diff --git a/changelogs/client_server/newsfragments/3331.clarification b/changelogs/client_server/newsfragments/3331.clarification
new file mode 100644
index 00000000..4e0836b8
--- /dev/null
+++ b/changelogs/client_server/newsfragments/3331.clarification
@@ -0,0 +1 @@
+Add auth property to UIA endpoint uploadCrossSigningKeys.
diff --git a/data/api/client-server/cross_signing.yaml b/data/api/client-server/cross_signing.yaml
index a0516ef2..c4c4d10f 100644
--- a/data/api/client-server/cross_signing.yaml
+++ b/data/api/client-server/cross_signing.yaml
@@ -66,6 +66,12 @@ paths:
                   request.
                 allOf:
                   - $ref: definitions/cross_signing_key.yaml
+              auth:
+                description: |-
+                  Additional authentication information for the
+                  user-interactive authentication API.
+                allOf:
+                - $ref: "definitions/auth_data.yaml"
             example: {
               "master_key": {
                 "user_id": "@alice:example.com",