diff --git a/changelog.d/7455.bugfix b/changelog.d/7455.bugfix
new file mode 100644
index 0000000000..d1693a7f22
--- /dev/null
+++ b/changelog.d/7455.bugfix
@@ -0,0 +1 @@
+Ensure that a user inteactive authentication session is tied to a single request.
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 7613e5b6ab..9c71702371 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -252,6 +252,7 @@ class AuthHandler(BaseHandler):
         clientdict: Dict[str, Any],
         clientip: str,
         description: str,
+        validate_clientdict: bool = True,
     ) -> Tuple[dict, dict, str]:
         """
         Takes a dictionary sent by the client in the login / registration
@@ -277,6 +278,10 @@ class AuthHandler(BaseHandler):
             description: A human readable string to be displayed to the user that
                          describes the operation happening on their account.
 
+            validate_clientdict: Whether to validate that the operation happening
+                                 on the account has not changed. If this is false,
+                                 the client dict is persisted instead of validated.
+
         Returns:
             A tuple of (creds, params, session_id).
 
@@ -317,30 +322,51 @@ class AuthHandler(BaseHandler):
             except StoreError:
                 raise SynapseError(400, "Unknown session ID: %s" % (sid,))
 
+            # If the client provides parameters, update what is persisted,
+            # otherwise use whatever was last provided.
+            #
+            # This was designed to allow the client to omit the parameters
+            # and just supply the session in subsequent calls so it split
+            # auth between devices by just sharing the session, (eg. so you
+            # could continue registration from your phone having clicked the
+            # email auth link on there). It's probably too open to abuse
+            # because it lets unauthenticated clients store arbitrary objects
+            # on a homeserver.
+            #
+            # Revisit: Assuming the REST APIs do sensible validation, the data
+            # isn't arbitrary.
+            #
+            # Note that the registration endpoint explicitly removes the
+            # "initial_device_display_name" parameter if it is provided
+            # without a "password" parameter. See the changes to
+            # synapse.rest.client.v2_alpha.register.RegisterRestServlet.on_POST
+            # in commit 544722bad23fc31056b9240189c3cbbbf0ffd3f9.
             if not clientdict:
-                # This was designed to allow the client to omit the parameters
-                # and just supply the session in subsequent calls so it split
-                # auth between devices by just sharing the session, (eg. so you
-                # could continue registration from your phone having clicked the
-                # email auth link on there). It's probably too open to abuse
-                # because it lets unauthenticated clients store arbitrary objects
-                # on a homeserver.
-                # Revisit: Assuming the REST APIs do sensible validation, the data
-                # isn't arbitrary.
                 clientdict = session.clientdict
 
             # Ensure that the queried operation does not vary between stages of
             # the UI authentication session. This is done by generating a stable
-            # comparator based on the URI, method, and body (minus the auth dict)
-            # and storing it during the initial query. Subsequent queries ensure
-            # that this comparator has not changed.
-            comparator = (uri, method, clientdict)
-            if (session.uri, session.method, session.clientdict) != comparator:
+            # comparator based on the URI, method, and client dict (minus the
+            # auth dict) and storing it during the initial query. Subsequent
+            # queries ensure that this comparator has not changed.
+            if validate_clientdict:
+                session_comparator = (session.uri, session.method, session.clientdict)
+                comparator = (uri, method, clientdict)
+            else:
+                session_comparator = (session.uri, session.method)  # type: ignore
+                comparator = (uri, method)  # type: ignore
+
+            if session_comparator != comparator:
                 raise SynapseError(
                     403,
                     "Requested operation has changed during the UI authentication session.",
                 )
 
+            # For backwards compatibility the registration endpoint persists
+            # changes to the client dict instead of validating them.
+            if not validate_clientdict:
+                await self.store.set_ui_auth_clientdict(sid, clientdict)
+
         if not authdict:
             raise InteractiveAuthIncompleteError(
                 self._auth_dict_for_flows(flows, session.session_id)
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index af08cc6cce..e77dd6bf92 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -516,6 +516,7 @@ class RegisterRestServlet(RestServlet):
             body,
             self.hs.get_ip_from_request(request),
             "register a new account",
+            validate_clientdict=False,
         )
 
