diff --git a/changelog.d/5309.bugfix b/changelog.d/5309.bugfix
new file mode 100644
index 0000000000..97b3527266
--- /dev/null
+++ b/changelog.d/5309.bugfix
@@ -0,0 +1 @@
+Prevent users from setting huge displaynames and avatar URLs.
diff --git a/synapse/handlers/profile.py b/synapse/handlers/profile.py
index 91fc718ff8..a5fc6c5dbf 100644
--- a/synapse/handlers/profile.py
+++ b/synapse/handlers/profile.py
@@ -31,6 +31,9 @@ from ._base import BaseHandler
 
 logger = logging.getLogger(__name__)
 
+MAX_DISPLAYNAME_LEN = 100
+MAX_AVATAR_URL_LEN = 1000
+
 
 class BaseProfileHandler(BaseHandler):
     """Handles fetching and updating user profile information.
@@ -162,6 +165,11 @@ class BaseProfileHandler(BaseHandler):
         if not by_admin and target_user != requester.user:
             raise AuthError(400, "Cannot set another user's displayname")
 
+        if len(new_displayname) > MAX_DISPLAYNAME_LEN:
+            raise SynapseError(
+                400, "Displayname is too long (max %i)" % (MAX_DISPLAYNAME_LEN, ),
+            )
+
         if new_displayname == '':
             new_displayname = None
 
@@ -217,6 +225,11 @@ class BaseProfileHandler(BaseHandler):
         if not by_admin and target_user != requester.user:
             raise AuthError(400, "Cannot set another user's avatar_url")
 
+        if len(new_avatar_url) > MAX_AVATAR_URL_LEN:
+            raise SynapseError(
+                400, "Avatar URL is too long (max %i)" % (MAX_AVATAR_URL_LEN, ),
+            )
+
         yield self.store.set_profile_avatar_url(
             target_user.localpart, new_avatar_url
         )
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index e83ee24f10..9a388ea013 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -531,6 +531,8 @@ class RegistrationHandler(BaseHandler):
             A tuple of (user_id, access_token).
         Raises:
             RegistrationError if there was a problem registering.
+
+        NB this is only used in tests. TODO: move it to the test package!
         """
         if localpart is None:
             raise SynapseError(400, "Request must include user id")
diff --git a/tests/rest/client/v1/test_profile.py b/tests/rest/client/v1/test_profile.py
index 769c37ce52..f4d0d48dad 100644
--- a/tests/rest/client/v1/test_profile.py
+++ b/tests/rest/client/v1/test_profile.py
@@ -14,6 +14,8 @@
 # limitations under the License.
 
 """Tests REST events for /profile paths."""
+import json
+
 from mock import Mock
 
 from twisted.internet import defer
@@ -31,8 +33,11 @@ myid = "@1234ABCD:test"
 PATH_PREFIX = "/_matrix/client/api/v1"
 
 
-class ProfileTestCase(unittest.TestCase):
-    """ Tests profile management. """
+class MockHandlerProfileTestCase(unittest.TestCase):
+    """ Tests rest layer of profile management.
+
+    Todo: move these into ProfileTestCase
+    """
 
     @defer.inlineCallbacks
     def setUp(self):
@@ -159,6 +164,59 @@ class ProfileTestCase(unittest.TestCase):
         self.assertEquals(mocked_set.call_args[0][2], "http://my.server/pic.gif")
 
 
+class ProfileTestCase(unittest.HomeserverTestCase):
+
+    servlets = [
+        admin.register_servlets_for_client_rest_resource,
+        login.register_servlets,
+        profile.register_servlets,
+    ]
+
+    def make_homeserver(self, reactor, clock):
+        self.hs = self.setup_test_homeserver()
+        return self.hs
+
+    def prepare(self, reactor, clock, hs):
+        self.owner = self.register_user("owner", "pass")
+        self.owner_tok = self.login("owner", "pass")
+
+    def test_set_displayname(self):
+        request, channel = self.make_request(
+            "PUT",
+            "/profile/%s/displayname" % (self.owner, ),
+            content=json.dumps({"displayname": "test"}),
+            access_token=self.owner_tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.code, 200, channel.result)
+
+        res = self.get_displayname()
+        self.assertEqual(res, "test")
+
+    def test_set_displayname_too_long(self):
+        """Attempts to set a stupid displayname should get a 400"""
+        request, channel = self.make_request(
+            "PUT",
+            "/profile/%s/displayname" % (self.owner, ),
+            content=json.dumps({"displayname": "test" * 100}),
+            access_token=self.owner_tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.code, 400, channel.result)
+
+        res = self.get_displayname()
+        self.assertEqual(res, "owner")
+
+    def get_displayname(self):
+        request, channel = self.make_request(
+            "GET",
+            "/profile/%s/displayname" % (self.owner, ),
+        )
+        self.render(request)
+        self.assertEqual(channel.code, 200, channel.result)
+        return channel.json_body["displayname"]
+
+
 class ProfilesRestrictedTestCase(unittest.HomeserverTestCase):
 
     servlets = [