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<h1 class="page-title">PEP 466 – Network Security Enhancements for Python 2.7.x</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Alyssa Coghlan <ncoghlan at gmail.com></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd>
|
||
<dt class="field-odd">Type<span class="colon">:</span></dt>
|
||
<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
|
||
<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">23-Mar-2014</dd>
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||
<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.7.9</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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||
<dd class="field-even">23-Mar-2014, 24-Mar-2014, 25-Mar-2014, 26-Mar-2014, 16-Apr-2014</dd>
|
||
<dt class="field-odd">Resolution<span class="colon">:</span></dt>
|
||
<dd class="field-odd"><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-April/134163.html">Python-Dev message</a></dd>
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||
</dl>
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<hr class="docutils" />
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<section id="contents">
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||
<details><summary>Table of Contents</summary><ul class="simple">
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#new-security-related-features-in-python-2-7-maintenance-releases">New security related features in Python 2.7 maintenance releases</a></li>
|
||
<li><a class="reference internal" href="#implementation-status">Implementation status</a></li>
|
||
<li><a class="reference internal" href="#backwards-compatibility-considerations">Backwards compatibility considerations</a><ul>
|
||
<li><a class="reference internal" href="#openssl-compatibility">OpenSSL compatibility</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#other-considerations">Other Considerations</a><ul>
|
||
<li><a class="reference internal" href="#maintainability">Maintainability</a></li>
|
||
<li><a class="reference internal" href="#security-releases">Security releases</a></li>
|
||
<li><a class="reference internal" href="#integration-testing">Integration testing</a></li>
|
||
<li><a class="reference internal" href="#handling-lower-security-environments-with-low-risk-tolerance">Handling lower security environments with low risk tolerance</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#motivation-and-rationale">Motivation and Rationale</a><ul>
|
||
<li><a class="reference internal" href="#why-these-particular-changes">Why these particular changes?</a></li>
|
||
<li><a class="reference internal" href="#rejected-alternative-just-advise-developers-to-migrate-to-python-3">Rejected alternative: just advise developers to migrate to Python 3</a></li>
|
||
<li><a class="reference internal" href="#rejected-alternative-create-and-release-python-2-8">Rejected alternative: create and release Python 2.8</a></li>
|
||
<li><a class="reference internal" href="#rejected-alternative-distribute-the-security-enhancements-via-pypi">Rejected alternative: distribute the security enhancements via PyPI</a></li>
|
||
<li><a class="reference internal" href="#rejected-variant-provide-a-legacy-ssl-infrastructure-branch">Rejected variant: provide a “legacy SSL infrastructure” branch</a></li>
|
||
<li><a class="reference internal" href="#rejected-variant-synchronise-particular-modules-entirely-with-python-3">Rejected variant: synchronise particular modules entirely with Python 3</a></li>
|
||
<li><a class="reference internal" href="#rejected-variant-open-ended-backport-policy">Rejected variant: open ended backport policy</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#disclosure-of-interest">Disclosure of Interest</a></li>
|
||
<li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
</details></section>
|
||
<section id="abstract">
|
||
<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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 upcoming Python 2.7.x maintenance releases.</p>
|
||
<p>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 <em>does</em> 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.</p>
|
||
</section>
|
||
<section id="new-security-related-features-in-python-2-7-maintenance-releases">
|
||
<h2><a class="toc-backref" href="#new-security-related-features-in-python-2-7-maintenance-releases" role="doc-backlink">New security related features in Python 2.7 maintenance releases</a></h2>
|
||
<p>Under this proposal, the following features will be backported from Python
|
||
3.4 to upcoming Python 2.7.x maintenance releases:</p>
|
||
<ul class="simple">
|
||
<li>in the <code class="docutils literal notranslate"><span class="pre">os</span></code> module:<ul>
|
||
<li>persistent file descriptor for <code class="docutils literal notranslate"><span class="pre">os.urandom()</span></code>.</li>
|
||
</ul>
|
||
</li>
|
||
<li>in the <code class="docutils literal notranslate"><span class="pre">hmac</span></code> module:<ul>
|
||
<li>constant time comparison function (<code class="docutils literal notranslate"><span class="pre">hmac.compare_digest()</span></code>).</li>
|
||
</ul>
|
||
</li>
|
||
<li>in the <code class="docutils literal notranslate"><span class="pre">hashlib</span></code> module:<ul>
|
||
<li>password hashing function (<code class="docutils literal notranslate"><span class="pre">hashlib.pbkdf2_hmac()</span></code>).</li>
|
||
<li>details of hash algorithm availability (<code class="docutils literal notranslate"><span class="pre">hashlib.algorithms_guaranteed</span></code>
|
||
and <code class="docutils literal notranslate"><span class="pre">hashlib.algorithms_available</span></code>).</li>
|
||
</ul>
|
||
</li>
|
||
<li>in the <code class="docutils literal notranslate"><span class="pre">ssl</span></code> module:<ul>
|
||
<li>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.</li>
|
||
<li>the only <code class="docutils literal notranslate"><span class="pre">ssl</span></code> module features <em>not</em> backported under this policy are
|
||
the <code class="docutils literal notranslate"><span class="pre">ssl.RAND_*</span></code> functions that provide access to OpenSSL’s random
|
||
number generation capabilities - use <code class="docutils literal notranslate"><span class="pre">os.urandom()</span></code> instead.</li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
<p>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.</p>
|
||
<p>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 an independently updated backport on the Python Package Index
|
||
instead is not an acceptable solution.</p>
|
||
</section>
|
||
<section id="implementation-status">
|
||
<h2><a class="toc-backref" href="#implementation-status" role="doc-backlink">Implementation status</a></h2>
|
||
<p>This PEP originally proposed adding all listed features to the Python 2.7.7
|
||
maintenance release. That approach proved to be too ambitious given the
|
||
limited time frame between the original creation and acceptance of the PEP
|
||
and the release of Python 2.7.7rc1. Instead, the progress of each individual
|
||
accepted feature backport is being tracked as an independent enhancement
|
||
targeting Python 2.7.</p>
|
||
<p>Implemented for Python 2.7.7:</p>
|
||
<ul class="simple">
|
||
<li><a class="reference external" href="http://bugs.python.org/issue21306">Issue #21306</a>: backport <code class="docutils literal notranslate"><span class="pre">hmac.compare_digest</span></code></li>
|
||
<li><a class="reference external" href="http://bugs.python.org/issue21462">Issue #21462</a>: upgrade OpenSSL in the Python 2.7 Windows installers</li>
|
||
</ul>
|
||
<p>Implemented for Python 2.7.8:</p>
|
||
<ul class="simple">
|
||
<li><a class="reference external" href="http://bugs.python.org/issue21304">Issue #21304</a>: backport <code class="docutils literal notranslate"><span class="pre">hashlib.pbkdf2</span></code></li>
|
||
</ul>
|
||
<p>Implemented for Python 2.7.9 (in development):</p>
|
||
<ul class="simple">
|
||
<li><a class="reference external" href="http://bugs.python.org/issue21308">Issue #21308</a>: backport specified <code class="docutils literal notranslate"><span class="pre">ssl</span></code> module features</li>
|
||
<li><a class="reference external" href="http://bugs.python.org/issue21307">Issue #21307</a>: backport remaining specified <code class="docutils literal notranslate"><span class="pre">hashlib</span></code> module features</li>
|
||
<li><a class="reference external" href="http://bugs.python.org/issue21305">Issue #21305</a>: backport <code class="docutils literal notranslate"><span class="pre">os.urandom</span></code> shared file descriptor change</li>
|
||
</ul>
|
||
</section>
|
||
<section id="backwards-compatibility-considerations">
|
||
<h2><a class="toc-backref" href="#backwards-compatibility-considerations" role="doc-backlink">Backwards compatibility considerations</a></h2>
|
||
<p>As in the Python 3 series, the backported <code class="docutils literal notranslate"><span class="pre">ssl.create_default_context()</span></code>
|
||
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.</p>
|
||
<p>This PEP does <em>not</em> 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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>The backported APIs allow library and application code to perform the
|
||
following actions after detecting the presence of a relevant
|
||
network security related feature:</p>
|
||
<ul class="simple">
|
||
<li>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)</li>
|
||
<li>explicitly opt in to less secure settings (to allow the use of newer Python
|
||
feature releases in lower security environments)</li>
|
||
<li>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)</li>
|
||
</ul>
|
||
<p>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 <a class="reference internal" href="#motivation-and-rationale">Motivation and Rationale</a> section for a review of the characteristics
|
||
that make the secure networking infrastructure worthy of special
|
||
consideration.</p>
|
||
<section id="openssl-compatibility">
|
||
<h3><a class="toc-backref" href="#openssl-compatibility" role="doc-backlink">OpenSSL compatibility</a></h3>
|
||
<p>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.</p>
|
||
<p>For the Windows binary installers, the <code class="docutils literal notranslate"><span class="pre">_ssl</span></code> and <code class="docutils literal notranslate"><span class="pre">_hashlib</span></code> 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
|
||
<code class="docutils literal notranslate"><span class="pre">egenix-pyopenssl</span></code> binaries has not resulted in any reported compatibility
|
||
issues <a class="footnote-reference brackets" href="#id11" id="id1">[3]</a></p>
|
||
<p>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 <a class="footnote-reference brackets" href="#id12" id="id2">[4]</a></p>
|
||
</section>
|
||
</section>
|
||
<section id="other-considerations">
|
||
<h2><a class="toc-backref" href="#other-considerations" role="doc-backlink">Other Considerations</a></h2>
|
||
<section id="maintainability">
|
||
<h3><a class="toc-backref" href="#maintainability" role="doc-backlink">Maintainability</a></h3>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
</section>
|
||
<section id="security-releases">
|
||
<h3><a class="toc-backref" href="#security-releases" role="doc-backlink">Security releases</a></h3>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>Whether or not redistributors choose to exercise that option will be up
|
||
to the individual redistributor.</p>
|
||
</section>
|
||
<section id="integration-testing">
|
||
<h3><a class="toc-backref" href="#integration-testing" role="doc-backlink">Integration testing</a></h3>
|
||
<p>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.</p>
|
||
</section>
|
||
<section id="handling-lower-security-environments-with-low-risk-tolerance">
|
||
<h3><a class="toc-backref" href="#handling-lower-security-environments-with-low-risk-tolerance" role="doc-backlink">Handling lower security environments with low risk tolerance</a></h3>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
</section>
|
||
</section>
|
||
<section id="motivation-and-rationale">
|
||
<h2><a class="toc-backref" href="#motivation-and-rationale" role="doc-backlink">Motivation and Rationale</a></h2>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>As one example, the Python 2 <code class="docutils literal notranslate"><span class="pre">ssl</span></code> module does not support the Server
|
||
Name Indication standard. While it is possible to obtain SNI support
|
||
by using the third party <code class="docutils literal notranslate"><span class="pre">requests</span></code> client library, actually doing so
|
||
currently requires using not only <code class="docutils literal notranslate"><span class="pre">requests</span></code> 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.</p>
|
||
<p>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 <code class="docutils literal notranslate"><span class="pre">requests</span></code> or <code class="docutils literal notranslate"><span class="pre">backports.ssl_match_hostname</span></code> to
|
||
obtain that functionality in Python 2.</p>
|
||
<p>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
|
||
<code class="docutils literal notranslate"><span class="pre">hmac.compare_digest()</span></code> 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 <em>does</em> make the barrier to resolution much lower once the problem is
|
||
pointed out.</p>
|
||
<p>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 <em>requires</em>
|
||
the ability to add new features, even while retaining backwards compatibility
|
||
for existing interfaces.</p>
|
||
<p>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 <em>series</em>. 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.</p>
|
||
<p>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.</p>
|
||
<section id="why-these-particular-changes">
|
||
<h3><a class="toc-backref" href="#why-these-particular-changes" role="doc-backlink">Why these particular changes?</a></h3>
|
||
<p>The key requirement for a feature to be considered for inclusion in this
|
||
proposal was that it must have security implications <em>beyond</em> the specific
|
||
application that is written in Python and the system that application is
|
||
running on. Thus the focus on network security protocols, password storage
|
||
and related cryptographic infrastructure - Python is a popular choice for
|
||
the development of web services and clients, and thus the capabilities of
|
||
widely used Python versions have implications for the security design of
|
||
other services that may themselves be using newer versions of Python or
|
||
other development languages, but need to interoperate with clients or
|
||
servers written using older versions of Python.</p>
|
||
<p>The intent behind this requirement was to minimise any impact that the
|
||
introduction of this policy may have on the stability and compatibility of
|
||
maintenance releases, while still addressing some key security concerns
|
||
relating to the particular aspects of Python 2.7. It would be thoroughly
|
||
