mercurial/sslutil.py

# 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 re
import ssl

from .i18n import _
from .pycompat import getattr
from .node import hex
from . import (
    encoding,
    error,
    pycompat,
    util,
)
from .utils import (
    hashutil,
    resourceutil,
    stringutil,
)

# 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.
#
# We require in setup.py the presence of ssl.SSLContext, which indicates modern
# SSL/TLS support.

configprotocols = {
    b'tls1.0',
    b'tls1.1',
    b'tls1.2',
}

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

# ssl.HAS_TLSv1* are preferred to check support but they were added in Python
# 3.7. Prior to CPython commit 6e8cda91d92da72800d891b2fc2073ecbc134d98
# (backported to the 3.7 branch), ssl.PROTOCOL_TLSv1_1 / ssl.PROTOCOL_TLSv1_2
# were defined only if compiled against a OpenSSL version with TLS 1.1 / 1.2
# support. At the mentioned commit, they were unconditionally defined.
supportedprotocols = set()
if getattr(ssl, 'HAS_TLSv1', util.safehasattr(ssl, 'PROTOCOL_TLSv1')):
    supportedprotocols.add(b'tls1.0')
if getattr(ssl, 'HAS_TLSv1_1', util.safehasattr(ssl, 'PROTOCOL_TLSv1_1')):
    supportedprotocols.add(b'tls1.1')
if getattr(ssl, 'HAS_TLSv1_2', util.safehasattr(ssl, 'PROTOCOL_TLSv1_2')):
    supportedprotocols.add(b'tls1.2')


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

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

    # Allow minimum TLS protocol to be specified in the config.
    def validateprotocol(protocol, key):
        if protocol not in configprotocols:
            raise error.Abort(
                _(b'unsupported protocol from hostsecurity.%s: %s')
                % (key, protocol),
                hint=_(b'valid protocols: %s')
                % b' '.join(sorted(configprotocols)),
            )

    # We default to TLS 1.1+ because TLS 1.0 has known vulnerabilities (like
    # BEAST and POODLE). We allow users to downgrade to TLS 1.0+ via config
    # options in case a legacy server is encountered.

    # setup.py checks that TLS 1.1 or TLS 1.2 is present, so the following
    # assert should not fail.
    assert supportedprotocols - {b'tls1.0'}
    defaultminimumprotocol = b'tls1.1'

    key = b'minimumprotocol'
    minimumprotocol = ui.config(b'hostsecurity', key, defaultminimumprotocol)
    validateprotocol(minimumprotocol, key)

    key = b'%s:minimumprotocol' % bhostname
    minimumprotocol = ui.config(b'hostsecurity', key, minimumprotocol)
    validateprotocol(minimumprotocol, key)

    # If --insecure is used, we allow the use of TLS 1.0 despite config options.
    # We always print a "connection security to %s is disabled..." message when
    # --insecure is used. So no need to print anything more here.
    if ui.insecureconnections:
        minimumprotocol = b'tls1.0'

    s[b'minimumprotocol'] = minimumprotocol

    ciphers = ui.config(b'hostsecurity', b'ciphers')
    ciphers = ui.config(b'hostsecurity', b'%s:ciphers' % bhostname, ciphers)
    s[b'ciphers'] = ciphers

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

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

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

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

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

    if ui.configbool(b'devel', b'disableloaddefaultcerts'):
        s[b'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(b'hostsecurity', b'%s:verifycertsfile' % bhostname)
    if s[b'certfingerprints'] and cafile:
        ui.warn(
            _(
                b'(hostsecurity.%s:verifycertsfile ignored when host '
                b'fingerprints defined; using host fingerprints for '
                b'verification)\n'
            )
            % bhostname
        )