         # Check that we're not trying to register a denied 3pid.
diff --git a/synapse/storage/data_stores/main/ui_auth.py b/synapse/storage/data_stores/main/ui_auth.py
index c8eebc9378..1d8ee22fb1 100644
--- a/synapse/storage/data_stores/main/ui_auth.py
+++ b/synapse/storage/data_stores/main/ui_auth.py
@@ -172,6 +172,27 @@ class UIAuthWorkerStore(SQLBaseStore):
 
         return results
 
+    async def set_ui_auth_clientdict(
+        self, session_id: str, clientdict: JsonDict
+    ) -> None:
+        """
+        Store an updated clientdict for a given session ID.
+
+        Args:
+            session_id: The ID of this session as returned from check_auth
+            clientdict:
+                The dictionary from the client root level, not the 'auth' key.
+        """
+        # The clientdict gets stored as JSON.
+        clientdict_json = json.dumps(clientdict)
+
+        self.db.simple_update_one(
+            table="ui_auth_sessions",
+            keyvalues={"session_id": session_id},
+            updatevalues={"clientdict": clientdict_json},
+            desc="set_ui_auth_client_dict",
+        )
+
     async def set_ui_auth_session_data(self, session_id: str, key: str, value: Any):
         """
         Store a key-value pair into the sessions data associated with this
diff --git a/tests/rest/client/v2_alpha/test_auth.py b/tests/rest/client/v2_alpha/test_auth.py
index 587be7b2e7..a56c50a5b7 100644
--- a/tests/rest/client/v2_alpha/test_auth.py
+++ b/tests/rest/client/v2_alpha/test_auth.py
@@ -12,16 +12,20 @@
 # 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.
-
+from typing import List, Union
 
 from twisted.internet.defer import succeed
 
 import synapse.rest.admin
 from synapse.api.constants import LoginType
 from synapse.handlers.ui_auth.checkers import UserInteractiveAuthChecker
-from synapse.rest.client.v2_alpha import auth, register
+from synapse.http.site import SynapseRequest
+from synapse.rest.client.v1 import login
+from synapse.rest.client.v2_alpha import auth, devices, register
+from synapse.types import JsonDict
 
 from tests import unittest
+from tests.server import FakeChannel
 
 
 class DummyRecaptchaChecker(UserInteractiveAuthChecker):
@@ -34,11 +38,15 @@ class DummyRecaptchaChecker(UserInteractiveAuthChecker):
         return succeed(True)
 
 
+class DummyPasswordChecker(UserInteractiveAuthChecker):
+    def check_auth(self, authdict, clientip):
+        return succeed(authdict["identifier"]["user"])
+
+
 class FallbackAuthTests(unittest.HomeserverTestCase):
 
     servlets = [
         auth.register_servlets,
-        synapse.rest.admin.register_servlets_for_client_rest_resource,
         register.register_servlets,
     ]
     hijack_auth = False
@@ -59,18 +67,51 @@ class FallbackAuthTests(unittest.HomeserverTestCase):
         auth_handler = hs.get_auth_handler()
         auth_handler.checkers[LoginType.RECAPTCHA] = self.recaptcha_checker
 
-    @unittest.INFO
-    def test_fallback_captcha(self):
+    def register(self, expected_response: int, body: JsonDict) -> FakeChannel:
+        """Make a register request."""
+        request, channel = self.make_request(
+            "POST", "register", body
+        )  # type: SynapseRequest, FakeChannel
+        self.render(request)
+
+        self.assertEqual(request.code, expected_response)
+        return channel
+
+    def recaptcha(
+        self, session: str, expected_post_response: int, post_session: str = None
+    ) -> None:
+        """Get and respond to a fallback recaptcha. Returns the second request."""
+        if post_session is None:
+            post_session = session
+
+        request, channel = self.make_request(
+            "GET", "auth/m.login.recaptcha/fallback/web?session=" + session
+        )  # type: SynapseRequest, FakeChannel
+        self.render(request)
+        self.assertEqual(request.code, 200)
 
         request, channel = self.make_request(
             "POST",
-            "register",
-            {"username": "user", "type": "m.login.password", "password": "bar"},
+            "auth/m.login.recaptcha/fallback/web?session="
+            + post_session
+            + "&g-recaptcha-response=a",
         )
         self.render(request)
+        self.assertEqual(request.code, expected_post_response)
 