counterproductive if end users became as cautious about updating to new
|
||
Python 2.7 maintenance releases as they are about updating to new feature
|
||
releases within the same release series.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">ssl</span></code> module changes are included in this proposal to bring the
|
||
Python 2 series up to date with the past 4 years of evolution in network
|
||
security standards, and make it easier for those standards to be broadly
|
||
adopted in both servers and clients. Similarly the hash algorithm
|
||
availability indicators in <code class="docutils literal notranslate"><span class="pre">hashlib</span></code> are included to make it easier for
|
||
applications to detect and employ appropriate hash definitions across both
|
||
Python 2 and 3.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">hmac.compare_digest()</span></code> and <code class="docutils literal notranslate"><span class="pre">hashlib.pbkdf2_hmac()</span></code> are included to
|
||
help lower the barriers to secure password storage and checking in Python 2
|
||
server applications.</p>
|
||
<p>The <code class="docutils literal notranslate"><span class="pre">os.urandom()</span></code> change has been included in this proposal to further
|
||
encourage users to leave the task of providing high quality random numbers
|
||
for cryptographic use cases to operating system vendors. The use of
|
||
insufficiently random numbers has the potential to compromise <em>any</em>
|
||
cryptographic system, and operating system developers have more tools
|
||
available to address that problem adequately than the typical Python
|
||
application runtime.</p>
|
||
</section>
|
||
<section id="rejected-alternative-just-advise-developers-to-migrate-to-python-3">
|
||
<h3><a class="toc-backref" href="#rejected-alternative-just-advise-developers-to-migrate-to-python-3" role="doc-backlink">Rejected alternative: just advise developers to migrate to Python 3</a></h3>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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).</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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 <em>anyway</em>, 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.</p>
|
||
<p>The problem is real, so <em>something</em> needs to change, and this PEP describes
|
||
my preferred approach to addressing the situation.</p>
|
||
</section>
|
||
<section id="rejected-alternative-create-and-release-python-2-8">
|
||
<h3><a class="toc-backref" href="#rejected-alternative-create-and-release-python-2-8" role="doc-backlink">Rejected alternative: create and release Python 2.8</a></h3>
|
||
<p>With sufficient corporate support, it likely <em>would</em> 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 <em>relatively
|
||
low impact</em> way to incorporate enhanced security features into integrated
|
||
products and deployments that make use of Python 2.</p>
|
||
<p>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.</p>
|
||
<p>Attempting to create a Python 2.8 release would also bring in suggestions
|
||
to backport many additional features from Python 3 (such as <code class="docutils literal notranslate"><span class="pre">tracemalloc</span></code>
|
||
and the improved coroutine support), making the migration from Python 2.7
|
||
to this hypothetical 2.8 release even riskier and more disruptive.</p>
|
||
<p>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).</p>
|
||
<p>Furthermore, while I can’t make any commitments to actually addressing
|
||
this issue on Red Hat platforms, I <em>can</em> categorically rule out the idea
|
||
of a Python 2.8 being of any use to me in even attempting to get it
|
||
addressed.</p>
|
||
</section>
|
||
<section id="rejected-alternative-distribute-the-security-enhancements-via-pypi">
|
||
<h3><a class="toc-backref" href="#rejected-alternative-distribute-the-security-enhancements-via-pypi" role="doc-backlink">Rejected alternative: distribute the security enhancements via PyPI</a></h3>
|
||
<p>While this initially appears to be an attractive and easier to manage
|
||
approach, it actually suffers from several significant problems.</p>
|
||
<p>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.</p>
|
||
<p>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
|
||
<code class="docutils literal notranslate"><span class="pre">pip</span></code> based software distribution ecosystem. Relying on a third party
|
||
binary dependency also creates potential compatibility problems for <code class="docutils literal notranslate"><span class="pre">pip</span></code>
|
||
when running on other interpreters like <code class="docutils literal notranslate"><span class="pre">PyPy</span></code>.</p>
|
||
<p>Another practical problem with the idea is the fact that <code class="docutils literal notranslate"><span class="pre">pip</span></code> itself
|
||
relies on the <code class="docutils literal notranslate"><span class="pre">ssl</span></code> support in the standard library (with some additional
|
||
