diff --git a/sydent/http/servlets/emailservlet.py b/sydent/http/servlets/emailservlet.py
index 393377cb..79ac54b4 100644
--- a/sydent/http/servlets/emailservlet.py
+++ b/sydent/http/servlets/emailservlet.py
@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import logging
 from twisted.web.resource import Resource
 
 from sydent.util.emailutils import EmailAddressException, EmailSendException
@@ -22,10 +23,13 @@
     IncorrectClientSecretException,
     NextLinkValidationException,
 )
+from sydent.validators.common import validate_next_link
 from sydent.util.stringutils import is_valid_client_secret
 
 from sydent.http.servlets import get_args, jsonwrap, send_cors
 
+logger = logging.getLogger(__name__)
+
 
 class EmailRequestCodeServlet(Resource):
     isLeaf = True
@@ -57,9 +61,18 @@ def render_POST(self, request):
         ipaddress = self.sydent.ip_from_request(request)
 
         nextLink = None
-        if 'next_link' in args and not args['next_link'].startswith("file:///"):
+        if 'next_link' in args:
             nextLink = args['next_link']
 
+            if not validate_next_link(self.sydent, nextLink):
+                logger.warning(
+                    "Validation attempt rejected as provided 'next_link' value is not "
+                    "http(s) or domain does not match "
+                    "general.next_link.domain_whitelist config value: %s",
+                    nextLink,
+                )
+                return {'errcode': 'M_INVALID_PARAM', 'error': 'Invalid next_link'}
+
         resp = None
 
         try:
diff --git a/sydent/sydent.py b/sydent/sydent.py
index 05f5f9f8..e63fca6a 100644
--- a/sydent/sydent.py
+++ b/sydent/sydent.py
@@ -20,6 +20,7 @@
 import logging
 import logging.handlers
 import os
+import re
 
 import twisted.internet.reactor
 from twisted.python import log
@@ -65,10 +66,11 @@
 
 logger = logging.getLogger(__name__)
 
-def list_from_comma_sep_string(rawstr):
+
+def set_from_comma_sep_string(rawstr):
     if rawstr == '':
-        return []
-    return [x.strip() for x in rawstr.split(',')]
+        return set()
+    return {x.strip() for x in rawstr.split(',')}
 
 
 CONFIG_DEFAULTS = {
@@ -86,6 +88,10 @@ def list_from_comma_sep_string(rawstr):
         # Path to file detailing the configuration of the /info and /internal-info servlets.
         # More information can be found in docs/info.md.
         'info_path': 'info.yaml',
+        # A comma-separated domain whitelist used to validate the next_link query parameter
+        # provided by the client to the /requestToken and /submitToken endpoints
+        # If empty, no whitelist is applied
+        'next_link.domain_whitelist': ''
     },
     'db': {
         'db.file': 'sydent.db',
@@ -180,9 +186,15 @@ def sighup(signum, stack):
         self.shadow_hs_master = self.cfg.get('general', 'shadow.hs.master')
         self.shadow_hs_slave  = self.cfg.get('general', 'shadow.hs.slave')
 
-        self.user_dir_allowed_hses = set(list_from_comma_sep_string(
+        self.user_dir_allowed_hses = set_from_comma_sep_string(
             self.cfg.get('userdir', 'userdir.allowed_homeservers', '')
-        ))
+        )
+
+        next_link_whitelist = self.cfg.get('general', 'next_link.domain_whitelist')
+        if next_link_whitelist == '':
+            self.next_link_domain_whitelist = None
+        else:
+            self.next_link_domain_whitelist = set_from_comma_sep_string(next_link_whitelist)
 
         self.invites_validity_period = parse_duration(
             self.cfg.get('general', 'invites.validity_period'),
diff --git a/sydent/validators/common.py b/sydent/validators/common.py
index 512db7fc..52f33ef8 100644
--- a/sydent/validators/common.py
+++ b/sydent/validators/common.py
@@ -9,6 +9,7 @@
 )
 from sydent.util import time_msec
 from sqlite3 import IntegrityError
+from urlparse import urlparse
 
 
 logger = logging.getLogger(__name__)
@@ -61,13 +62,23 @@ def validateSessionWithToken(sydent, sid, clientSecret, token, next_link=None):
     # If so, and the next_link this time around is different, then the
     # user may be getting phished. Reject the validation attempt.
     if next_link and valSessionStore.next_link_differs(sid, token, next_link):
-        logger.info(
+        logger.warning(
             "Validation attempt rejected as provided 'next_link' is different "
             "from that in a previous, successful validation attempt with this "
             "session id"
         )
         raise NextLinkValidationException()
 
+    # Validate the value of next_link
+    if next_link and not validate_next_link(sydent, next_link):
+        logger.warning(
+            "Validation attempt rejected as provided 'next_link' value is not "
+            "http(s) or domain does not match "
+            "general.next_link.domain_whitelist config value: %s",
+            next_link,
+        )
+        raise NextLinkValidationException()
+
     # TODO once we can validate the token oob
     #if tokenObj.validated and clientSecret == tokenObj.clientSecret:
     #    return True
@@ -116,3 +127,34 @@ def validateSessionWithToken(sydent, sid, clientSecret, token, next_link=None):
     else:
         logger.info("Incorrect token submitted")
         return False
+
+
+def validate_next_link(sydent, next_link):
+    """Return whether a given next_link value is valid. next_link is valid if the scheme is
+    http(s) and the next_link.domain_whitelist config option is either empty or contains a
+    domain that matches the one in the given next_link
+
+    :param sydent: A global sydent object
+    :type sydent: Sydent
+
+    :param next_link: The value of the next_link query parameter
+    :type next_link: str
+
+    :returns: Whether a given next_link value is valid
+    :rtype: bool
+    """
+    # Parse the contents of the URL
+    next_link_parsed = urlparse(next_link)
+
+    # Scheme must be http(s)
+    if next_link_parsed.scheme not in ["http", "https"]:
+        return False
+
+    # If the domain whitelist is set, the domain must be in it
+    if (
+        sydent.next_link_domain_whitelist is not None
+        and next_link_parsed.hostname not in sydent.next_link_domain_whitelist
+    ):
+        return False
+
+    return True