+        # The recaptcha handler is called with the response given
+        attempts = self.recaptcha_checker.recaptcha_attempts
+        self.assertEqual(len(attempts), 1)
+        self.assertEqual(attempts[0][0]["response"], "a")
+
+    @unittest.INFO
+    def test_fallback_captcha(self):
+        """Ensure that fallback auth via a captcha works."""
         # Returns a 401 as per the spec
-        self.assertEqual(request.code, 401)
+        channel = self.register(
+            401, {"username": "user", "type": "m.login.password", "password": "bar"},
+        )
+
         # Grab the session
         session = channel.json_body["session"]
         # Assert our configured public key is being given
@@ -78,60 +119,32 @@ class FallbackAuthTests(unittest.HomeserverTestCase):
             channel.json_body["params"]["m.login.recaptcha"]["public_key"], "brokencake"
         )
 
-        request, channel = self.make_request(
-            "GET", "auth/m.login.recaptcha/fallback/web?session=" + session
-        )
-        self.render(request)
-        self.assertEqual(request.code, 200)
-
-        request, channel = self.make_request(
-            "POST",
-            "auth/m.login.recaptcha/fallback/web?session="
-            + session
-            + "&g-recaptcha-response=a",
-        )
-        self.render(request)
-        self.assertEqual(request.code, 200)
-
-        # The recaptcha handler is called with the response given
-        attempts = self.recaptcha_checker.recaptcha_attempts
-        self.assertEqual(len(attempts), 1)
-        self.assertEqual(attempts[0][0]["response"], "a")
+        # Complete the recaptcha step.
+        self.recaptcha(session, 200)
 
         # also complete the dummy auth
-        request, channel = self.make_request(
-            "POST", "register", {"auth": {"session": session, "type": "m.login.dummy"}}
-        )
-        self.render(request)
+        self.register(200, {"auth": {"session": session, "type": "m.login.dummy"}})
 
         # Now we should have fulfilled a complete auth flow, including
         # the recaptcha fallback step, we can then send a
         # request to the register API with the session in the authdict.
-        request, channel = self.make_request(
-            "POST", "register", {"auth": {"session": session}}
-        )
-        self.render(request)
-        self.assertEqual(channel.code, 200)
+        channel = self.register(200, {"auth": {"session": session}})
 
         # We're given a registered user.
         self.assertEqual(channel.json_body["user_id"], "@user:test")
 
-    def test_cannot_change_operation(self):
+    def test_legacy_registration(self):
         """
-        The initial requested operation cannot be modified during the user interactive authentication session.
+        Registration allows the parameters to vary through the process.
         """
 
         # Make the initial request to register. (Later on a different password
         # will be used.)
-        request, channel = self.make_request(
-            "POST",
-            "register",
-            {"username": "user", "type": "m.login.password", "password": "bar"},
-        )
-        self.render(request)
-
         # Returns a 401 as per the spec
-        self.assertEqual(request.code, 401)
+        channel = self.register(
+            401, {"username": "user", "type": "m.login.password", "password": "bar"},
+        )
+
         # Grab the session
         session = channel.json_body["session"]
         # Assert our configured public key is being given
@@ -139,65 +152,39 @@ class FallbackAuthTests(unittest.HomeserverTestCase):
             channel.json_body["params"]["m.login.recaptcha"]["public_key"], "brokencake"
         )
 
