Disable TLS compression to protect against CRIME-like attacks

[Jason Gerard DeRose]

Thanks to prodding from David Jordan, I've looked into the CRIME attack more closely, and as a result figured out how to disable compression via SSLContext.options.

This article is a decent overview of the attack:

http://arstechnica.com/security/2012/09/crime-hijacks-https-sessions/

Python 3.3 adds the `ssl.OP_NO_COMPRESSION` constant. So my fix is to monkey patch Python3.2 to add this, and then to change build_ssl_context() so that it sets the appropriate options like this:

    ctx = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
    ctx.verify_mode = ssl.CERT_REQUIRED
    ctx.options |= ssl.OP_NO_COMPRESSION

Although Novacut and Dmedia use client side certs rather than sending a secret session cookie or password with each request, these are use cases Microfiber supports so we should use good, secure defaults. And you never know in what ways this compression attack might be extended.

[David Jordan (dmj726)]

Thanks, it never hurts to be careful!

[Jason Gerard DeRose (jderose)]

Interestingly, it's not possible to disable TLS compression under Apache 2.2, something we should keep in mind for our Apache SSL frontend servers:

https://issues.apache.org/bugzilla/show_bug.cgi?id=53219

From testing I've done, I've confirmed you can disabled it from the client-side, so we're covered as far as Microfiber goes. But we also have to worry about the CouchDB replicator.