mercurial/sslutil.py

sslutil: abort when unable to verify peer connection (BC)

Previously, when we connected to a server and were unable to verify
its certificate against a trusted certificate authority we would
issue a warning and continue to connect. This is obviously not
great behavior because the x509 certificate model is based upon
trust of specific CAs. Failure to enforce that trust erodes security.
This behavior was defined several years ago when Python did not
support loading the system trusted CA store (Python 2.7.9's
backports of Python 3's improvements to the "ssl" module enabled
this).

This commit changes behavior when connecting to abort if the peer
certificate can't be validated. With an empty/default Mercurial
configuration, the peer certificate can be validated if Python is
able to load the system trusted CA store. Environments able to load
the system trusted CA store include:

* Python 2.7.9+ on most platforms and installations
* Python 2.7 distributions with a modern ssl module (e.g. RHEL7's
  patched 2.7.5 package)
* Python shipped on OS X

Environments unable to load the system trusted CA store include:

* Python 2.6
* Python 2.7 on many existing Linux installs (because they don't
  ship 2.7.9+ or haven't backported modern ssl module)
* Python 2.7.9+ on some installs where Python is unable to locate
  the system CA store (this is hopefully rare)

Users of these Pythongs will need to configure Mercurial to load the
system CA store using web.cacerts. This should ideally be performed
by packagers (by setting web.cacerts in the global/system hgrc file).
Where Mercurial packagers aren't setting this, the linked URL in the
new abort message can contain instructions for users.

In the future, we may want to add more code for finding the system
CA store. For example, many Linux distributions have the CA store
at well-known locations (such as /etc/ssl/certs/ca-certificates.crt
in the case of Ubuntu). This will enable CA loading to "just work"
on more Python configurations and will be best for our users since
they won't have to change anything after upgrading to a Mercurial
with this patch.

We may also want to consider distributing a trusted CA store with
Mercurial. Although we should think long and hard about that because
most systems have a global CA store and Mercurial should almost
certainly use the same store used by everything else on the system.

# sslutil.py - SSL handling for mercurial
#
# Copyright 2005, 2006, 2007, 2008 Matt Mackall <[e-mail]>
# Copyright 2006, 2007 Alexis S. L. Carvalho <[e-mail]>
# Copyright 2006 Vadim Gelfer <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from __future__ import absolute_import

import hashlib
import os
import ssl
import sys

from .i18n import _
from . import (
    error,
    util,
)

# Python 2.7.9+ overhauled the built-in SSL/TLS features of Python. It added
# support for TLS 1.1, TLS 1.2, SNI, system CA stores, etc. These features are
# all exposed via the "ssl" module.
#
# Depending on the version of Python being used, SSL/TLS support is either
# modern/secure or legacy/insecure. Many operations in this module have
# separate code paths depending on support in Python.

hassni = getattr(ssl, 'HAS_SNI', False)

try:
    OP_NO_SSLv2 = ssl.OP_NO_SSLv2
    OP_NO_SSLv3 = ssl.OP_NO_SSLv3
except AttributeError:
    OP_NO_SSLv2 = 0x1000000
    OP_NO_SSLv3 = 0x2000000

try:
    # ssl.SSLContext was added in 2.7.9 and presence indicates modern
    # SSL/TLS features are available.
    SSLContext = ssl.SSLContext
    modernssl = True
    _canloaddefaultcerts = util.safehasattr(SSLContext, 'load_default_certs')
except AttributeError:
    modernssl = False
    _canloaddefaultcerts = False

    # We implement SSLContext using the interface from the standard library.
    class SSLContext(object):
        # ssl.wrap_socket gained the "ciphers" named argument in 2.7.
        _supportsciphers = sys.version_info >= (2, 7)

        def __init__(self, protocol):
            # From the public interface of SSLContext
            self.protocol = protocol
            self.check_hostname = False
            self.options = 0
            self.verify_mode = ssl.CERT_NONE

            # Used by our implementation.
            self._certfile = None
            self._keyfile = None
            self._certpassword = None
            self._cacerts = None
            self._ciphers = None

        def load_cert_chain(self, certfile, keyfile=None, password=None):
            self._certfile = certfile
            self._keyfile = keyfile
            self._certpassword = password

        def load_default_certs(self, purpose=None):
            pass

        def load_verify_locations(self, cafile=None, capath=None, cadata=None):
            if capath:
                raise error.Abort(_('capath not supported'))
            if cadata:
                raise error.Abort(_('cadata not supported'))

            self._cacerts = cafile

        def set_ciphers(self, ciphers):
            if not self._supportsciphers:
                raise error.Abort(_('setting ciphers not supported'))

            self._ciphers = ciphers

        def wrap_socket(self, socket, server_hostname=None, server_side=False):
            # server_hostname is unique to SSLContext.wrap_socket and is used
            # for SNI in that context. So there's nothing for us to do with it
            # in this legacy code since we don't support SNI.

            args = {
                'keyfile': self._keyfile,
                'certfile': self._certfile,
                'server_side': server_side,
                'cert_reqs': self.verify_mode,
                'ssl_version': self.protocol,
                'ca_certs': self._cacerts,
            }

            if self._supportsciphers:
                args['ciphers'] = self._ciphers

            return ssl.wrap_socket(socket, **args)

def _hostsettings(ui, hostname):
    """Obtain security settings for a hostname.

    Returns a dict of settings relevant to that hostname.
    """
    s = {
        # Whether we should attempt to load default/available CA certs
        # if an explicit ``cafile`` is not defined.
        'allowloaddefaultcerts': True,
        # List of 2-tuple of (hash algorithm, hash).
        'certfingerprints': [],
        # Path to file containing concatenated CA certs. Used by
        # SSLContext.load_verify_locations().
        'cafile': None,
        # Whether certificate verification should be disabled.
        'disablecertverification': False,
        # Whether the legacy [hostfingerprints] section has data for this host.
        'legacyfingerprint': False,
        # ssl.CERT_* constant used by SSLContext.verify_mode.
        'verifymode': None,
    }

    # Look for fingerprints in [hostsecurity] section. Value is a list
    # of <alg>:<fingerprint> strings.
    fingerprints = ui.configlist('hostsecurity', '%s:fingerprints' % hostname,
                                 [])
    for fingerprint in fingerprints:
        if not (fingerprint.startswith(('sha1:', 'sha256:', 'sha512:'))):
            raise error.Abort(_('invalid fingerprint for %s: %s') % (
                                hostname, fingerprint),
                              hint=_('must begin with "sha1:", "sha256:", '
                                     'or "sha512:"'))

        alg, fingerprint = fingerprint.split(':', 1)
        fingerprint = fingerprint.replace(':', '').lower()
        s['certfingerprints'].append((alg, fingerprint))

    # Fingerprints from [hostfingerprints] are always SHA-1.
    for fingerprint in ui.configlist('hostfingerprints', hostname, []):
        fingerprint = fingerprint.replace(':', '').lower()
        s['certfingerprints'].append(('sha1', fingerprint))
        s['legacyfingerprint'] = True

    # If a host cert fingerprint is defined, it is the only thing that
    # matters. No need to validate CA certs.
    if s['certfingerprints']:
        s['verifymode'] = ssl.CERT_NONE

    # If --insecure is used, don't take CAs into consideration.
    elif ui.insecureconnections:
        s['disablecertverification'] = True
        s['verifymode'] = ssl.CERT_NONE

    if ui.configbool('devel', 'disableloaddefaultcerts'):
        s['allowloaddefaultcerts'] = False

    # If both fingerprints and a per-host ca file are specified, issue a warning
    # because users should not be surprised about what security is or isn't
    # being performed.
    cafile = ui.config('hostsecurity', '%s:verifycertsfile' % hostname)
    if s['certfingerprints'] and cafile:
        ui.warn(_('(hostsecurity.%s:verifycertsfile ignored when host '
                  'fingerprints defined; using host fingerprints for '
                  'verification)\n') % hostname)

    # Try to hook up CA certificate validation unless something above
    # makes it not necessary.
    if s['verifymode'] is None:
        # Look at per-host ca file first.
        if cafile:
            cafile = util.expandpath(cafile)
            if not os.path.exists(cafile):
                raise error.Abort(_('path specified by %s does not exist: %s') %
                                  ('hostsecurity.%s:verifycertsfile' % hostname,
                                   cafile))
            s['cafile'] = cafile
        else:
            # Find global certificates file in config.
            cafile = ui.config('web', 'cacerts')

            if cafile:
                cafile = util.expandpath(cafile)
                if not os.path.exists(cafile):
                    raise error.Abort(_('could not find web.cacerts: %s') %
                                      cafile)
            else:
                # No global CA certs. See if we can load defaults.
                cafile = _defaultcacerts()
                if cafile:
                    ui.debug('using %s to enable OS X system CA\n' % cafile)

            s['cafile'] = cafile

        # Require certificate validation if CA certs are being loaded and
        # verification hasn't been disabled above.
        if cafile or (_canloaddefaultcerts and s['allowloaddefaultcerts']):
            s['verifymode'] = ssl.CERT_REQUIRED
        else:
            # At this point we don't have a fingerprint, aren't being
            # explicitly insecure, and can't load CA certs. Connecting
            # is insecure. We allow the connection and abort during
            # validation (once we have the fingerprint to print to the
            # user).
            s['verifymode'] = ssl.CERT_NONE

    assert s['verifymode'] is not None

    return s

def wrapsocket(sock, keyfile, certfile, ui, serverhostname=None):
    """Add SSL/TLS to a socket.

    This is a glorified wrapper for ``ssl.wrap_socket()``. It makes sane
    choices based on what security options are available.

    In addition to the arguments supported by ``ssl.wrap_socket``, we allow
    the following additional arguments:

    * serverhostname - The expected hostname of the remote server. If the
      server (and client) support SNI, this tells the server which certificate
      to use.
    """
    if not serverhostname:
        raise error.Abort(_('serverhostname argument is required'))

    settings = _hostsettings(ui, serverhostname)

    # Despite its name, PROTOCOL_SSLv23 selects the highest protocol
    # that both ends support, including TLS protocols. On legacy stacks,
    # the highest it likely goes in TLS 1.0. On modern stacks, it can
    # support TLS 1.2.
    #
    # The PROTOCOL_TLSv* constants select a specific TLS version
    # only (as opposed to multiple versions). So the method for
    # supporting multiple TLS versions is to use PROTOCOL_SSLv23 and
    # disable protocols via SSLContext.options and OP_NO_* constants.
    # However, SSLContext.options doesn't work unless we have the
    # full/real SSLContext available to us.
    #
    # SSLv2 and SSLv3 are broken. We ban them outright.
    if modernssl:
        protocol = ssl.PROTOCOL_SSLv23
    else:
        protocol = ssl.PROTOCOL_TLSv1

    # TODO use ssl.create_default_context() on modernssl.
    sslcontext = SSLContext(protocol)

    # This is a no-op on old Python.
    sslcontext.options |= OP_NO_SSLv2 | OP_NO_SSLv3

    # This still works on our fake SSLContext.
    sslcontext.verify_mode = settings['verifymode']

    if certfile is not None:
        def password():
            f = keyfile or certfile
            return ui.getpass(_('passphrase for %s: ') % f, '')
        sslcontext.load_cert_chain(certfile, keyfile, password)

    if settings['cafile'] is not None:
        sslcontext.load_verify_locations(cafile=settings['cafile'])
        caloaded = True
    elif settings['allowloaddefaultcerts']:
        # This is a no-op on old Python.
        sslcontext.load_default_certs()
        caloaded = True
    else:
        caloaded = False

    sslsocket = sslcontext.wrap_socket(sock, server_hostname=serverhostname)
    # check if wrap_socket failed silently because socket had been
    # closed
    # - see http://bugs.python.org/issue13721
    if not sslsocket.cipher():
        raise error.Abort(_('ssl connection failed'))

    sslsocket._hgstate = {
        'caloaded': caloaded,
        'hostname': serverhostname,
        'settings': settings,
        'ui': ui,
    }

    return sslsocket

def _verifycert(cert, hostname):
    '''Verify that cert (in socket.getpeercert() format) matches hostname.
    CRLs is not handled.