    # Try to hook up CA certificate validation unless something above
    # makes it not necessary.
    if s[b'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(
                    _(b'path specified by %s does not exist: %s')
                    % (
                        b'hostsecurity.%s:verifycertsfile' % (bhostname,),
                        cafile,
                    )
                )
            s[b'cafile'] = cafile
        else:
            # Find global certificates file in config.
            cafile = ui.config(b'web', b'cacerts')

            if cafile:
                cafile = util.expandpath(cafile)
                if not os.path.exists(cafile):
                    raise error.Abort(
                        _(b'could not find web.cacerts: %s') % cafile
                    )
            elif s[b'allowloaddefaultcerts']:
                # CAs not defined in config. Try to find system bundles.
                cafile = _defaultcacerts(ui)
                if cafile:
                    ui.debug(b'using %s for CA file\n' % cafile)

            s[b'cafile'] = cafile

        # Require certificate validation if CA certs are being loaded and
        # verification hasn't been disabled above.
        if cafile or s[b'allowloaddefaultcerts']:
            s[b'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[b'verifymode'] = ssl.CERT_NONE

    assert s[b'verifymode'] is not None

    return s


def commonssloptions(minimumprotocol):
    """Return SSLContext options common to servers and clients."""
    if minimumprotocol not in configprotocols:
        raise ValueError(b'protocol value not supported: %s' % minimumprotocol)

    # SSLv2 and SSLv3 are broken. We ban them outright.
    options = ssl.OP_NO_SSLv2 | ssl.OP_NO_SSLv3

    if minimumprotocol == b'tls1.0':
        # Defaults above are to use TLS 1.0+
        pass
    elif minimumprotocol == b'tls1.1':
        options |= ssl.OP_NO_TLSv1
    elif minimumprotocol == b'tls1.2':
        options |= ssl.OP_NO_TLSv1 | ssl.OP_NO_TLSv1_1
    else:
        raise error.Abort(_(b'this should not happen'))

    # Prevent CRIME.
    # There is no guarantee this attribute is defined on the module.
    options |= getattr(ssl, 'OP_NO_COMPRESSION', 0)

    return options


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(_(b'serverhostname argument is required'))

    if b'SSLKEYLOGFILE' in encoding.environ:
        try:
            import sslkeylog

            sslkeylog.set_keylog(
                pycompat.fsdecode(encoding.environ[b'SSLKEYLOGFILE'])
            )
            ui.warnnoi18n(
                b'sslkeylog enabled by SSLKEYLOGFILE environment variable\n'
            )
        except ImportError:
            ui.warnnoi18n(
                b'sslkeylog module missing, '
                b'but SSLKEYLOGFILE set in environment\n'
            )

    for f in (keyfile, certfile):
        if f and not os.path.exists(f):
            raise error.Abort(
                _(b'certificate file (%s) does not exist; cannot connect to %s')
                % (f, pycompat.bytesurl(serverhostname)),
                hint=_(
                    b'restore missing file or fix references '
                    b'in Mercurial config'
                ),
            )

    settings = _hostsettings(ui, serverhostname)

    # We can't use ssl.create_default_context() because it calls
    # load_default_certs() unless CA arguments are passed to it. We want to
    # have explicit control over CA loading because implicitly loading
    # CAs may undermine the user's intent. For example, a user may define a CA
    # bundle with a specific CA cert removed. If the system/default CA bundle
    # is loaded and contains that removed CA, you've just undone the user's
    # choice.
    #
    # Despite its name, PROTOCOL_SSLv23 selects the highest protocol that both
    # ends support, including TLS protocols. commonssloptions() restricts the
    # set of allowed protocols.
    sslcontext = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
    sslcontext.options |= commonssloptions(settings[b'minimumprotocol'])
    sslcontext.verify_mode = settings[b'verifymode']

    if settings[b'ciphers']:
        try:
            sslcontext.set_ciphers(pycompat.sysstr(settings[b'ciphers']))
        except ssl.SSLError as e:
            raise error.Abort(
                _(b'could not set ciphers: %s')
                % stringutil.forcebytestr(e.args[0]),
                hint=_(b'change cipher string (%s) in config')
                % settings[b'ciphers'],
            )

