Add ability to perform password reset via email without trusting the identity server (#5377)

Sends password reset emails from the homeserver instead of proxying to the identity server. This is now the default behaviour for security reasons. If you wish to continue proxying password reset requests to the identity server you must now enable the email.trust_identity_server_for_password_resets option.

This PR is a culmination of 3 smaller PRs which have each been separately reviewed:

* #5308
* #5345
* #5368

17 files changed, 871 insertions, 60 deletions

diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index e91697049c..66201d6efe 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -339,6 +339,15 @@ class UnsupportedRoomVersionError(SynapseError):
         )
 
 
+class ThreepidValidationError(SynapseError):
+    """An error raised when there was a problem authorising an event."""
+
+    def __init__(self, *args, **kwargs):
+        if "errcode" not in kwargs:
+            kwargs["errcode"] = Codes.FORBIDDEN
+        super(ThreepidValidationError, self).__init__(*args, **kwargs)
+
+
 class IncompatibleRoomVersionError(SynapseError):
     """A server is trying to join a room whose version it does not support.
 
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 1045d28949..df524a23dd 100755
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -176,6 +176,7 @@ class SynapseHomeServer(HomeServer):
 
             resources.update({
                 "/_matrix/client/api/v1": client_resource,
+                "/_synapse/password_reset": client_resource,
                 "/_matrix/client/r0": client_resource,
                 "/_matrix/client/unstable": client_resource,
                 "/_matrix/client/v2_alpha": client_resource,
diff --git a/synapse/config/emailconfig.py b/synapse/config/emailconfig.py
index 8400471f40..ae04252906 100644
--- a/synapse/config/emailconfig.py
+++ b/synapse/config/emailconfig.py
@@ -50,6 +50,11 @@ class EmailConfig(Config):
         else:
             self.email_app_name = "Matrix"
 
+        # TODO: Rename notif_from to something more generic, or have a separate
+        # from for password resets, message notifications, etc?
+        # Currently the email section is a bit bogged down with settings for
+        # multiple functions. Would be good to split it out into separate
+        # sections and only put the common ones under email:
         self.email_notif_from = email_config.get("notif_from", None)
         if self.email_notif_from is not None:
             # make sure it's valid
@@ -74,7 +79,28 @@ class EmailConfig(Config):
             "account_validity", {},
         ).get("renew_at")
 
-        if self.email_enable_notifs or account_validity_renewal_enabled:
+        email_trust_identity_server_for_password_resets = email_config.get(
+            "trust_identity_server_for_password_resets", False,
+        )
+        self.email_password_reset_behaviour = (
+            "remote" if email_trust_identity_server_for_password_resets else "local"
+        )
+        if self.email_password_reset_behaviour == "local" and email_config == {}:
+            logger.warn(
+                "User password resets have been disabled due to lack of email config"
+            )
+            self.email_password_reset_behaviour = "off"
+
+        # Get lifetime of a validation token in milliseconds
+        self.email_validation_token_lifetime = self.parse_duration(
+            email_config.get("validation_token_lifetime", "1h")
+        )
+
+        if (
+            self.email_enable_notifs
+            or account_validity_renewal_enabled
+            or self.email_password_reset_behaviour == "local"
+        ):
             # make sure we can import the required deps
             import jinja2
             import bleach
@@ -82,6 +108,67 @@ class EmailConfig(Config):
             jinja2
             bleach
 
+        if self.email_password_reset_behaviour == "local":
+            required = [
+                "smtp_host",
+                "smtp_port",
+                "notif_from",
+            ]
+
+            missing = []
+            for k in required:
+                if k not in email_config:
+                    missing.append(k)
+
+            if (len(missing) > 0):
+                raise RuntimeError(
+                    "email.password_reset_behaviour is set to 'local' "
+                    "but required keys are missing: %s" %
+                    (", ".join(["email." + k for k in missing]),)
+                )
+
+            # Templates for password reset emails
+            self.email_password_reset_template_html = email_config.get(
+                "password_reset_template_html", "password_reset.html",
+            )
+            self.email_password_reset_template_text = email_config.get(
+                "password_reset_template_text", "password_reset.txt",
+            )
+            self.email_password_reset_failure_template = email_config.get(
+                "password_reset_failure_template", "password_reset_failure.html",
+            )
+            # This template does not support any replaceable variables, so we will
+            # read it from the disk once during setup
+            email_password_reset_success_template = email_config.get(
+                "password_reset_success_template", "password_reset_success.html",
+            )
+
+            # Check templates exist
+            for f in [self.email_password_reset_template_html,
+                      self.email_password_reset_template_text,
+                      self.email_password_reset_failure_template,
+                      email_password_reset_success_template]:
+                p = os.path.join(self.email_template_dir, f)
+                if not os.path.isfile(p):
+                    raise ConfigError("Unable to find template file %s" % (p, ))
+
+            # Retrieve content of web templates
+            filepath = os.path.join(
+                self.email_template_dir,
+                email_password_reset_success_template,
+            )
+            self.email_password_reset_success_html_content = self.read_file(
+                filepath,
+                "email.password_reset_template_success_html",
+            )
+
+            if config.get("public_baseurl") is None:
+                raise RuntimeError(
+                    "email.password_reset_behaviour is set to 'local' but no "
+                    "public_baseurl is set. This is necessary to generate password "
+                    "reset links"
+                )
+
         if self.email_enable_notifs:
             required = [
                 "smtp_host",
@@ -121,10 +208,6 @@ class EmailConfig(Config):
             self.email_riot_base_url = email_config.get(
                 "riot_base_url", None
             )
-        else:
-            self.email_enable_notifs = False
-            # Not much point setting defaults for the rest: it would be an
-            # error for them to be used.
 