-        request, channel = self.make_request(
-            "GET", "auth/m.login.recaptcha/fallback/web?session=" + session
-        )
-        self.render(request)
-        self.assertEqual(request.code, 200)
-
-        request, channel = self.make_request(
-            "POST",
-            "auth/m.login.recaptcha/fallback/web?session="
-            + session
-            + "&g-recaptcha-response=a",
-        )
-        self.render(request)
-        self.assertEqual(request.code, 200)
-
-        # The recaptcha handler is called with the response given
-        attempts = self.recaptcha_checker.recaptcha_attempts
-        self.assertEqual(len(attempts), 1)
-        self.assertEqual(attempts[0][0]["response"], "a")
+        # Complete the recaptcha step.
+        self.recaptcha(session, 200)
 
         # also complete the dummy auth
-        request, channel = self.make_request(
-            "POST", "register", {"auth": {"session": session, "type": "m.login.dummy"}}
-        )
-        self.render(request)
+        self.register(200, {"auth": {"session": session, "type": "m.login.dummy"}})
 
         # Now we should have fulfilled a complete auth flow, including
         # the recaptcha fallback step. Make the initial request again, but
-        # with a different password. This causes the request to fail since the
-        # operaiton was modified during the ui auth session.
-        request, channel = self.make_request(
-            "POST",
-            "register",
+        # with a changed password. This still completes.
+        channel = self.register(
+            200,
             {
                 "username": "user",
                 "type": "m.login.password",
-                "password": "foo",  # Note this doesn't match the original request.
+                "password": "foo",  # Note that this is different.
                 "auth": {"session": session},
             },
         )
-        self.render(request)
-        self.assertEqual(channel.code, 403)
+
+        # We're given a registered user.
+        self.assertEqual(channel.json_body["user_id"], "@user:test")
 
     def test_complete_operation_unknown_session(self):
         """
         Attempting to mark an invalid session as complete should error.
         """
-
         # Make the initial request to register. (Later on a different password
         # will be used.)
-        request, channel = self.make_request(
-            "POST",
-            "register",
-            {"username": "user", "type": "m.login.password", "password": "bar"},
-        )
-        self.render(request)
-
         # Returns a 401 as per the spec
-        self.assertEqual(request.code, 401)
+        channel = self.register(
+            401, {"username": "user", "type": "m.login.password", "password": "bar"}
+        )
+
         # Grab the session
         session = channel.json_body["session"]
         # Assert our configured public key is being given
@@ -205,19 +192,160 @@ class FallbackAuthTests(unittest.HomeserverTestCase):
             channel.json_body["params"]["m.login.recaptcha"]["public_key"], "brokencake"
         )
 
-        request, channel = self.make_request(
-            "GET", "auth/m.login.recaptcha/fallback/web?session=" + session
-        )
-        self.render(request)
-        self.assertEqual(request.code, 200)
+        # Attempt to complete the recaptcha step with an unknown session.
+        # This results in an error.
+        self.recaptcha(session, 400, session + "unknown")
 
