https-everywhere-checker 0.1.0

HTTPS Everywhere Rule Checker
=============================

Author: Ondrej Mikle, CZ.NIC (ondrej.mikle \|at\_sign\| nic.cz)

Installation and requirements
-----------------------------

::

pip install https-everywhere-checker

Configuration
-------------

Copy ``checker.config.sample`` to ``checker.config`` and change the
``rulesdir`` under ``[rulesets]`` to point to a directory with the XML
files of HTTPS Everywhere rules (usually the
``src/chrome/content/rules`` of locally checked out git tree of HTTPS
Everywhere).

Running
-------

Once you have modified the config, run:

::

check-https-rules checker.config

Output will be written to selected log file, infos/warnings/errors
contain the useful information.

Features
--------

- Attempts to follow Firefox behavior as closely as possible (including
rewriting HTTP redirects according to rules; well except for
Javascript and meta-redirects)
- IDN domain support
- Currently two metrics on "distance" of two resources implemented, one
is purely string-based, the other tries to measure "similarity of the
shape of DOM tree"
- Multi-threaded scanner
- Support for various "platforms" (e.g. CAcert), i.e. sets of CA
certificate sets which can be switched during following of redirects
- set of used CA certificates can be statically restricted to one CA
certificate set (see ``static_ca_path`` in config file)

What errors in rulesets can be detected
---------------------------------------

- big difference in HTML page structure
- error in ruleset - declared target that no rule rewrites, bad regexps
(usually capture groups are wrong), incomplete FQDNs, non-existent
domains
- HTTP 200 in original page, while rewritten page returns 4xx/5xx
- cycle detection in redirects
- transvalid certificates (incomplete chains)
- other invalid certificate detection (self-signed, expired, CN
mismatch...)

False positives and shortcomings
--------------------------------

- Some pages deliberately have different HTTP and HTTPS page, some for
example redirect to different page under https
- URLs to scan are naively guessed from target hosts, having test set
of URLs in a ruleset would improve it (better coverage)

Known bugs
----------

CURL+NSS can't handle hosts with SNI sharing same IP address
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PyCURL and NSS incorrectly handle the case when two FQDNs have identical
IP address, use Server Name Indication and try to resume TLS session
with the same session ID. Even turning off SSL session cache via setting
``pycurl.SSL_SESSIONID_CACHE`` to zero won't help (it's ignored by
libcurl/pycurl for some reason). PyCURL+NSS fail to see that server
didn't acknowledge SNI in response (see RFC 4366 reference below), thus
'Host' header in HTTP and SNI seen by server are different, thus HTTP
404.

This one issue was especially insidious bug, many thanks to Pavel Janík
for helping hunt this bug down.

Testcase
^^^^^^^^

See ``curl_test_nss/curl_testcase_nss_sni.py`` script that demonstrates
the bug.

Technical details
^^^^^^^^^^^^^^^^^

PyCURL sends TLS handshake with SNI for the first host. This works.
Connection is then closed, but PyCURL+NSS remembers the SSL session ID.
It will attempt to use the same session ID when later connecting to
second host on the same IP.

However, the server won't acknowledge what client requested with new
SNI, because client attempts to resume during TLS handshake using the
incorrect session ID. Thus the session is "resumed" to the first host's
SNI.

Side observation: When validation is turned off in PyCURL+NSS, it also
turns off session resume as a side effect (the code is in curl's nss.c).

Workaround
^^^^^^^^^^

Set config to use SSLv3 instead of default TLSv1 (option ``ssl_version``
under ``http`` section).

Normative reference
^^^^^^^^^^^^^^^^^^^

See last four paragraphs of `RFC 4366, section
3.1 <https://tools.ietf.org/html/rfc4366#section-3.1>`__. Contrast with
`RFC 6066 section 3 <https://tools.ietf.org/html/rfc6066#section-3>`__,
last two paragraphs. In TLS 1.2 the logic is reversed - server must not
resume such connection and must go through full handshake again.

At most 9 capture groups in rule supported
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is a workaround for ambiguous rewrites in rules such as:

::

<rule from="^http://(www\.)?01\.org/" to="https://$101.org/" />

The ``$101`` would actually mean 101-st group, so we assume that only first digit after ``$``
denotes the group (which is how it seems to work in javascript).

May not work under Windows
~~~~~~~~~~~~~~~~~~~~~~~~~~

According to `PyCURL
documentation <http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTCAPATH>`__,
using CAPATH may not work under Windows. I'd guess it's due to openssl's
``c_rehash`` utility that creates symlinks to PEM certificates.
Hypothetically it could work if the symlinks were replaced by regular
files with identical names, but haven't tried.

Threading bugs and workarounds
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are some race conditions with Python threads and OpenSSL/GnuTLS
that cause about due to SIGPIPE or SIGSEGV. While libcurl code seems to
have implemented the necessary callbacks, there's a bug somewhere :-)

Workaround: set ``fetch_in_subprocess`` under ``http`` section in config
to true when using multiple threads for fetching. Using subprocess is on
by default.

You might have to set PYTHONPATH if working dir is different from code
dir with python scripts.

If underlying SSL library is NSS, threading looks fine.

As a side effect, the CURL+NSS SNI bug does not happen with subprocesses
(SSL session ID cache is not kept among process invocations).

If pure-threaded version starts eating too much memory (like 1 GB in a
minute), turn on the ``fetch_in_subprocess`` option metioned above. Some
combinations of CURL and SSL library versions do that. Spawning separate
subprocesses prevents any caches building up and eating too much memory.

Using subprocess hypothetically might cause a deadlock due to
insufficient buffer size when exchanging data through stdin/stdout in
case of a large HTML page, but hasn't happened for any of the rules
(I've tried to run them on the complete batch of rulesets contained in
HTTPS Everywhere Nov 2 2012 commit
c343f230a49d960dba90424799c3bacc2325fc94). Though in case deadlock
happens, increase buffer size in ``subprocess.Popen`` invocation in
``http_client.py``.

Generic bugs/quirks of SSL libraries
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Each of the three possible libraries (OpenSSL, GnuTLS, NSS) has
different set of quirks. GnuTLS seems to be the most strict one
regarding relevant RFCs and will not for instance tolerate certificate
chain in wrong order or forgive server not sending ``close_notify``
alert.

Thus it's entirely possible that while a server chain and SSL/TLS
handshake seems OK when using one lib, it may break with the other.

Transvalid certificates (transitive closure of root and intermediate certs)
---------------------------------------------------------------------------

The ``platform_certs/FF_transvalid.tar.bz2`` attempts to simulate common
browser behavior of caching intermediate certs. The directory contains
FF's builtin certs and all intermediate certs that validate from FF's
builtin certs (a transitive closure).

The certs above are in a tarball (need to be unpacked and c\_rehash'd
for use).

The script is in ``certs_transitive_closure/build_closure.sh`` and is
rather crude, definitely needs some double-checking of sanity (see
comments inside the script).

Quick outline of the script's algorithm:

1. IntermediateSet\_0 := {trusted builtin certs from clean install of
Firefox}
2. Certs that have basic constraints CA=true or are X509 version 1 are
exported from some DB like SSL Observatory
3. Iterate over all exported certs, add new unique certificates not yet
contained in IntermediateSet\_n validate against latest
IntermediateSet\_n, forming IntermediateSet\_{n+1}
4. n += 1
5. If any certs were added in step 3, goto 3, else end

Last IntermediateSet is the closure.




History
-------

0.1.0 (unreleased)
++++++++++++++++++

* First release on PyPI.