         if account_validity_renewal_enabled:
             self.email_expiry_template_html = email_config.get(
@@ -141,10 +224,8 @@ class EmailConfig(Config):
 
     def default_config(self, config_dir_path, server_name, **kwargs):
         return """
-        # Enable sending emails for notification events or expiry notices
-        # Defining a custom URL for Riot is only needed if email notifications
-        # should contain links to a self-hosted installation of Riot; when set
-        # the "app_name" setting is ignored.
+        # Enable sending emails for password resets, notification events or
+        # account expiry notices
         #
         # If your SMTP server requires authentication, the optional smtp_user &
         # smtp_pass variables should be used
@@ -152,20 +233,62 @@ class EmailConfig(Config):
         #email:
         #   enable_notifs: false
         #   smtp_host: "localhost"
-        #   smtp_port: 25
+        #   smtp_port: 25 # SSL: 465, STARTTLS: 587
         #   smtp_user: "exampleusername"
         #   smtp_pass: "examplepassword"
         #   require_transport_security: False
         #   notif_from: "Your Friendly %(app)s Home Server <noreply@example.com>"
         #   app_name: Matrix
-        #   # if template_dir is unset, uses the example templates that are part of
-        #   # the Synapse distribution.
+        #
+        #   # Enable email notifications by default
+        #   notif_for_new_users: True
+        #
+        #   # Defining a custom URL for Riot is only needed if email notifications
+        #   # should contain links to a self-hosted installation of Riot; when set
+        #   # the "app_name" setting is ignored
+        #   riot_base_url: "http://localhost/riot"
+        #
+        #   # Enable sending password reset emails via the configured, trusted
+        #   # identity servers
+        #   #
+        #   # IMPORTANT! This will give a malicious or overtaken identity server
+        #   # the ability to reset passwords for your users! Make absolutely sure
+        #   # that you want to do this! It is strongly recommended that password
+        #   # reset emails be sent by the homeserver instead
+        #   #
+        #   # If this option is set to false and SMTP options have not been
+        #   # configured, resetting user passwords via email will be disabled
+        #   #trust_identity_server_for_password_resets: false
+        #
+        #   # Configure the time that a validation email or text message code
+        #   # will expire after sending
+        #   #
+        #   # This is currently used for password resets
+        #   #validation_token_lifetime: 1h
+        #
+        #   # Template directory. All template files should be stored within this
+        #   # directory
+        #   #
         #   #template_dir: res/templates
+        #
+        #   # Templates for email notifications
+        #   #
         #   notif_template_html: notif_mail.html
         #   notif_template_text: notif_mail.txt
-        #   # Templates for account expiry notices.
+        #
+        #   # Templates for account expiry notices
+        #   #
         #   expiry_template_html: notice_expiry.html
         #   expiry_template_text: notice_expiry.txt
-        #   notif_for_new_users: True
-        #   riot_base_url: "http://localhost/riot"
+        #
+        #   # Templates for password reset emails sent by the homeserver
+        #   #
+        #   #password_reset_template_html: password_reset.html
+        #   #password_reset_template_text: password_reset.txt
+        #
+        #   # Templates for password reset success and failure pages that a user
+        #   # will see after attempting to reset their password
+        #   #
+        #   #password_reset_template_success_html: password_reset_success.html
+        #   #password_reset_template_failure_html: password_reset_failure.html
         """
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index aa5d89a9ac..7f8ddc99c6 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -162,7 +162,7 @@ class AuthHandler(BaseHandler):
         defer.returnValue(params)
 
     @defer.inlineCallbacks
-    def check_auth(self, flows, clientdict, clientip):
+    def check_auth(self, flows, clientdict, clientip, password_servlet=False):
         """
         Takes a dictionary sent by the client in the login / registration
         protocol and handles the User-Interactive Auth flow.
@@ -186,6 +186,16 @@ class AuthHandler(BaseHandler):
 
             clientip (str): The IP address of the client.
 
+            password_servlet (bool): Whether the request originated from
+                PasswordRestServlet.
+                XXX: This is a temporary hack to distinguish between checking
+                for threepid validations locally (in the case of password
+                resets) and using the identity server (in the case of binding
+                a 3PID during registration). Once we start using the
+                homeserver for both tasks, this distinction will no longer be
+                necessary.
+
+
         Returns:
             defer.Deferred[dict, dict, str]: a deferred tuple of
                 (creds, params, session_id).
@@ -241,7 +251,9 @@ class AuthHandler(BaseHandler):
         if 'type' in authdict:
             login_type = authdict['type']
             try:
-                result = yield self._check_auth_dict(authdict, clientip)
+                result = yield self._check_auth_dict(
+                    authdict, clientip, password_servlet=password_servlet,
+                )
                 if result:
                     creds[login_type] = result
                     self._save_session(session)
@@ -351,7 +363,7 @@ class AuthHandler(BaseHandler):
         return sess.setdefault('serverdict', {}).get(key, default)
 
     @defer.inlineCallbacks
-    def _check_auth_dict(self, authdict, clientip):
+    def _check_auth_dict(self, authdict, clientip, password_servlet=False):
         """Attempt to validate the auth dict provided by a client
 
         Args:
@@ -369,7 +381,13 @@ class AuthHandler(BaseHandler):
         login_type = authdict['type']
         checker = self.checkers.get(login_type)
         if checker is not None:
-            res = yield checker(authdict, clientip)
+            # XXX: Temporary workaround for having Synapse handle password resets
+            # See AuthHandler.check_auth for further details
+            res = yield checker(
+                authdict,
+                clientip=clientip,
+                password_servlet=password_servlet,
+            )
             defer.returnValue(res)
 
         # build a v1-login-style dict out of the authdict and fall back to the
@@ -383,7 +401,7 @@ class AuthHandler(BaseHandler):
         defer.returnValue(canonical_id)
 
     @defer.inlineCallbacks
-    def _check_recaptcha(self, authdict, clientip):
+    def _check_recaptcha(self, authdict, clientip, **kwargs):
         try:
             user_response = authdict["response"]
         except KeyError:
@@ -429,20 +447,20 @@ class AuthHandler(BaseHandler):
                 defer.returnValue(True)
         raise LoginError(401, "", errcode=Codes.UNAUTHORIZED)
 
-    def _check_email_identity(self, authdict, _):
-        return self._check_threepid('email', authdict)
+    def _check_email_identity(self, authdict, **kwargs):
+        return self._check_threepid('email', authdict, **kwargs)
 
-    def _check_msisdn(self, authdict, _):
+    def _check_msisdn(self, authdict, **kwargs):
         return self._check_threepid('msisdn', authdict)
 
-    def _check_dummy_auth(self, authdict, _):
+    def _check_dummy_auth(self, authdict, **kwargs):
         return defer.succeed(True)
 
-    def _check_terms_auth(self, authdict, _):
+    def _check_terms_auth(self, authdict, **kwargs):
         return defer.succeed(True)
 
     @defer.inlineCallbacks
-    def _check_threepid(self, medium, authdict):
+    def _check_threepid(self, medium, authdict, password_servlet=False, **kwargs):
         if 'threepid_creds' not in authdict:
             raise LoginError(400, "Missing threepid_creds", Codes.MISSING_PARAM)
 
@@ -451,7 +469,29 @@ class AuthHandler(BaseHandler):
         identity_handler = self.hs.get_handlers().identity_handler
 
         logger.info("Getting validated threepid. threepidcreds: %r", (threepid_creds,))
-        threepid = yield identity_handler.threepid_from_creds(threepid_creds)
+        if (
+            not password_servlet
+            or self.hs.config.email_password_reset_behaviour == "remote"
+        ):
+            threepid = yield identity_handler.threepid_from_creds(threepid_creds)
+        elif self.hs.config.email_password_reset_behaviour == "local":
+            row = yield self.store.get_threepid_validation_session(
+                medium,
+                threepid_creds["client_secret"],
+                sid=threepid_creds["sid"],
+            )
+
+            threepid = {
+                "medium": row["medium"],
+                "address": row["address"],
+                "validated_at": row["validated_at"],
+            } if row else None
+
+            if row:
+                # Valid threepid returned, delete from the db
+                yield self.store.delete_threepid_session(threepid_creds["sid"])
+        else:
+            raise SynapseError(400, "Password resets are not enabled on this homeserver")
 
         if not threepid:
             raise LoginError(401, "", errcode=Codes.UNAUTHORIZED)
diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index 22469486d7..04caf65793 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -247,7 +247,14 @@ class IdentityHandler(BaseHandler):
         defer.returnValue(changed)
 
     @defer.inlineCallbacks
-    def requestEmailToken(self, id_server, email, client_secret, send_attempt, **kwargs):
+    def requestEmailToken(
+        self,
+        id_server,
+        email,
+        client_secret,
+        send_attempt,
+        next_link=None,
+    ):
         if not self._should_trust_id_server(id_server):
             raise SynapseError(
                 400, "Untrusted ID server '%s'" % id_server,
@@ -259,7 +266,9 @@ class IdentityHandler(BaseHandler):
             'client_secret': client_secret,
             'send_attempt': send_attempt,
         }
-        params.update(kwargs)
+
+        if next_link:
+            params.update({'next_link': next_link})
 
         try:
             data = yield self.http_client.post_json_get_json(
diff --git a/synapse/push/mailer.py b/synapse/push/mailer.py
index c269bcf4a4..4bc9eb7313 100644
--- a/synapse/push/mailer.py
+++ b/synapse/push/mailer.py
@@ -80,10 +80,10 @@ ALLOWED_ATTRS = {
 
 
 class Mailer(object):
-    def __init__(self, hs, app_name, notif_template_html, notif_template_text):
+    def __init__(self, hs, app_name, template_html, template_text):
         self.hs = hs
-        self.notif_template_html = notif_template_html
-        self.notif_template_text = notif_template_text
+        self.template_html = template_html
+        self.template_text = template_text
 
         self.sendmail = self.hs.get_sendmail()
         self.store = self.hs.get_datastore()
@@ -94,21 +94,48 @@ class Mailer(object):
         logger.info("Created Mailer for app_name %s" % app_name)
 
     @defer.inlineCallbacks
-    def send_notification_mail(self, app_id, user_id, email_address,
-                               push_actions, reason):
-        try:
-            from_string = self.hs.config.email_notif_from % {
-                "app": self.app_name
-            }
-        except TypeError:
-            from_string = self.hs.config.email_notif_from
+    def send_password_reset_mail(
+        self,
+        email_address,
+        token,
+        client_secret,
+        sid,
+    ):
+        """Send an email with a password reset link to a user
+
+        Args:
+            email_address (str): Email address we're sending the password
+                reset to
+            token (str): Unique token generated by the server to verify
+                password reset email was received
+            client_secret (str): Unique token generated by the client to
+                group together multiple email sending attempts
+            sid (str): The generated session ID
+        """
+        if email.utils.parseaddr(email_address)[1] == '':
+            raise RuntimeError("Invalid 'to' email address")
+
+        link = (
+            self.hs.config.public_baseurl +
+            "_synapse/password_reset/email/submit_token"
+            "?token=%s&client_secret=%s&sid=%s" %
+            (token, client_secret, sid)
+        )
 
-        raw_from = email.utils.parseaddr(from_string)[1]
-        raw_to = email.utils.parseaddr(email_address)[1]
+        template_vars = {
+            "link": link,
+        }
 
-        if raw_to == '':
-            raise RuntimeError("Invalid 'to' address")
+        yield self.send_email(
+            email_address,
+            "[%s] Password Reset Email" % self.hs.config.server_name,
+            template_vars,
+        )
 
+    @defer.inlineCallbacks
+    def send_notification_mail(self, app_id, user_id, email_address,
+                               push_actions, reason):
+        """Send email regarding a user's room notifications"""
         rooms_in_order = deduped_ordered_list(
             [pa['room_id'] for pa in push_actions]
         )
@@ -176,14 +203,36 @@ class Mailer(object):
             "reason": reason,
         }
 
-        html_text = self.notif_template_html.render(**template_vars)
+        yield self.send_email(
+            email_address,
+            "[%s] %s" % (self.app_name, summary_text),
+            template_vars,
+        )
+
+    @defer.inlineCallbacks
+    def send_email(self, email_address, subject, template_vars):
+        """Send an email with the given information and template text"""
+        try:
+            from_string = self.hs.config.email_notif_from % {
+                "app": self.app_name
+            }
+        except TypeError:
+            from_string = self.hs.config.email_notif_from
+
+        raw_from = email.utils.parseaddr(from_string)[1]
+        raw_to = email.utils.parseaddr(email_address)[1]
+
+        if raw_to == '':
+            raise RuntimeError("Invalid 'to' address")
+
+        html_text = self.template_html.render(**template_vars)
         html_part = MIMEText(html_text, "html", "utf8")
 
-        plain_text = self.notif_template_text.render(**template_vars)
+        plain_text = self.template_text.render(**template_vars)
         text_part = MIMEText(plain_text, "plain", "utf8")
 
         multipart_msg = MIMEMultipart('alternative')
-        multipart_msg['Subject'] = "[%s] %s" % (self.app_name, summary_text)
+        multipart_msg['Subject'] = subject
         multipart_msg['From'] = from_string
         multipart_msg['To'] = email_address
         multipart_msg['Date'] = email.utils.formatdate()
diff --git a/synapse/push/pusher.py b/synapse/push/pusher.py
index 14bc7823cf..aff85daeb5 100644
--- a/synapse/push/pusher.py
+++ b/synapse/push/pusher.py
@@ -70,8 +70,8 @@ class PusherFactory(object):
             mailer = Mailer(
                 hs=self.hs,
                 app_name=app_name,
-                notif_template_html=self.notif_template_html,
-                notif_template_text=self.notif_template_text,
+                template_html=self.notif_template_html,
+                template_text=self.notif_template_text,
             )
             self.mailers[app_name] = mailer
         return EmailPusher(self.hs, pusherdict, mailer)
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index f64baa4d58..c78f2cb15e 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -77,7 +77,7 @@ REQUIREMENTS = [
 ]
 
 CONDITIONAL_REQUIREMENTS = {
-    "email.enable_notifs": ["Jinja2>=2.9", "bleach>=1.4.2"],
+    "email": ["Jinja2>=2.9", "bleach>=1.4.2"],
     "matrix-synapse-ldap3": ["matrix-synapse-ldap3>=0.1"],
 
     # we use execute_batch, which arrived in psycopg 2.7.
diff --git a/synapse/res/templates/password_reset.html b/synapse/res/templates/password_reset.html
new file mode 100644
index 0000000000..4fa7b36734
--- /dev/null
+++ b/synapse/res/templates/password_reset.html
@@ -0,0 +1,9 @@
+<html>
+<body>
+    <p>A password reset request has been received for your Matrix account. If this was you, please click the link below to confirm resetting your password:</p>
+
+    <a href="{{ link }}">{{ link }}</a>
+
+    <p>If this was not you, please disregard this email and contact your server administrator. Thank you.</p>
+</body>
+</html>
diff --git a/synapse/res/templates/password_reset.txt b/synapse/res/templates/password_reset.txt
new file mode 100644
index 0000000000..f0deff59a7
--- /dev/null
+++ b/synapse/res/templates/password_reset.txt
@@ -0,0 +1,7 @@
+A password reset request has been received for your Matrix account. If this
+was you, please click the link below to confirm resetting your password:
+
+{{ link }}
+
+If this was not you, please disregard this email and contact your server
+administrator. Thank you.
diff --git a/synapse/res/templates/password_reset_failure.html b/synapse/res/templates/password_reset_failure.html
new file mode 100644
index 0000000000..0b132cf8db
--- /dev/null
+++ b/synapse/res/templates/password_reset_failure.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+<p>{{ failure_reason }}. Your password has not been reset.</p>
+</body>
+</html>
diff --git a/synapse/res/templates/password_reset_success.html b/synapse/res/templates/password_reset_success.html
new file mode 100644
index 0000000000..7b6fa5e6f0
--- /dev/null
+++ b/synapse/res/templates/password_reset_success.html
@@ -0,0 +1,6 @@
+<html>
+<head></head>
+<body>
+<p>Your password was successfully reset. You may now close this window.</p>
+</body>
+</html>
diff --git a/synapse/rest/client/v2_alpha/account.py b/synapse/rest/client/v2_alpha/account.py
index ca35dc3c83..e4c63b69b9 100644
--- a/synapse/rest/client/v2_alpha/account.py
+++ b/synapse/rest/client/v2_alpha/account.py
@@ -15,19 +15,25 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import re
 
 from six.moves import http_client
 
+import jinja2
+
 from twisted.internet import defer
 
 from synapse.api.constants import LoginType
-from synapse.api.errors import Codes, SynapseError
+from synapse.api.errors import Codes, SynapseError, ThreepidValidationError
+from synapse.http.server import finish_request
 from synapse.http.servlet import (
     RestServlet,
     assert_params_in_dict,
     parse_json_object_from_request,
+    parse_string,
 )
 from synapse.util.msisdn import phone_number_to_msisdn
+from synapse.util.stringutils import random_string
 from synapse.util.threepids import check_3pid_allowed
 
 from ._base import client_patterns, interactive_auth_handler
@@ -41,17 +47,42 @@ class EmailPasswordRequestTokenRestServlet(RestServlet):
     def __init__(self, hs):
         super(EmailPasswordRequestTokenRestServlet, self).__init__()
         self.hs = hs
+        self.datastore = hs.get_datastore()
+        self.config = hs.config
         self.identity_handler = hs.get_handlers().identity_handler
 
+        if self.config.email_password_reset_behaviour == "local":
+            from synapse.push.mailer import Mailer, load_jinja2_templates
+            templates = load_jinja2_templates(
+                config=hs.config,
+                template_html_name=hs.config.email_password_reset_template_html,
+                template_text_name=hs.config.email_password_reset_template_text,
+            )
+            self.mailer = Mailer(
+                hs=self.hs,
+                app_name=self.config.email_app_name,
+                template_html=templates[0],
+                template_text=templates[1],
+            )
+
     @defer.inlineCallbacks
     def on_POST(self, request):
+        if self.config.email_password_reset_behaviour == "off":
+            raise SynapseError(400, "Password resets have been disabled on this server")
+
         body = parse_json_object_from_request(request)
 
         assert_params_in_dict(body, [
-            'id_server', 'client_secret', 'email', 'send_attempt'
+            'client_secret', 'email', 'send_attempt'
         ])
 
-        if not check_3pid_allowed(self.hs, "email", body['email']):
+        # Extract params from body
+        client_secret = body["client_secret"]
+        email = body["email"]
+        send_attempt = body["send_attempt"]
+        next_link = body.get("next_link")  # Optional param
+
+        if not check_3pid_allowed(self.hs, "email", email):
             raise SynapseError(
                 403,
                 "Your email domain is not authorized on this server",
@@ -59,15 +90,100 @@ class EmailPasswordRequestTokenRestServlet(RestServlet):
             )
 
         existingUid = yield self.hs.get_datastore().get_user_id_by_threepid(
-            'email', body['email']
+            'email', email,
         )
 
         if existingUid is None:
             raise SynapseError(400, "Email not found", Codes.THREEPID_NOT_FOUND)
 
-        ret = yield self.identity_handler.requestEmailToken(**body)
+        if self.config.email_password_reset_behaviour == "remote":
+            if 'id_server' not in body:
+                raise SynapseError(400, "Missing 'id_server' param in body")
+
+            # Have the identity server handle the password reset flow
+            ret = yield self.identity_handler.requestEmailToken(
+                body["id_server"], email, client_secret, send_attempt, next_link,
+            )
+        else:
+            # Send password reset emails from Synapse
+            sid = yield self.send_password_reset(
+                email, client_secret, send_attempt, next_link,
+            )
+
+            # Wrap the session id in a JSON object
+            ret = {"sid": sid}
+
         defer.returnValue((200, ret))
 
+    @defer.inlineCallbacks
+    def send_password_reset(
+        self,
+        email,
+        client_secret,
+        send_attempt,
+        next_link=None,
+    ):
+        """Send a password reset email
+
+        Args:
+            email (str): The user's email address
+            client_secret (str): The provided client secret
+            send_attempt (int): Which send attempt this is
+
+        Returns:
+            The new session_id upon success
+
+        Raises:
+            SynapseError is an error occurred when sending the email
+        """
+        # Check that this email/client_secret/send_attempt combo is new or
+        # greater than what we've seen previously
+        session = yield self.datastore.get_threepid_validation_session(
+            "email", client_secret, address=email, validated=False,
+        )
+
+        # Check to see if a session already exists and that it is not yet
+        # marked as validated
+        if session and session.get("validated_at") is None:
+            session_id = session['session_id']
+            last_send_attempt = session['last_send_attempt']
+
+            # Check that the send_attempt is higher than previous attempts
+            if send_attempt <= last_send_attempt:
+                # If not, just return a success without sending an email
+                defer.returnValue(session_id)
+        else:
+            # An non-validated session does not exist yet.
+            # Generate a session id
+            session_id = random_string(16)
+
+        # Generate a new validation token
+        token = random_string(32)
+
+        # Send the mail with the link containing the token, client_secret
+        # and session_id
+        try:
+            yield self.mailer.send_password_reset_mail(
+                email, token, client_secret, session_id,
+            )
+        except Exception:
+            logger.exception(
+                "Error sending a password reset email to %s", email,
+            )
+            raise SynapseError(
+                500, "An error was encountered when sending the password reset email"
+            )
+
+        token_expires = (self.hs.clock.time_msec() +
+                         self.config.email_validation_token_lifetime)
+
+        yield self.datastore.start_or_continue_validation_session(
+            "email", email, session_id, client_secret, send_attempt,
+            next_link, token, token_expires,
+        )
+
+        defer.returnValue(session_id)
+
 
 class MsisdnPasswordRequestTokenRestServlet(RestServlet):
     PATTERNS = client_patterns("/account/password/msisdn/requestToken$")
@@ -80,6 +196,9 @@ class MsisdnPasswordRequestTokenRestServlet(RestServlet):
 
     @defer.inlineCallbacks
     def on_POST(self, request):
+        if not self.config.email_password_reset_behaviour == "off":
+            raise SynapseError(400, "Password resets have been disabled on this server")
+
         body = parse_json_object_from_request(request)
 
         assert_params_in_dict(body, [
@@ -107,6 +226,118 @@ class MsisdnPasswordRequestTokenRestServlet(RestServlet):
         defer.returnValue((200, ret))
 
 
+class PasswordResetSubmitTokenServlet(RestServlet):
+    """Handles 3PID validation token submission"""
+    PATTERNS = [
+        re.compile("^/_synapse/password_reset/(?P<medium>[^/]*)/submit_token/*$"),
+    ]
+
+    def __init__(self, hs):
+        """
+        Args:
+            hs (synapse.server.HomeServer): server
+        """
+        super(PasswordResetSubmitTokenServlet, self).__init__()
+        self.hs = hs
+        self.auth = hs.get_auth()
+        self.config = hs.config
+        self.clock = hs.get_clock()
+        self.datastore = hs.get_datastore()
+
+    @defer.inlineCallbacks
+    def on_GET(self, request, medium):
+        if medium != "email":
+            raise SynapseError(
+                400,
+                "This medium is currently not supported for password resets",
+            )
+
+        sid = parse_string(request, "sid")
+        client_secret = parse_string(request, "client_secret")
+        token = parse_string(request, "token")
+
+        # Attempt to validate a 3PID sesssion
+        try:
+            # Mark the session as valid
+            next_link = yield self.datastore.validate_threepid_session(
+                sid,
+                client_secret,
+                token,
+                self.clock.time_msec(),
+            )
+
+            # Perform a 302 redirect if next_link is set
+            if next_link:
+                if next_link.startswith("file:///"):
+                    logger.warn(
+                        "Not redirecting to next_link as it is a local file: address"
+                    )
+                else:
+                    request.setResponseCode(302)
+                    request.setHeader("Location", next_link)
+                    finish_request(request)
+                    defer.returnValue(None)
+
+            # Otherwise show the success template
+            html = self.config.email_password_reset_success_html_content
+            request.setResponseCode(200)
+        except ThreepidValidationError as e:
+            # Show a failure page with a reason
+            html = self.load_jinja2_template(
+                self.config.email_template_dir,
+                self.config.email_password_reset_failure_template,
+                template_vars={
+                    "failure_reason": e.msg,
+                }
+            )
+            request.setResponseCode(e.code)
+
+        request.write(html.encode('utf-8'))
+        finish_request(request)
+        defer.returnValue(None)
+
+    def load_jinja2_template(self, template_dir, template_filename, template_vars):
+        """Loads a jinja2 template with variables to insert
+
+        Args:
+            template_dir (str): The directory where templates are stored
+            template_filename (str): The name of the template in the template_dir
+            template_vars (Dict): Dictionary of keys in the template
+                alongside their values to insert
+
+        Returns:
+            str containing the contents of the rendered template
+        """
+        loader = jinja2.FileSystemLoader(template_dir)
+        env = jinja2.Environment(loader=loader)
+
+        template = env.get_template(template_filename)
+        return template.render(**template_vars)
+
+    @defer.inlineCallbacks
+    def on_POST(self, request, medium):
+        if medium != "email":
+            raise SynapseError(
+                400,
+                "This medium is currently not supported for password resets",
+            )
+
+        body = parse_json_object_from_request(request)
+        assert_params_in_dict(body, [
+            'sid', 'client_secret', 'token',
+        ])
+
+        valid, _ = yield self.datastore.validate_threepid_validation_token(
+            body['sid'],
+            body['client_secret'],
+            body['token'],
+            self.clock.time_msec(),
+        )
+        response_code = 200 if valid else 400
+
+        defer.returnValue((response_code, {"success": valid}))
+
+
 class PasswordRestServlet(RestServlet):
     PATTERNS = client_patterns("/account/password$")
 
@@ -144,6 +375,7 @@ class PasswordRestServlet(RestServlet):
             result, params, _ = yield self.auth_handler.check_auth(
                 [[LoginType.EMAIL_IDENTITY], [LoginType.MSISDN]],
                 body, self.hs.get_ip_from_request(request),
+                password_servlet=True,
             )
 
             if LoginType.EMAIL_IDENTITY in result:
@@ -417,6 +649,7 @@ class WhoamiRestServlet(RestServlet):
 def register_servlets(hs, http_server):
     EmailPasswordRequestTokenRestServlet(hs).register(http_server)
     MsisdnPasswordRequestTokenRestServlet(hs).register(http_server)
+    PasswordResetSubmitTokenServlet(hs).register(http_server)
     PasswordRestServlet(hs).register(http_server)
     DeactivateAccountRestServlet(hs).register(http_server)
     EmailThreepidRequestTokenRestServlet(hs).register(http_server)
diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 52891bb9eb..ae891aa332 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -588,6 +588,10 @@ class SQLBaseStore(object):
         Args:
             table : string giving the table name
             values : dict of new column names and values for them
+            or_ignore : bool stating whether an exception should be raised
+                when a conflicting row already exists. If True, False will be
+                returned by the function instead
+            desc : string giving a description of the transaction
 
         Returns:
             bool: Whether the row was inserted or not. Only useful when
@@ -1228,8 +1232,8 @@ class SQLBaseStore(object):
         )
 
         txn.execute(select_sql, list(keyvalues.values()))
-
         row = txn.fetchone()
+
         if not row:
             if allow_none:
                 return None
diff --git a/synapse/storage/prepare_database.py b/synapse/storage/prepare_database.py
index c1711bc8bd..23a4baa484 100644
--- a/synapse/storage/prepare_database.py
+++ b/synapse/storage/prepare_database.py
@@ -25,7 +25,7 @@ logger = logging.getLogger(__name__)
 
 # Remember to update this number every time a change is made to database
 # schema files, so the users will be informed on server restarts.
-SCHEMA_VERSION = 54
+SCHEMA_VERSION = 55
 
 dir_path = os.path.abspath(os.path.dirname(__file__))
 
diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index 4cf159ba81..9b41cbd757 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -17,17 +17,20 @@
 
 import re
 
+from six import iterkeys
 from six.moves import range
 
 from twisted.internet import defer
 
 from synapse.api.constants import UserTypes
-from synapse.api.errors import Codes, StoreError
+from synapse.api.errors import Codes, StoreError, ThreepidValidationError
 from synapse.storage import background_updates
 from synapse.storage._base import SQLBaseStore
 from synapse.types import UserID
 from synapse.util.caches.descriptors import cached, cachedInlineCallbacks
 
+THIRTY_MINUTES_IN_MS = 30 * 60 * 1000
+
 
 class RegistrationWorkerStore(SQLBaseStore):
     def __init__(self, db_conn, hs):
@@ -422,7 +425,7 @@ class RegistrationWorkerStore(SQLBaseStore):
         defer.returnValue(None)
 
     @defer.inlineCallbacks
-    def get_user_id_by_threepid(self, medium, address):
+    def get_user_id_by_threepid(self, medium, address, require_verified=False):
         """Returns user id from threepid
 
         Args:
@@ -595,6 +598,11 @@ class RegistrationStore(
             "user_threepids_grandfather", self._bg_user_threepids_grandfather,
         )
 
+        # Create a background job for culling expired 3PID validity tokens
+        hs.get_clock().looping_call(
+            self.cull_expired_threepid_validation_tokens, THIRTY_MINUTES_IN_MS,
+        )
+
     @defer.inlineCallbacks
     def add_access_token_to_user(self, user_id, token, device_id=None):
         """Adds an access token for the given user.
@@ -963,7 +971,6 @@ class RegistrationStore(
         We do this by grandfathering in existing user threepids assuming that
         they used one of the server configured trusted identity servers.
         """
-
         id_servers = set(self.config.trusted_third_party_id_servers)
 
         def _bg_user_threepids_grandfather_txn(txn):
@@ -984,3 +991,280 @@ class RegistrationStore(
         yield self._end_background_update("user_threepids_grandfather")
 
         defer.returnValue(1)
+
+    def get_threepid_validation_session(
+        self,
+        medium,
+        client_secret,
+        address=None,
+        sid=None,
+        validated=None,
+    ):
+        """Gets a session_id and last_send_attempt (if available) for a
+        client_secret/medium/(address|session_id) combo
+
+        Args:
+            medium (str|None): The medium of the 3PID
+            address (str|None): The address of the 3PID
+            sid (str|None): The ID of the validation session
+            client_secret (str|None): A unique string provided by the client to
+                help identify this validation attempt
+            validated (bool|None): Whether sessions should be filtered by
+                whether they have been validated already or not. None to
+                perform no filtering
+
+        Returns:
+            deferred {str, int}|None: A dict containing the
+                latest session_id and send_attempt count for this 3PID.
+                Otherwise None if there hasn't been a previous attempt
+        """
+        keyvalues = {
+            "medium": medium,
+            "client_secret": client_secret,
+        }
+        if address:
+            keyvalues["address"] = address
+        if sid:
+            keyvalues["session_id"] = sid
+
+        assert(address or sid)
+
+        def get_threepid_validation_session_txn(txn):
+            sql = """
+                SELECT address, session_id, medium, client_secret,
+                last_send_attempt, validated_at
+                FROM threepid_validation_session WHERE %s
+                """ % (" AND ".join("%s = ?" % k for k in iterkeys(keyvalues)),)
+
+            if validated is not None:
+                sql += " AND validated_at IS " + ("NOT NULL" if validated else "NULL")
+
+            sql += " LIMIT 1"
+
+            txn.execute(sql, list(keyvalues.values()))
+            rows = self.cursor_to_dict(txn)
+            if not rows:
+                return None
+
+            return rows[0]
+
+        return self.runInteraction(
+            "get_threepid_validation_session",
+            get_threepid_validation_session_txn,
+        )
+
+    def validate_threepid_session(
+        self,
+        session_id,
+        client_secret,
+        token,
+        current_ts,
+    ):
+        """Attempt to validate a threepid session using a token
+
+        Args:
+            session_id (str): The id of a validation session
+            client_secret (str): A unique string provided by the client to
+                help identify this validation attempt
+            token (str): A validation token
+            current_ts (int): The current unix time in milliseconds. Used for
+                checking token expiry status
+
+        Returns:
+            deferred str|None: A str representing a link to redirect the user
+            to if there is one.
+        """
+        # Insert everything into a transaction in order to run atomically
+        def validate_threepid_session_txn(txn):
+            row = self._simple_select_one_txn(
+                txn,
+                table="threepid_validation_session",
+                keyvalues={"session_id": session_id},
+                retcols=["client_secret", "validated_at"],
+                allow_none=True,
+            )
+
+            if not row:
+                raise ThreepidValidationError(400, "Unknown session_id")
+            retrieved_client_secret = row["client_secret"]
+            validated_at = row["validated_at"]
+
+            if retrieved_client_secret != client_secret:
+                raise ThreepidValidationError(
+                    400, "This client_secret does not match the provided session_id",
+                )
+
+            row = self._simple_select_one_txn(
+                txn,
+                table="threepid_validation_token",
+                keyvalues={"session_id": session_id, "token": token},
+                retcols=["expires", "next_link"],
+                allow_none=True,
+            )
+
+            if not row:
+                raise ThreepidValidationError(
+                    400, "Validation token not found or has expired",
+                )
+            expires = row["expires"]
+            next_link = row["next_link"]
+
+            # If the session is already validated, no need to revalidate
+            if validated_at:
+                return next_link
+
+            if expires <= current_ts:
+                raise ThreepidValidationError(
+                    400, "This token has expired. Please request a new one",
+                )
+
+            # Looks good. Validate the session
+            self._simple_update_txn(
+                txn,
+                table="threepid_validation_session",
+                keyvalues={"session_id": session_id},
+                updatevalues={"validated_at": self.clock.time_msec()},
+            )
+
+            return next_link
+
+        # Return next_link if it exists
+        return self.runInteraction(
+            "validate_threepid_session_txn",
+            validate_threepid_session_txn,
+        )
+
+    def upsert_threepid_validation_session(
+        self,
+        medium,
+        address,
+        client_secret,
+        send_attempt,
+        session_id,
+        validated_at=None,
+    ):
+        """Upsert a threepid validation session
+        Args:
+            medium (str): The medium of the 3PID
+            address (str): The address of the 3PID
+            client_secret (str): A unique string provided by the client to
+                help identify this validation attempt
+            send_attempt (int): The latest send_attempt on this session
+            session_id (str): The id of this validation session
+            validated_at (int|None): The unix timestamp in milliseconds of
+                when the session was marked as valid
+        """
+        insertion_values = {
+            "medium": medium,
+            "address": address,
+            "client_secret": client_secret,
+        }
+
+        if validated_at:
+            insertion_values["validated_at"] = validated_at
+
+        return self._simple_upsert(
+            table="threepid_validation_session",
+            keyvalues={"session_id": session_id},
+            values={"last_send_attempt": send_attempt},
+            insertion_values=insertion_values,
+            desc="upsert_threepid_validation_session",
+        )
+
+    def start_or_continue_validation_session(
+        self,
+        medium,
+        address,
+        session_id,
+        client_secret,
+        send_attempt,
+        next_link,
+        token,
+        token_expires,
+    ):
+        """Creates a new threepid validation session if it does not already
+        exist and associates a new validation token with it
+
+        Args:
+            medium (str): The medium of the 3PID
+            address (str): The address of the 3PID
+            session_id (str): The id of this validation session
+            client_secret (str): A unique string provided by the client to
+                help identify this validation attempt
+            send_attempt (int): The latest send_attempt on this session
+            next_link (str|None): The link to redirect the user to upon
+                successful validation
+            token (str): The validation token
+            token_expires (int): The timestamp for which after the token
+                will no longer be valid
+        """
+        def start_or_continue_validation_session_txn(txn):
+            # Create or update a validation session
+            self._simple_upsert_txn(
+                txn,
+                table="threepid_validation_session",
+                keyvalues={"session_id": session_id},
+                values={"last_send_attempt": send_attempt},
+                insertion_values={
+                    "medium": medium,
+                    "address": address,
+                    "client_secret": client_secret,
+                },
+            )
+
+            # Create a new validation token with this session ID
+            self._simple_insert_txn(
+                txn,
+                table="threepid_validation_token",
+                values={
+                    "session_id": session_id,
+                    "token": token,
+                    "next_link": next_link,
+                    "expires": token_expires,
+                },
+            )
+
+        return self.runInteraction(
+            "start_or_continue_validation_session",
+            start_or_continue_validation_session_txn,
+        )
+
+    def cull_expired_threepid_validation_tokens(self):
+        """Remove threepid validation tokens with expiry dates that have passed"""
+        def cull_expired_threepid_validation_tokens_txn(txn, ts):
+            sql = """
+            DELETE FROM threepid_validation_token WHERE
+            expires < ?
+            """
+            return txn.execute(sql, (ts,))
+
+        return self.runInteraction(
+            "cull_expired_threepid_validation_tokens",
+            cull_expired_threepid_validation_tokens_txn,
+            self.clock.time_msec(),
+        )
+
+    def delete_threepid_session(self, session_id):
+        """Removes a threepid validation session from the database. This can
+        be done after validation has been performed and whatever action was
+        waiting on it has been carried out
+
+        Args:
+            session_id (str): The ID of the session to delete
+        """
+        def delete_threepid_session_txn(txn):
+            self._simple_delete_txn(
+                txn,
+                table="threepid_validation_token",
+                keyvalues={"session_id": session_id},
+            )
+            self._simple_delete_txn(
+                txn,
+                table="threepid_validation_session",
+                keyvalues={"session_id": session_id},
+            )
+
+        return self.runInteraction(
+            "delete_threepid_session",
+            delete_threepid_session_txn,
+        )
diff --git a/synapse/storage/schema/delta/55/track_threepid_validations.sql b/synapse/storage/schema/delta/55/track_threepid_validations.sql
new file mode 100644
index 0000000000..a8eced2e0a
--- /dev/null
+++ b/synapse/storage/schema/delta/55/track_threepid_validations.sql
@@ -0,0 +1,31 @@
+/* Copyright 2019 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+CREATE TABLE IF NOT EXISTS threepid_validation_session (
+    session_id TEXT PRIMARY KEY,
+    medium TEXT NOT NULL,
+    address TEXT NOT NULL,
+    client_secret TEXT NOT NULL,
+    last_send_attempt BIGINT NOT NULL,
+    validated_at BIGINT
+);
+
+CREATE TABLE IF NOT EXISTS threepid_validation_token (
+    token TEXT PRIMARY KEY,
+    session_id TEXT NOT NULL,
+    next_link TEXT,
+    expires BIGINT NOT NULL
+);
+
+CREATE INDEX threepid_validation_token_session_id ON threepid_validation_token(session_id);