-        # Attempt to complete an unknown session, which should return an error.
-        unknown_session = session + "unknown"
+
+class UIAuthTests(unittest.HomeserverTestCase):
+    servlets = [
+        auth.register_servlets,
+        devices.register_servlets,
+        login.register_servlets,
+        synapse.rest.admin.register_servlets_for_client_rest_resource,
+        register.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, hs):
+        auth_handler = hs.get_auth_handler()
+        auth_handler.checkers[LoginType.PASSWORD] = DummyPasswordChecker(hs)
+
+        self.user_pass = "pass"
+        self.user = self.register_user("test", self.user_pass)
+        self.user_tok = self.login("test", self.user_pass)
+
+    def get_device_ids(self) -> List[str]:
+        # Get the list of devices so one can be deleted.
         request, channel = self.make_request(
-            "POST",
-            "auth/m.login.recaptcha/fallback/web?session="
-            + unknown_session
-            + "&g-recaptcha-response=a",
-        )
+            "GET", "devices", access_token=self.user_tok,
+        )  # type: SynapseRequest, FakeChannel
         self.render(request)
-        self.assertEqual(request.code, 400)
+
+        # Get the ID of the device.
+        self.assertEqual(request.code, 200)
+        return [d["device_id"] for d in channel.json_body["devices"]]
+
+    def delete_device(
+        self, device: str, expected_response: int, body: Union[bytes, JsonDict] = b""
+    ) -> FakeChannel:
+        """Delete an individual device."""
+        request, channel = self.make_request(
+            "DELETE", "devices/" + device, body, access_token=self.user_tok
+        )  # type: SynapseRequest, FakeChannel
+        self.render(request)
+
+        # Ensure the response is sane.
+        self.assertEqual(request.code, expected_response)
+
+        return channel
+
+    def delete_devices(self, expected_response: int, body: JsonDict) -> FakeChannel:
+        """Delete 1 or more devices."""
+        # Note that this uses the delete_devices endpoint so that we can modify
+        # the payload half-way through some tests.
+        request, channel = self.make_request(
+            "POST", "delete_devices", body, access_token=self.user_tok,
+        )  # type: SynapseRequest, FakeChannel
+        self.render(request)
+
+        # Ensure the response is sane.
+        self.assertEqual(request.code, expected_response)
+
+        return channel
+
+    def test_ui_auth(self):
+        """
+        Test user interactive authentication outside of registration.
+        """
+        device_id = self.get_device_ids()[0]
+
+        # Attempt to delete this device.
+        # Returns a 401 as per the spec
+        channel = self.delete_device(device_id, 401)
+
+        # Grab the session
+        session = channel.json_body["session"]
+        # Ensure that flows are what is expected.
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        # Make another request providing the UI auth flow.
+        self.delete_device(
+            device_id,
+            200,
+            {
+                "auth": {
+                    "type": "m.login.password",
+                    "identifier": {"type": "m.id.user", "user": self.user},
+                    "password": self.user_pass,
+                    "session": session,
+                },
+            },
+        )
+
+    def test_cannot_change_body(self):
+        """
+        The initial requested client dict cannot be modified during the user interactive authentication session.
+        """
+        # Create a second login.
+        self.login("test", self.user_pass)
+
+        device_ids = self.get_device_ids()
+        self.assertEqual(len(device_ids), 2)
+
+        # Attempt to delete the first device.
+        # Returns a 401 as per the spec
+        channel = self.delete_devices(401, {"devices": [device_ids[0]]})
+
+        # Grab the session
+        session = channel.json_body["session"]
+        # Ensure that flows are what is expected.
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        # Make another request providing the UI auth flow, but try to delete the
+        # second device. This results in an error.
+        self.delete_devices(
+            403,
+            {
+                "devices": [device_ids[1]],
+                "auth": {
+                    "type": "m.login.password",
+                    "identifier": {"type": "m.id.user", "user": self.user},
+                    "password": self.user_pass,
+                    "session": session,
+                },
+            },
+        )
+
+    def test_cannot_change_uri(self):
+        """
+        The initial requested URI cannot be modified during the user interactive authentication session.
+        """
+        # Create a second login.
+        self.login("test", self.user_pass)
+
+        device_ids = self.get_device_ids()
+        self.assertEqual(len(device_ids), 2)
+
+        # Attempt to delete the first device.
+        # Returns a 401 as per the spec
+        channel = self.delete_device(device_ids[0], 401)
+
+        # Grab the session
+        session = channel.json_body["session"]
+        # Ensure that flows are what is expected.
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        # Make another request providing the UI auth flow, but try to delete the
+        # second device. This results in an error.
+        self.delete_device(
+            device_ids[1],
+            403,
+            {
+                "auth": {
+                    "type": "m.login.password",
+                    "identifier": {"type": "m.id.user", "user": self.user},
+                    "password": self.user_pass,
+                    "session": session,
+                },
+            },
+        )
diff --git a/tox.ini b/tox.ini
index eccc44e436..8aef52021d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -207,6 +207,7 @@ commands = mypy \
             synapse/util/caches/stream_change_cache.py \
             tests/replication/tcp/streams \
             tests/test_utils \
+            tests/rest/client/v2_alpha/test_auth.py \
             tests/util/test_stream_change_cache.py
 
 # To find all folders that pass mypy you run: