On Mar 23, 2014, at 3:07 AM, Nick Coghlan email@example.com wrote:
Several significant changes in this revision:
- scope narrowed to just Python 2.7 plus permission for commercial
redistributors to use the same strategy in their long term support releases
- far more explicit that this is about inviting potential corporate
contributors to address the situation for the benefit of the overall Python ecosystem, not offering to fix it for them for free
- clarified that third party integration testing services would need
to be updated to support testing against multiple Python 2.7 minor releases
For what it’s worth, I have an outstanding PR against Travis CI that would make this trivial for them at least. They are a pretty popular CI service especially for OSS projects. I made that PR for unrelated reasons but it could at least serve as a template for other projects to do the same thing.
- explicit sections on why I don't think the status quo is
sustainable, why I don't think Python 2.8 would actually solve the problem, and why I think a PyPI based solution not only wouldn't solve the problem, but would be rather difficult to get working in the first place
- be completely explicit that I am *not* speaking on behalf of Red Hat
at this point and have no authority to make commitments on their behalf. Instead, I'm looking for upstream consensus that 1) this is a genuine problem that needs to be solved; 2) we're open to corporate assistance in solving it; and 3) we have a pretty good idea what help we actually want. If all that happens, *then* I can take up the issue internally to try to get us some help in maintaining the proposed solution (hopefully other folks with corporate influence can do the same, and we may actually get some ongoing assistance with upstream maintenance out of this, rather than having our downstream redistributors continue to take us for granted).
Advance warning: while I was able to get this revision turned around pretty quickly, future revisions are likely to take a fair bit longer. It was already a rather busy month before I decided to start this discussion on top of everything else :)
PEP: 466 Title: Network Security Enhancement Exception for Python 2.7 Version: $Revision$ Last-Modified: $Date$ Author: Nick Coghlan firstname.lastname@example.org, Status: Draft Type: Informational Content-Type: text/x-rst Created: 23-Mar-2014 Post-History: 23-Mar-2014
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 commercial use cases where upgrading to Python 3 in the near term may not be practical.
Accordingly, this PEP relaxes the normal restrictions by allowing enhancements to be applied in Python 2.7 maintenance releases for standard library components that have implications for the overall security of the internet. In particular, the exception will apply to:
- the ``ssl`` module
- the ``hashlib`` module
- the ``hmac`` module
- the ``sha`` module (Python 2 only)
- the components of other networking modules that make use of these modules
- the components of the ``random`` and ``os`` modules that are relevant to
- the version of OpenSSL bundled with the binary installers
Proposed backports for these modules will still need to undergo normal backwards compatibility assessments, but new features will be permitted where appropriate, making it easier to implement secure networked software in Python, even for software that needs to remain compatible with older feature releases of Python.
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 adopt a similar approach to ensuring that the secure networking infrastructure keeps pace with the evolution of the internet, or else disclaim support for the use of older versions in roles that involving connecting directly to the public internet.
Under this policy, the following network security related modules are granted a blanket exemption to the restriction against adding new features in maintenance releases, for the purpose of keeping their APIs aligned with their counterparts in the latest feature release of Python 3:
- the ``ssl`` module
- the ``hashlib`` module
- the ``hmac`` module
- the ``sha`` module (Python 2 only)
This exemption applies to *all* proposals to backport backwards compatible changes in these modules to Python 2.7 maintenance releases. This choice is made deliberately to ensure that the "feature or fix?" argument isn't simply replaced by a "security related or not?" argument. These particular modules are inherently security related, and all enhancements to them improve Python's capabilities as a platform for development of secure networked software.
As part of this policy, permission is also granted to upgrade to newer feature releases of OpenSSL when preparing the binary installers for new maintenance releases of CPython.
In addition to the above blanket exemption, a conditional exemption is granted for these modules that may include some network security related features:
- the ``os`` module (specifically ``os.urandom``)
- the ``random`` module
- networking related modules that depend on one or more of the network
security related modules listed above
This more limited exemption for these modules requires that the *specific* enhancement being proposed for backporting needs to be justified as being network security related. If the enhancement under discussion is designed to take advantage of a new feature in one of the network security related modules, then that will be taken as implying that the enhancement is security related.
Backwards Compatibility Considerations
This PEP does *not* grant any general exemptions to the usual backwards compatibility policy for maintenance releases. Instead, by explicitly encouraging the use of feature based checks and explicitly opting in to less secure configurations, it is designed to make it easier to provide more "secure by default" behaviour in future feature releases, while still limiting the risk of breaking currently working software when upgrading to a new maintenance release.
In *all* cases where this policy is applied to backport enhancements to maintenance releases, it MUST be possible to write cross-version compatible code that operates by "feature detection" (for example, checking for particular attributes in the 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 continue operating using a slightly degraded security configuration.
Affected APIs SHOULD be designed to 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)
- 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 MUST NOT require checking for a specific maintenance release)
Security related changes to other modules (such as 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 needs to evolve faster than the 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.
This policy does NOT represent a commitment by volunteer contributors to actually backport network security related changes from the Python 3 series to the Python 2 series. Rather, it is intended to send a clear signal to potential corporate contributors that the core development team are willing to review and merge corporate contributions that put this policy into effect.
Backporting security related fixes and enhancements to earlier versions is a common service for commercial redistributors to offer to their customers. This policy represents an explicit invitation to contribute some of those changes back to the upstream community in cases where they are likely to have a broad impact that helps improve the security of the internet as a whole.
All modules that take advantage of this policy to backport network security related enhancements to earlier Python versions MUST include a "Security Considerations" section in their documentation.
In addition to any other module specific contents, this section MUST enumerate key security enhancements and fixes (with CVE identifiers where applicable), along with the feature and maintenance releases that first included them.
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 applying this policy to any Python release series that is either in security fix only mode, or has been declared "end of life" by the core development team.
Whether or not redistributors choose to exercise that option will be up to the redistributor.
Third party integration testing services would likely need to start offering users a choice of multiple Python 2.7.x versions to test against, to ensure that the application is correctly degrading gracefully if it attempts to use newer networking features on maintenance releases that are too old to provide them.
Evolution of this Policy
The key requirement for a module to be considered for inclusion in this policy (whether under a blanket or conditional exemption) is that it must have security implications *beyond* the specific application that is written in Python and the system that application is running on. Thus the focus on network security protocols 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 be using newer versions of Python or other development languages.
The intent behind this requirement is to minimise any impact that the introduction of this policy may have on the stability and compatibility of maintenance releases. It would be thoroughly counterproductive if end users became as cautious about updating to new Python maintenance releases as they are about updating to new feature releases.
Motivation and Rationale
This PEP can be seen as a more targeted version of the "faster standard library release cycle" proposals discussed in PEP 407 and PEP 413, focusing specifically on those areas which have implications beyond the Python community.
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, and 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, and 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 policy aims to acknowledge the fact that Python's popularity and adoption has now reached a level where 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 Identification 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, may users don't even consider the problem 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.
My position on the ongoing transition from Python 2 to Python 3 has long been that Python 2 remains a supported platform for the core development team, and that commercial support will remain available well after upstream maintenance ends. However, in the absence of this network security enhancement policy, that position is difficult to justify when it comes to software that operates over the public internet. Just as many developers consider it too difficult to develop truly secure modern networked software in C/C++ (largely due to the challenges associated with manual memory management), I anticipate that in the not too distant future, it will be considered too difficult to develop truly secure modern networked software using the Python 2 series (some developers would argue that we have already reached that point).
Alternative: advise developers of networked software 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.
Now that we're fully aware of the impact the limitations in Python 2 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 available in Python 3.
While (as far as I am aware) Ubuntu has successfully switched to Python 3.4 as its main Python interpreter for its 14.04 LTS release, Fedora still has a lot of 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 those changes won't affect the core tools regardless.
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 large enough that it is going to take quite some time to migrate.
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. The key point of this PEP is that those situations affect more people than just the developers and users of the affected application: their existence becomes something that developers of secure networked services need to take into account as part of their security design.
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 currently preferred approach to addressing the situation.
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).
This is not a recommended approach, as it would involve substantial additional work for a result that is actually less effective as a solution to the original problem (the widespread use of the aging network security infrastructure in Python 2).
Alternative: distribute the security enhancements via PyPI
While it initially appears to be an attractive and easier to manage approach, there are actually several significant problems with this idea.
Firstly, this PEP encompasses a non-trivial portion of the standard library. It's not just the underlying SSL support, but also the libraries for other network protocols like HTTP, FTP, IMAP, and POP3 that integrate with the SSL infrastructure to provide secure links, and that's just the protocols in the standard library. Even if an API compatible ``ssl2`` module was made available, it would need to be imported and injected into ``sys.modules`` as ``ssl`` before importing any other module that needed it.
Secondly, 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, 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 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 taking the risk of trying to upgrade 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 going around to 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 *opt out* of the security enhancements, which most of them are unlikely to do.
- What are the risks associated with allowing OpenSSL to be updated to
new feature versions in the Windows and Mac OS X binary installers for maintenance releases? Currently we just upgrade to the appropriate OpenSSL maintenance releases, rather than switching to the latest feature release. In particular, is it possible Windows C extensions may be linking against the Python provided OpenSSL module?
- Are there any other security relevant modules that should be covered
by either a blanket or conditional exemption?
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.
Thanks to Christian Heimes for his recent efforts on 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.
Christian and Donald Stufft also provided valuable feedback on a preliminary draft of this proposal.
Thanks also to participants in the python-dev mailing list thread _
This document has been placed in the public domain.
-- Nick Coghlan | email@example.com | Brisbane, Australia _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/donald%40stufft.io
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