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Python SSL Woes

Posted by Bjarni RĂșnar on March 24, 2016

There's not much to say about progress this week; things have been slow, but most of my hacking time has been spent getting frustrated by SSL and Python.

So following in our Grand Tradition of biting the hand that feeds us and publicly complaining about libraries and tools we rely upon, I am going to write a few words about the state of SSL/TLS support in Python.

This rant was (sort of) requested by Christian, one of the maintainers of the Python SSL module. Hi Christian, thanks for all your hard work! ;-)

Why We Care

SSL (or TLS) is the technology used to secure how Mailpile communicates with online mail servers, and is also used when downloading data from the web, whether for key discovery or things like user avatars and icons.

The e-mail world is a bit different from the web, in that the use of encryption is somewhat less mature. Self-signed certificates for mail servers are more common than on the web and use of encryption for e-mail in transit tends to be more opportunistic, so the "standard" SSL key management strategies employed by web-servers and web-browsers don't really apply.

The current iteration of the Mailpile code-base doesn't validate SSL certificate at all when connecting to SMTP or IMAP servers; this means any motivated attacker can easily perform a man-in-the-middle attack to listen in on, or modify, sent or downloaded e-mail.

Fixing this requires validating the server certificates somehow; for those that have a certificate issued by one of the known certificate authorities, this is largely the same process as on the web. However, a large number of e-mail servers use self-signed certificates, particularly within the enthusiast community which is a significant part of the people we expect to adopt Mailpile. So we need to handle those certificates gracefully as well.

While working on this, I yet again encountered limitations and bugs in Python's standard ssl code.

Issue 1: Python 2.6, 2.7.3, ...

It is incredibly frustrating that Python 2.7.9 was the first release of Python that had semi-functional SSL support in the standard library. Previous versions were almost completely insecure and did not expose the API hooks required to improve the situation.

Since I'm pretty sure nobody in the Python community has a time machine, there's not much the Python community can do about this. All we can do is thank the people that are working to fix things today - and get those fixes used!

On that note: distributions that are shipping Python 2.7 but refuse to issue a security fix to upgrade to the latest point release are failing to protect their users. At least back-port the affected modules, for crying out loud!

Angry Tweeting

In a moment of drama, I tweeted the following:

The more I use Python's ssl module, the more obvious it is that nobody ever used it for anything serious. Even the 2.7/3.x code just fails.

Those are fighting words and I'm sorry if they offended anyone. Christian responded with a very level-headed request for details, which in turn prompted this post.

The frustration I expressed there, was to a large degree related to Issue 1 above, but the comment on 2.7 and 3.x needs explaining. I'll talk about the big picture first, with critiques of individual methods further down.

This should by no means be considered an exhaustive review of the Python ssl module; but I hope at least some of my suggestions are actionable and constructive.

One caveat: I haven't checked the Python 2.7.11 code, this critique is based on reading the sources from 2.7.10 as installed by my distro's package manager. Some of these issues may have been fixed, which would bring me back to Issue 1.

Issue 2: import ssl, poplib, smtplib, imaplib

I am going to charitably assume that the ssl backport in 2.7.9 took care to preserve compatibility and that is why imaplib, poplib and smtplib (all used by Mailpile) still make insecure SSLv3 connections.

Unless I am reading the code completely wrong, the more secure new "default SSL contexts" are not actually used (see below for more details). This is fixed in Python 3.x.

Fair enough, backwards compatibility is important! When forced to choose between fixing a security hole and suddenly breaking apps that used to work, it's not obvious that security should always come first.

Unless this was a mistake?

Either way, the backported ssl module provides no mechanism for changing this behaviour, short of monkey-patching. That seems like a pretty serious oversight, and one easily corrected with a global module variable or two.

More generally, the ssl module would benefit greatly from an API that would allow an app to specify default behaviours and callbacks or hooks for "interesting events"; allow more control and more introspection without forcing all the other libraries to complicate their APIs.

Issue 3: Certificate Management

An e-mail client needs to be able to manage SSL certificates, including self-signed ones.