    if certfile is not None:

        def password():
            f = keyfile or certfile
            return ui.getpass(_(b'passphrase for %s: ') % f, b'')

        sslcontext.load_cert_chain(certfile, keyfile, password)

    if settings[b'cafile'] is not None:
        try:
            sslcontext.load_verify_locations(cafile=settings[b'cafile'])
        except ssl.SSLError as e:
            if len(e.args) == 1:  # pypy has different SSLError args
                msg = e.args[0]
            else:
                msg = e.args[1]
            raise error.Abort(
                _(b'error loading CA file %s: %s')
                % (settings[b'cafile'], stringutil.forcebytestr(msg)),
                hint=_(b'file is empty or malformed?'),
            )
        caloaded = True
    elif settings[b'allowloaddefaultcerts']:
        # This is a no-op on old Python.
        sslcontext.load_default_certs()
        caloaded = True
    else:
        caloaded = False

    try:
        sslsocket = sslcontext.wrap_socket(sock, server_hostname=serverhostname)
    except ssl.SSLError as e:
        # If we're doing certificate verification and no CA certs are loaded,
        # that is almost certainly the reason why verification failed. Provide
        # a hint to the user.
        # The exception handler is here to handle bugs around cert attributes:
        # https://bugs.python.org/issue20916#msg213479.  (See issues5313.)
        # When the main 20916 bug occurs, 'sslcontext.get_ca_certs()' is a
        # non-empty list, but the following conditional is otherwise True.
        try:
            if (
                caloaded
                and settings[b'verifymode'] == ssl.CERT_REQUIRED
                and not sslcontext.get_ca_certs()
            ):
                ui.warn(
                    _(
                        b'(an attempt was made to load CA certificates but '
                        b'none were loaded; see '
                        b'https://mercurial-scm.org/wiki/SecureConnections '
                        b'for how to configure Mercurial to avoid this '
                        b'error)\n'
                    )
                )
        except ssl.SSLError:
            pass

        # Try to print more helpful error messages for known failures.
        if util.safehasattr(e, b'reason'):
            # This error occurs when the client and server don't share a
            # common/supported SSL/TLS protocol. We've disabled SSLv2 and SSLv3
            # outright. Hopefully the reason for this error is that we require
            # TLS 1.1+ and the server only supports TLS 1.0. Whatever the
            # reason, try to emit an actionable warning.
            if e.reason == 'UNSUPPORTED_PROTOCOL':
                # We attempted TLS 1.0+.
                if settings[b'minimumprotocol'] == b'tls1.0':
                    # We support more than just TLS 1.0+. If this happens,
                    # the likely scenario is either the client or the server
                    # is really old. (e.g. server doesn't support TLS 1.0+ or
                    # client doesn't support modern TLS versions introduced
                    # several years from when this comment was written).
                    if supportedprotocols != {b'tls1.0'}:
                        ui.warn(
                            _(
                                b'(could not communicate with %s using security '
                                b'protocols %s; if you are using a modern Mercurial '
                                b'version, consider contacting the operator of this '
                                b'server; see '
                                b'https://mercurial-scm.org/wiki/SecureConnections '
                                b'for more info)\n'
                            )
                            % (
                                pycompat.bytesurl(serverhostname),
                                b', '.join(sorted(supportedprotocols)),
                            )
                        )
                    else:
                        ui.warn(
                            _(
                                b'(could not communicate with %s using TLS 1.0; the '
                                b'likely cause of this is the server no longer '
                                b'supports TLS 1.0 because it has known security '
                                b'vulnerabilities; see '
                                b'https://mercurial-scm.org/wiki/SecureConnections '
                                b'for more info)\n'
                            )
                            % pycompat.bytesurl(serverhostname)
                        )
                else:
                    # We attempted TLS 1.1+. We can only get here if the client
                    # supports the configured protocol. So the likely reason is
                    # the client wants better security than the server can
                    # offer.
                    ui.warn(
                        _(
                            b'(could not negotiate a common security protocol (%s+) '
                            b'with %s; the likely cause is Mercurial is configured '
                            b'to be more secure than the server can support)\n'
                        )
                        % (
                            settings[b'minimumprotocol'],
                            pycompat.bytesurl(serverhostname),
                        )
                    )
                    ui.warn(
                        _(
                            b'(consider contacting the operator of this '
                            b'server and ask them to support modern TLS '
                            b'protocol versions; or, set '
                            b'hostsecurity.%s:minimumprotocol=tls1.0 to allow '
                            b'use of legacy, less secure protocols when '
                            b'communicating with this server)\n'
                        )
                        % pycompat.bytesurl(serverhostname)
                    )
                    ui.warn(
                        _(
                            b'(see https://mercurial-scm.org/wiki/SecureConnections '
                            b'for more info)\n'
                        )
                    )

