cannot make any commitments on Red Hat's behalf.


Acknowledgements
================

Thanks to Christian Heimes and other for their efforts in greatly improving
Python's SSL support in the Python 3 series, and a variety of members of
the Python community for helping me to better understand the implications
of the default settings we provide in our SSL modules, and the impact that
tolerating the use of SSL infrastructure that was defined in 2010
(Python 2.7) or even 2008 (Python 2.6) potentially has for the security
of the web as a whole.

Thanks to Donald Stufft and Alex Gaynor for identifying a more limited set
of essential security features that allowed the proposal to be made more
fine-grained than backporting entire modules from Python 3.4 [7,8]_.

Christian and Donald also provided valuable feedback on a preliminary
draft of this proposal.

Thanks also to participants in the python-dev mailing list threads
[1,2,5,6]_, as well as the various folks I discussed this issue with at
PyCon 2014 in Montreal.


References
==========

.. [1] PEP 466 discussion (round 1)
   (https://mail.python.org/pipermail/python-dev/2014-March/133334.html)

.. [2] PEP 466 discussion (round 2)
   (https://mail.python.org/pipermail/python-dev/2014-March/133389.html)

.. [3] Marc-Andre Lemburg's OpenSSL feedback for Windows
   (https://mail.python.org/pipermail/python-dev/2014-March/133438.html)

.. [4] Ned Deily's OpenSSL feedback for Mac OS X
   (https://mail.python.org/pipermail/python-dev/2014-March/133347.html)

.. [5] PEP 466 discussion (round 3)
   (https://mail.python.org/pipermail/python-dev/2014-March/133442.html)

.. [6] PEP 466 discussion (round 4)
   (https://mail.python.org/pipermail/python-dev/2014-March/133472.html)

.. [7] Donald Stufft's recommended set of backported features
   (https://mail.python.org/pipermail/python-dev/2014-March/133500.html)

.. [8] Alex Gaynor's recommended set of backported features
   (https://mail.python.org/pipermail/python-dev/2014-March/133503.html)



Copyright
=========

This document has been placed in the public domain.

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

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