support from a bundled copy of <code class="docutils literal notranslate"><span class="pre">requests</span></code>, which in turn bundles
|
||
<code class="docutils literal notranslate"><span class="pre">backport.ssl_match_hostname</span></code>), and hence would require any replacement
|
||
module to also be bundled within <code class="docutils literal notranslate"><span class="pre">pip</span></code>. This wouldn’t pose any
|
||
insurmountable difficulties (it’s just another dependency to vendor), but
|
||
it <em>would</em> mean yet another copy of OpenSSL to keep up to date.</p>
|
||
<p>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.</p>
|
||
<p>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
|
||
<em>Python</em>, and a pre-existing set of additional packages. Getting new
|
||
packages added to the pre-existing set <em>can</em> 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 <em>opt out</em> of the
|
||
security enhancements by deliberately downgrading the provided network
|
||
security infrastructure, which most of them are unlikely to do.</p>
|
||
</section>
|
||
<section id="rejected-variant-provide-a-legacy-ssl-infrastructure-branch">
|
||
<h3><a class="toc-backref" href="#rejected-variant-provide-a-legacy-ssl-infrastructure-branch" role="doc-backlink">Rejected variant: provide a “legacy SSL infrastructure” branch</a></h3>
|
||
<p>Earlier versions of this PEP included the concept of a <code class="docutils literal notranslate"><span class="pre">2.7-legacy-ssl</span></code>
|
||
branch that preserved the exact feature set of the Python 2.7.6 network
|
||
security infrastructure.</p>
|
||
<p>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.</p>
|
||
<p>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.</p>
|
||
<p>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”.</p>
|
||
</section>
|
||
<section id="rejected-variant-synchronise-particular-modules-entirely-with-python-3">
|
||
<h3><a class="toc-backref" href="#rejected-variant-synchronise-particular-modules-entirely-with-python-3" role="doc-backlink">Rejected variant: synchronise particular modules entirely with Python 3</a></h3>
|
||
<p>Earlier versions of this PEP suggested synchronising the <code class="docutils literal notranslate"><span class="pre">hmac</span></code>,
|
||
<code class="docutils literal notranslate"><span class="pre">hashlib</span></code> and <code class="docutils literal notranslate"><span class="pre">ssl</span></code> modules entirely with their Python 3 counterparts.</p>
|
||
<p>This approach proved too vague to build a compelling case for the exception,
|
||
and has thus been replaced by the current more explicit proposal.</p>
|
||
</section>
|
||
<section id="rejected-variant-open-ended-backport-policy">
|
||
<h3><a class="toc-backref" href="#rejected-variant-open-ended-backport-policy" role="doc-backlink">Rejected variant: open ended backport policy</a></h3>
|
||
<p>Earlier versions of this PEP suggested a general policy change related to
|
||
future Python 3 enhancements that impact the general security of the
|
||
internet.</p>
|
||
<p>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.</p>
|
||
</section>
|
||
</section>
|
||
<section id="disclosure-of-interest">
|
||
<h2><a class="toc-backref" href="#disclosure-of-interest" role="doc-backlink">Disclosure of Interest</a></h2>
|
||
<p>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.</p>
|
||
</section>
|
||
<section id="acknowledgements">
|
||
<h2><a class="toc-backref" href="#acknowledgements" role="doc-backlink">Acknowledgements</a></h2>
|
||
<p>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.</p>
|
||
<p>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 (<a class="footnote-reference brackets" href="#id15" id="id3">[7]</a>, <a class="footnote-reference brackets" href="#id16" id="id4">[8]</a>).</p>
|
||
<p>Christian and Donald also provided valuable feedback on a preliminary
|
||
draft of this proposal.</p>
|
||
<p>Thanks also to participants in the python-dev mailing list threads
|
||
(<a class="footnote-reference brackets" href="#id9" id="id5">[1]</a>, <a class="footnote-reference brackets" href="#id10" id="id6">[2]</a>, <a class="footnote-reference brackets" href="#id13" id="id7">[5]</a>, <a class="footnote-reference brackets" href="#id14" id="id8">[6]</a>), as well as the various folks I discussed this issue with at
|
||
PyCon 2014 in Montreal.</p>
|
||
</section>
|
||
<section id="references">
|
||
<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
|
||
<aside class="footnote-list brackets">
|
||
<aside class="footnote brackets" id="id9" role="doc-footnote">
|
||
<dt class="label" id="id9">[<a href="#id5">1</a>]</dt>
|
||
<dd>PEP 466 discussion (round 1)
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133334.html">https://mail.python.org/pipermail/python-dev/2014-March/133334.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id10" role="doc-footnote">
|
||
<dt class="label" id="id10">[<a href="#id6">2</a>]</dt>
|
||
<dd>PEP 466 discussion (round 2)
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133389.html">https://mail.python.org/pipermail/python-dev/2014-March/133389.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id11" role="doc-footnote">