            elif e.reason == 'CERTIFICATE_VERIFY_FAILED' and pycompat.iswindows:

                ui.warn(
                    _(
                        b'(the full certificate chain may not be available '
                        b'locally; see "hg help debugssl")\n'
                    )
                )
        raise

    # check if wrap_socket failed silently because socket had been
    # closed
    # - see http://bugs.python.org/issue13721
    if not sslsocket.cipher():
        raise error.SecurityError(_(b'ssl connection failed'))

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

    return sslsocket


def wrapserversocket(
    sock, ui, certfile=None, keyfile=None, cafile=None, requireclientcert=False
):
    """Wrap a socket for use by servers.

    ``certfile`` and ``keyfile`` specify the files containing the certificate's
    public and private keys, respectively. Both keys can be defined in the same
    file via ``certfile`` (the private key must come first in the file).

    ``cafile`` defines the path to certificate authorities.

    ``requireclientcert`` specifies whether to require client certificates.

    Typically ``cafile`` is only defined if ``requireclientcert`` is true.
    """
    # This function is not used much by core Mercurial, so the error messaging
    # doesn't have to be as detailed as for wrapsocket().
    for f in (certfile, keyfile, cafile):
        if f and not os.path.exists(f):
            raise error.Abort(
                _(b'referenced certificate file (%s) does not exist') % f
            )

    # Despite its name, PROTOCOL_SSLv23 selects the highest protocol that both
    # ends support, including TLS protocols. commonssloptions() restricts the
    # set of allowed protocols.
    protocol = ssl.PROTOCOL_SSLv23
    options = commonssloptions(b'tls1.0')

    # This config option is intended for use in tests only. It is a giant
    # footgun to kill security. Don't define it.
    exactprotocol = ui.config(b'devel', b'serverexactprotocol')
    if exactprotocol == b'tls1.0':
        if b'tls1.0' not in supportedprotocols:
            raise error.Abort(_(b'TLS 1.0 not supported by this Python'))
        protocol = ssl.PROTOCOL_TLSv1
    elif exactprotocol == b'tls1.1':
        if b'tls1.1' not in supportedprotocols:
            raise error.Abort(_(b'TLS 1.1 not supported by this Python'))
        protocol = ssl.PROTOCOL_TLSv1_1
    elif exactprotocol == b'tls1.2':
        if b'tls1.2' not in supportedprotocols:
            raise error.Abort(_(b'TLS 1.2 not supported by this Python'))
        protocol = ssl.PROTOCOL_TLSv1_2
    elif exactprotocol:
        raise error.Abort(
            _(b'invalid value for serverexactprotocol: %s') % exactprotocol
        )

    # We /could/ use create_default_context() here since it doesn't load
    # CAs when configured for client auth. However, it is hard-coded to
    # use ssl.PROTOCOL_SSLv23 which may not be appropriate here.
    sslcontext = ssl.SSLContext(protocol)
    sslcontext.options |= options