    Returns error message if any problems are found and None on success.
    '''
    if not cert:
        return _('no certificate received')
    dnsname = hostname.lower()
    def matchdnsname(certname):
        return (certname == dnsname or
                '.' in dnsname and certname == '*.' + dnsname.split('.', 1)[1])

    san = cert.get('subjectAltName', [])
    if san:
        certnames = [value.lower() for key, value in san if key == 'DNS']
        for name in certnames:
            if matchdnsname(name):
                return None
        if certnames:
            return _('certificate is for %s') % ', '.join(certnames)

    # subject is only checked when subjectAltName is empty
    for s in cert.get('subject', []):
        key, value = s[0]
        if key == 'commonName':
            try:
                # 'subject' entries are unicode
                certname = value.lower().encode('ascii')
            except UnicodeEncodeError:
                return _('IDN in certificate not supported')
            if matchdnsname(certname):
                return None
            return _('certificate is for %s') % certname
    return _('no commonName or subjectAltName found in certificate')

def _plainapplepython():
    """return true if this seems to be a pure Apple Python that
    * is unfrozen and presumably has the whole mercurial module in the file
      system
    * presumably is an Apple Python that uses Apple OpenSSL which has patches
      for using system certificate store CAs in addition to the provided
      cacerts file
    """
    if sys.platform != 'darwin' or util.mainfrozen() or not sys.executable:
        return False
    exe = os.path.realpath(sys.executable).lower()
    return (exe.startswith('/usr/bin/python') or
            exe.startswith('/system/library/frameworks/python.framework/'))

def _defaultcacerts():
    """return path to default CA certificates or None."""
    if _plainapplepython():
        dummycert = os.path.join(os.path.dirname(__file__), 'dummycert.pem')
        if os.path.exists(dummycert):
            return dummycert

    return None

def validatesocket(sock):
    """Validate a socket meets security requiremnets.

    The passed socket must have been created with ``wrapsocket()``.
    """
    host = sock._hgstate['hostname']
    ui = sock._hgstate['ui']
    settings = sock._hgstate['settings']

    try:
        peercert = sock.getpeercert(True)
        peercert2 = sock.getpeercert()
    except AttributeError:
        raise error.Abort(_('%s ssl connection error') % host)

    if not peercert:
        raise error.Abort(_('%s certificate error: '
                           'no certificate received') % host)

    if settings['disablecertverification']:
        # We don't print the certificate fingerprint because it shouldn't
        # be necessary: if the user requested certificate verification be
        # disabled, they presumably already saw a message about the inability
        # to verify the certificate and this message would have printed the
        # fingerprint. So printing the fingerprint here adds little to no
        # value.
        ui.warn(_('warning: connection security to %s is disabled per current '
                  'settings; communication is susceptible to eavesdropping '
                  'and tampering\n') % host)
        return

    # If a certificate fingerprint is pinned, use it and only it to
    # validate the remote cert.
    peerfingerprints = {
        'sha1': hashlib.sha1(peercert).hexdigest(),
        'sha256': hashlib.sha256(peercert).hexdigest(),
        'sha512': hashlib.sha512(peercert).hexdigest(),
    }

    def fmtfingerprint(s):
        return ':'.join([s[x:x + 2] for x in range(0, len(s), 2)])

    nicefingerprint = 'sha256:%s' % fmtfingerprint(peerfingerprints['sha256'])

    if settings['certfingerprints']:
        for hash, fingerprint in settings['certfingerprints']:
            if peerfingerprints[hash].lower() == fingerprint:
                ui.debug('%s certificate matched fingerprint %s:%s\n' %
                         (host, hash, fmtfingerprint(fingerprint)))
                return

        # Pinned fingerprint didn't match. This is a fatal error.
        if settings['legacyfingerprint']:
            section = 'hostfingerprint'
            nice = fmtfingerprint(peerfingerprints['sha1'])
        else:
            section = 'hostsecurity'
            nice = '%s:%s' % (hash, fmtfingerprint(peerfingerprints[hash]))
        raise error.Abort(_('certificate for %s has unexpected '
                            'fingerprint %s') % (host, nice),
                          hint=_('check %s configuration') % section)

    # Security is enabled but no CAs are loaded. We can't establish trust
    # for the cert so abort.
    if not sock._hgstate['caloaded']:
        raise error.Abort(
            _('unable to verify security of %s (no loaded CA certificates); '
              'refusing to connect') % host,
            hint=_('see https://mercurial-scm.org/wiki/SecureConnections for '
                   'how to configure Mercurial to avoid this error or set '
                   'hostsecurity.%s:fingerprints=%s to trust this server') %
                   (host, nicefingerprint))

    msg = _verifycert(peercert2, host)
    if msg:
        raise error.Abort(_('%s certificate error: %s') % (host, msg),
                         hint=_('set hostsecurity.%s:certfingerprints=%s '
                                'config setting or use --insecure to connect '
                                'insecurely') %
                              (host, nicefingerprint))