diff --git a/api/client-server/definitions/device_keys.yaml b/api/client-server/definitions/device_keys.yaml
index ce770c09..4f7cffe7 100644
--- a/api/client-server/definitions/device_keys.yaml
+++ b/api/client-server/definitions/device_keys.yaml
@@ -54,6 +54,10 @@ properties:
 
        The signature is calculated using the process described at `Signing
        JSON`_.
+    additionalProperties:
+      type: object
+      additionalProperties:
+        type: string
     example:
       "@alice:example.com":
         "ed25519:JLAFKJWSCS": "dSO80A01XiigH3uBiDVx/EjzaoycHcjq9lfQX0uWsqxl2giMIiSPR8a4d291W1ihKJL/a+myXS367WT6NAIcBA"
diff --git a/api/client-server/definitions/third_party_signed.yaml b/api/client-server/definitions/third_party_signed.yaml
index c9c761a1..7ce1a1d1 100644
--- a/api/client-server/definitions/third_party_signed.yaml
+++ b/api/client-server/definitions/third_party_signed.yaml
@@ -33,6 +33,10 @@ properties:
     type: object
     description: A signatures object containing a signature of the entire signed object.
     title: Signatures
+    additionalProperties:
+      type: object
+      additionalProperties:
+        type: string
     example: {
       "example.org": {
         "ed25519:0": "some9signature"
diff --git a/api/server-server/definitions/keys.yaml b/api/server-server/definitions/keys.yaml
index 7e300402..306d4d00 100644
--- a/api/server-server/definitions/keys.yaml
+++ b/api/server-server/definitions/keys.yaml
@@ -81,6 +81,10 @@ properties:
       The signature is calculated using the process described at `Signing
       JSON`_.
     title: Signatures
+    additionalProperties:
+      type: object
+      additionalProperties:
+        type: string
   valid_until_ts:
     type: integer
     format: int64
diff --git a/api/server-server/third_party_invite.yaml b/api/server-server/third_party_invite.yaml
index 8897bdcf..fbd2a9d9 100644
--- a/api/server-server/third_party_invite.yaml
+++ b/api/server-server/third_party_invite.yaml
@@ -103,6 +103,10 @@ paths:
                           signatures:
                             type: object
                             title: Signatures
+                            additionalProperties:
+                              type: object
+                              additionalProperties:
+                                type: string
                             description: |-
                               The server signatures for this event.
 
diff --git a/api/server-server/user_keys.yaml b/api/server-server/user_keys.yaml
index 166f1ff0..6ed51af0 100644
--- a/api/server-server/user_keys.yaml
+++ b/api/server-server/user_keys.yaml
@@ -89,6 +89,10 @@ paths:
                           signatures:
                             type: object
                             title: Signatures
+                            additionalProperties:
+                              type: object
+                              additionalProperties:
+                                type: string
                             description: |-
                               Signature of the key object.
 
diff --git a/event-schemas/schema/m.room.member b/event-schemas/schema/m.room.member
index 1033c098..bff2002e 100644
--- a/event-schemas/schema/m.room.member
+++ b/event-schemas/schema/m.room.member
@@ -82,6 +82,10 @@ properties:
                 description: 'A single signature from the verifying server, in the format specified by the Signing Events section of the server-server API.'
                 title: Signatures
                 type: object
+                additionalProperties:
+                  type: object
+                  additionalProperties:
+                    type: string
               token:
                 description: The token property of the containing third_party_invite object.
                 type: string