    # Improve forward secrecy.
    sslcontext.options |= getattr(ssl, 'OP_SINGLE_DH_USE', 0)
    sslcontext.options |= getattr(ssl, 'OP_SINGLE_ECDH_USE', 0)

    # Use the list of more secure ciphers if found in the ssl module.
    if util.safehasattr(ssl, b'_RESTRICTED_SERVER_CIPHERS'):
        sslcontext.options |= getattr(ssl, 'OP_CIPHER_SERVER_PREFERENCE', 0)
        sslcontext.set_ciphers(ssl._RESTRICTED_SERVER_CIPHERS)

    if requireclientcert:
        sslcontext.verify_mode = ssl.CERT_REQUIRED
    else:
        sslcontext.verify_mode = ssl.CERT_NONE

    if certfile or keyfile:
        sslcontext.load_cert_chain(certfile=certfile, keyfile=keyfile)

    if cafile:
        sslcontext.load_verify_locations(cafile=cafile)

    return sslcontext.wrap_socket(sock, server_side=True)


class wildcarderror(Exception):
    """Represents an error parsing wildcards in DNS name."""


def _dnsnamematch(dn, hostname, maxwildcards=1):
    """Match DNS names according RFC 6125 section 6.4.3.

    This code is effectively copied from CPython's ssl._dnsname_match.

    Returns a bool indicating whether the expected hostname matches
    the value in ``dn``.
    """
    pats = []
    if not dn:
        return False
    dn = pycompat.bytesurl(dn)
    hostname = pycompat.bytesurl(hostname)

    pieces = dn.split(b'.')
    leftmost = pieces[0]
    remainder = pieces[1:]
    wildcards = leftmost.count(b'*')
    if wildcards > maxwildcards:
        raise wildcarderror(
            _(b'too many wildcards in certificate DNS name: %s') % dn
        )

    # speed up common case w/o wildcards
    if not wildcards:
        return dn.lower() == hostname.lower()

    # RFC 6125, section 6.4.3, subitem 1.
    # The client SHOULD NOT attempt to match a presented identifier in which
    # the wildcard character comprises a label other than the left-most label.
    if leftmost == b'*':
        # When '*' is a fragment by itself, it matches a non-empty dotless
        # fragment.
        pats.append(b'[^.]+')
    elif leftmost.startswith(b'xn--') or hostname.startswith(b'xn--'):
        # RFC 6125, section 6.4.3, subitem 3.
        # The client SHOULD NOT attempt to match a presented identifier
        # where the wildcard character is embedded within an A-label or
        # U-label of an internationalized domain name.
        pats.append(stringutil.reescape(leftmost))
    else:
        # Otherwise, '*' matches any dotless string, e.g. www*
        pats.append(stringutil.reescape(leftmost).replace(br'\*', b'[^.]*'))

    # add the remaining fragments, ignore any wildcards
    for frag in remainder:
        pats.append(stringutil.reescape(frag))

    pat = re.compile(br'\A' + br'\.'.join(pats) + br'\Z', re.IGNORECASE)
    return pat.match(hostname) is not None


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 _(b'no certificate received')

    dnsnames = []
    san = cert.get('subjectAltName', [])
    for key, value in san:
        if key == 'DNS':
            try:
                if _dnsnamematch(value, hostname):
                    return
            except wildcarderror as e:
                return stringutil.forcebytestr(e.args[0])

            dnsnames.append(value)

    if not dnsnames:
        # The subject is only checked when there is no DNS in subjectAltName.
        for sub in cert.get('subject', []):
            for key, value in sub:
                # According to RFC 2818 the most specific Common Name must
                # be used.
                if key == 'commonName':
                    # 'subject' entries are unicode.
                    try:
                        value = value.encode('ascii')
                    except UnicodeEncodeError:
                        return _(b'IDN in certificate not supported')