Showing my ignorance here, I couldn't figure out how to do this with the Python ssl module until I got some helpful replies to my angry tweet. Even then, the APIs I will need to work with if I stick with the standard python ssl module are very awkward.

I think it would be time very well spent to improve both the documentation and the APIs (see below) for this use-case.

Googling for Python ssl examples will in the vast majority of cases show people disabling certificate verification entirely. Surely that is not what we want people to do! The official API documentation is the most effective place to combat these bad examples.

Issues 4..N

These are random warts in the ssl API. Some major, some minor.

pydoc ssl

A typo: the method "fetch_server_certificate" does not exist.

ssl.get_server_certificate

If asked to validate, this method may throw an exception instead of returning the certificate. This is unhelpful.

The data returned by this method is a binary blob; the library does not provide any utilities for decoding the contents.

ssl.SSLError

This exception is raised when a certificate fails to validate. However, it doesn't tell you anything about the certificate itself.

Including the certificate as an attribute of the exception would allow for more nuanced and meaningful error handling.

Details as to why the validation failed would be nice too.

ssl.SSLSocket.getpeercert

As far as I can tell, this is the only method provided which parses an SSL certificate, making any certificate handling code reliant on the network. This is bad, but isn't the only problem here.

Aside from crypto geeks, few consumers of this API are likely to care about the details of a validated certificate. It's trusted, let's get on with things! It's the certificate that fails to validate that you need to know more about, so you can tell the user something useful.

Unfortunately, this method completely refuses to return structured information about a certificate unless it has been validated first. And of course, if validation is enabled then an exception will be thrown before this method can be called.

It appears the workaround is to first use ssl.get_server_certificate and then provide the certificate as CA to ssl.SSLContext or ssl.wrap_socket, so getpeercert will return actual information.

This is both non-obvious and quite convoluted.

This method should always return structured data when requested, but backwards compatibility concerns will probably preclude that. :-(

Either way, an standalone method that just parses certificates would be very nice to have.

ssl.create_default_context

This method confused me.

At first I thought this method would be used by various Python libraries that support the SSL protocol... and wanted to figure out how to customize the output of this so my app would have control over how SSL was done.

Turns out I was completely wrong on all counts.

I read the ssl library source and found that the stdlib modules are expected to use the undocumented variable _create_stdlib_context which maps to the undocumented method named _create_unverified_context. Except for http.client, which should use _create_default_https_context which maps to ssl.create_default_context after all...

Feeling somewhat confused, I read the Python 2.7.10 sources for imaplib, smtplib and poplib. Turns out none of them use SSL contexts at all! They all just call ssl.wrap_socket, which in turn uses none of the above methods...

Argh. Okay. :-(

It looks like those comments were just back-ported from 3.x, without the libraries themselves getting updated. It's confusing, but the main take-away is that the backport appears to have failed to secure any of the standard libraries except for httplib (and ftplib).

I raise this issue to illustrate how hard it is for an app developer to truly understand how SSL/TLS is being used within their app, let alone have control over it. It's frustrating, to say the least.

My original wish, to be able to control the default context returned by this method, would be a great feature request if this method were actually used! It's not. However, I think the feature request still makes sense - it's just more work!

What is Mailpile Doing About it?

We are:

  1. Talking to the Python devs (complaining)
  2. Planning to publish a TOFU-for-TLS module
  3. Planning to publish our monkey-patching magic

Step 1 is pretty much done; I submitted an early draft of this blog post to the current maintainers of the Python ssl module. In spite of the fact that it contained an extended and distracting rant about the SSL community in general (which I in the end decided not to post), they responded well to the technical points and gave constructive feedback.

The main take-away was that apps like Mailpile, that need control over how SSL connections are made and want to implement non-standard (non-web) certificate verification models, are not well served by the standard Python modules. So we'll be using pyOpenSSL instead and patching or subclassing the protocol libraries.

The TOFU code and policy management logic will be released separately from Mailpile and under a liberal license, so the community can benefit or point and laugh, whichever seems more appropriate. I don't know whether this will happen before or after 1.0.

Stay tuned. :-)



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