|
||
<dt class="label" id="id11">[<a href="#id1">3</a>]</dt>
|
||
<dd>Marc-Andre Lemburg’s OpenSSL feedback for Windows
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133438.html">https://mail.python.org/pipermail/python-dev/2014-March/133438.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id12" role="doc-footnote">
|
||
<dt class="label" id="id12">[<a href="#id2">4</a>]</dt>
|
||
<dd>Ned Deily’s OpenSSL feedback for Mac OS X
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133347.html">https://mail.python.org/pipermail/python-dev/2014-March/133347.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id13" role="doc-footnote">
|
||
<dt class="label" id="id13">[<a href="#id7">5</a>]</dt>
|
||
<dd>PEP 466 discussion (round 3)
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133442.html">https://mail.python.org/pipermail/python-dev/2014-March/133442.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id14" role="doc-footnote">
|
||
<dt class="label" id="id14">[<a href="#id8">6</a>]</dt>
|
||
<dd>PEP 466 discussion (round 4)
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133472.html">https://mail.python.org/pipermail/python-dev/2014-March/133472.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id15" role="doc-footnote">
|
||
<dt class="label" id="id15">[<a href="#id3">7</a>]</dt>
|
||
<dd>Donald Stufft’s recommended set of backported features
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133500.html">https://mail.python.org/pipermail/python-dev/2014-March/133500.html</a>)</aside>
|
||
<aside class="footnote brackets" id="id16" role="doc-footnote">
|
||
<dt class="label" id="id16">[<a href="#id4">8</a>]</dt>
|
||
<dd>Alex Gaynor’s recommended set of backported features
|
||
(<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2014-March/133503.html">https://mail.python.org/pipermail/python-dev/2014-March/133503.html</a>)</aside>
|
||
</aside>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document has been placed in the public domain.</p>
|
||
</section>
|
||
</section>
|
||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0466.rst">https://github.com/python/peps/blob/main/peps/pep-0466.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0466.rst">2023-10-11 12:05:51 GMT</a></p>
|
||
|
||
</article>
|
||
<nav id="pep-sidebar">
|
||
<h2>Contents</h2>
|
||
<ul>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#new-security-related-features-in-python-2-7-maintenance-releases">New security related features in Python 2.7 maintenance releases</a></li>
|
||
<li><a class="reference internal" href="#implementation-status">Implementation status</a></li>
|
||
<li><a class="reference internal" href="#backwards-compatibility-considerations">Backwards compatibility considerations</a><ul>
|
||
<li><a class="reference internal" href="#openssl-compatibility">OpenSSL compatibility</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#other-considerations">Other Considerations</a><ul>
|
||
<li><a class="reference internal" href="#maintainability">Maintainability</a></li>
|
||
<li><a class="reference internal" href="#security-releases">Security releases</a></li>
|
||
<li><a class="reference internal" href="#integration-testing">Integration testing</a></li>
|
||
<li><a class="reference internal" href="#handling-lower-security-environments-with-low-risk-tolerance">Handling lower security environments with low risk tolerance</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#motivation-and-rationale">Motivation and Rationale</a><ul>
|
||
<li><a class="reference internal" href="#why-these-particular-changes">Why these particular changes?</a></li>
|
||
<li><a class="reference internal" href="#rejected-alternative-just-advise-developers-to-migrate-to-python-3">Rejected alternative: just advise developers to migrate to Python 3</a></li>
|
||
<li><a class="reference internal" href="#rejected-alternative-create-and-release-python-2-8">Rejected alternative: create and release Python 2.8</a></li>
|
||
<li><a class="reference internal" href="#rejected-alternative-distribute-the-security-enhancements-via-pypi">Rejected alternative: distribute the security enhancements via PyPI</a></li>
|
||
<li><a class="reference internal" href="#rejected-variant-provide-a-legacy-ssl-infrastructure-branch">Rejected variant: provide a “legacy SSL infrastructure” branch</a></li>
|
||
<li><a class="reference internal" href="#rejected-variant-synchronise-particular-modules-entirely-with-python-3">Rejected variant: synchronise particular modules entirely with Python 3</a></li>
|
||
<li><a class="reference internal" href="#rejected-variant-open-ended-backport-policy">Rejected variant: open ended backport policy</a></li>
|
||
</ul>
|
||
</li>
|
||
<li><a class="reference internal" href="#disclosure-of-interest">Disclosure of Interest</a></li>
|
||
<li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
||
<br>
|
||
<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0466.rst">Page Source (GitHub)</a>
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