diff --git a/changelog.d/5309.bugfix b/changelog.d/5309.bugfix
new file mode 100644
index 000000000..97b352726
--- /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 91fc718ff..a5fc6c5db 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 e83ee24f1..9a388ea01 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")