diff --git a/changelog.d/6334.feature b/changelog.d/6334.feature
new file mode 100644
index 0000000000..eaf69ef3f6
--- /dev/null
+++ b/changelog.d/6334.feature
@@ -0,0 +1 @@
+Limit the length of data returned by url previews, to prevent DoS attacks.
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index 4d4b3c1462..ec9c4619c9 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -274,7 +274,8 @@ class PreviewUrlResource(DirectServeResource):
         # filter out any stupidly long values
         keys_to_remove = []
         for k, v in og.items():
-            if len(k) > OG_TAG_NAME_MAXLEN or len(v) > OG_TAG_VALUE_MAXLEN:
+            # values can be numeric as well as strings, hence the cast to str
+            if len(k) > OG_TAG_NAME_MAXLEN or len(str(v)) > OG_TAG_VALUE_MAXLEN:
                 logger.warning(
                     "Pruning overlong tag %s from OG data", k[:OG_TAG_NAME_MAXLEN]
                 )