                    try:
                        if _dnsnamematch(value, hostname):
                            return
                    except wildcarderror as e:
                        return stringutil.forcebytestr(e.args[0])

                    dnsnames.append(value)

    dnsnames = [pycompat.bytesurl(d) for d in dnsnames]
    if len(dnsnames) > 1:
        return _(b'certificate is for %s') % b', '.join(dnsnames)
    elif len(dnsnames) == 1:
        return _(b'certificate is for %s') % dnsnames[0]
    else:
        return _(b'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 (
        not pycompat.isdarwin
        or resourceutil.mainfrozen()
        or not pycompat.sysexecutable
    ):
        return False
    exe = os.path.realpath(pycompat.sysexecutable).lower()
    return exe.startswith(b'/usr/bin/python') or exe.startswith(
        b'/system/library/frameworks/python.framework/'
    )


def _defaultcacerts(ui):
    """return path to default CA certificates or None.

    It is assumed this function is called when the returned certificates
    file will actually be used to validate connections. Therefore this
    function may print warnings or debug messages assuming this usage.

    We don't print a message when the Python is able to load default
    CA certs because this scenario is detected at socket connect time.
    """
    # The "certifi" Python package provides certificates. If it is installed
    # and usable, assume the user intends it to be used and use it.
    try:
        import certifi

        certs = certifi.where()
        if os.path.exists(certs):
            ui.debug(b'using ca certificates from certifi\n')
            return pycompat.fsencode(certs)
    except (ImportError, AttributeError):
        pass

    # Apple's OpenSSL has patches that allow a specially constructed certificate
    # to load the system CA store. If we're running on Apple Python, use this
    # trick.
    if _plainapplepython():
        dummycert = os.path.join(
            os.path.dirname(pycompat.fsencode(__file__)), b'dummycert.pem'
        )
        if os.path.exists(dummycert):
            return dummycert

    return None


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

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

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

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

    if settings[b'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(
            _(
                b'warning: connection security to %s is disabled per current '
                b'settings; communication is susceptible to eavesdropping '
                b'and tampering\n'
            )
            % host
        )
        return

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

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

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

    if settings[b'certfingerprints']:
        for hash, fingerprint in settings[b'certfingerprints']:
            if peerfingerprints[hash].lower() == fingerprint:
                ui.debug(
                    b'%s certificate matched fingerprint %s:%s\n'
                    % (host, hash, fmtfingerprint(fingerprint))
                )
                if settings[b'legacyfingerprint']:
                    ui.warn(
                        _(
                            b'(SHA-1 fingerprint for %s found in legacy '
                            b'[hostfingerprints] section; '
                            b'if you trust this fingerprint, remove the old '
                            b'SHA-1 fingerprint from [hostfingerprints] and '
                            b'add the following entry to the new '
                            b'[hostsecurity] section: %s:fingerprints=%s)\n'
                        )
                        % (host, host, nicefingerprint)
                    )
                return

        # Pinned fingerprint didn't match. This is a fatal error.
        if settings[b'legacyfingerprint']:
            section = b'hostfingerprint'
            nice = fmtfingerprint(peerfingerprints[b'sha1'])
        else:
            section = b'hostsecurity'
            nice = b'%s:%s' % (hash, fmtfingerprint(peerfingerprints[hash]))
        raise error.SecurityError(
            _(b'certificate for %s has unexpected fingerprint %s')
            % (host, nice),
            hint=_(b'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[b'caloaded']:
        raise error.SecurityError(
            _(
                b'unable to verify security of %s (no loaded CA certificates); '
                b'refusing to connect'
            )
            % host,
            hint=_(
                b'see https://mercurial-scm.org/wiki/SecureConnections for '
                b'how to configure Mercurial to avoid this error or set '
                b'hostsecurity.%s:fingerprints=%s to trust this server'
            )
            % (host, nicefingerprint),
        )

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