diff --git a/changelogs/server_server/newsfragments/2284.clarification b/changelogs/server_server/newsfragments/2284.clarification
new file mode 100644
index 00000000..31d94bf4
--- /dev/null
+++ b/changelogs/server_server/newsfragments/2284.clarification
@@ -0,0 +1 @@
+Specified input validation for PDUs passed to federation membership endpoints.
diff --git a/data/api/server-server/invites-v1.yaml b/data/api/server-server/invites-v1.yaml
index beb44b84..31cc4eeb 100644
--- a/data/api/server-server/invites-v1.yaml
+++ b/data/api/server-server/invites-v1.yaml
@@ -172,6 +172,17 @@ paths:
                   }
         "400":
           description: |-
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The invite event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `invite`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not a user ID on the receiving server.
+
+            Servers MUST apply the validation above to the invite event before
+            signing it regardless of room version.
+
             The `M_MISSING_PARAM` error code is used to indicate one or more of
             the following:
 
@@ -186,9 +197,9 @@ paths:
             Servers MAY apply the validation above to room versions 1 through 11,
             and SHOULD apply the validation above to all other room versions.
 
-            If `M_MISSING_PARAM` is returned and the request is associated with a
-            Client-Server API request, the Client-Server API request SHOULD fail
-            with a 5xx error rather than being passed through.
+            If `M_MISSING_PARAM` or `M_INVALID_PARAM` is returned and the request
+            is associated with a Client-Server API request, the Client-Server API
+            request SHOULD fail with a 5xx error rather than being passed through.
           content:
             application/json:
               schema:
diff --git a/data/api/server-server/invites-v2.yaml b/data/api/server-server/invites-v2.yaml
index 7b71b472..9c3a474b 100644
--- a/data/api/server-server/invites-v2.yaml
+++ b/data/api/server-server/invites-v2.yaml
@@ -154,6 +154,17 @@ paths:
             The error should be passed through to clients so that they
             may give better feedback to users.
 
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The invite event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `invite`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not a user ID on the receiving server.
+
+            Servers MUST apply the validation above to the invite event before
+            signing it regardless of room version.
+
             The `M_MISSING_PARAM` error code is used to indicate one or more of
             the following:
 
@@ -168,9 +179,9 @@ paths:
             Servers MAY apply the validation above to room versions 1 through 11,
             and SHOULD apply the validation above to all other room versions.
 
-            If `M_MISSING_PARAM` is returned and the request is associated with a
-            Client-Server API request, the Client-Server API request SHOULD fail
-            with a 5xx error rather than being passed through.
+            If `M_MISSING_PARAM` or `M_INVALID_PARAM` is returned and the request
+            is associated with a Client-Server API request, the Client-Server API
+            request SHOULD fail with a 5xx error rather than being passed through.
           content:
             application/json:
               schema:
diff --git a/data/api/server-server/joins-v1.yaml b/data/api/server-server/joins-v1.yaml
index de671ef9..ae62411f 100644
--- a/data/api/server-server/joins-v1.yaml
+++ b/data/api/server-server/joins-v1.yaml
@@ -36,7 +36,7 @@ paths:
             type: string
         - in: path
           name: userId
-          description: The user ID the join event will be for.
+          description: The user ID the join event will be for. This MUST be a user ID on the origin server.
           required: true
           example: "@someone:example.org"
           schema:
@@ -388,6 +388,43 @@ paths:
                       }
                     }
                   ]
+        "400":
+          description: |-
+            The request is invalid in some way.
+
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The join event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `join`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not equal to the `sender`.
+
+            Servers MUST apply the validation above to the join event.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_INVALID_PARAM",
+                    "error": "Not a join event."
+                  }
+        "403":
+          description: |-
+            The room that the joining server is attempting to join does not permit the user
+            to join.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_FORBIDDEN",
+                    "error": "You are not invited to this room"
+                  }
 servers:
   - url: "{protocol}://{hostname}{basePath}"
     variables:
diff --git a/data/api/server-server/joins-v2.yaml b/data/api/server-server/joins-v2.yaml
index 91e6a83e..51687fb5 100644
--- a/data/api/server-server/joins-v2.yaml
+++ b/data/api/server-server/joins-v2.yaml
@@ -247,6 +247,16 @@ paths:
             The error should be passed through to clients so that they
             may give better feedback to users.
 
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The join event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `join`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not equal to the `sender`.
+
+            Servers MUST apply the validation above to the join event.
+
             New in `v1.2`, the following error conditions might happen:
 
             If the room is [restricted](/client-server-api/#restricted-rooms)
diff --git a/data/api/server-server/knocks.yaml b/data/api/server-server/knocks.yaml
index 38092085..6d0e2d78 100644
--- a/data/api/server-server/knocks.yaml
+++ b/data/api/server-server/knocks.yaml
@@ -36,7 +36,7 @@ paths:
             type: string
         - in: path
           name: userId
-          description: The user ID the knock event will be for.
+          description: The user ID the knock event will be for. This MUST be a user ID on the origin server.
           required: true
           example: "@someone:example.org"
           schema:
@@ -330,6 +330,27 @@ paths:
                       "$ref": "./examples/invite_or_knock_state.json"
                     }
                   }
+        "400":
+          description: |-
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The knock event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `knock`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not equal to the `sender`.
+
+            Servers MUST apply the validation above to the knock event.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_INVALID_PARAM",
+                    "error": "Not a knock event."
+                  }
         "403":
           description: |-
             The knocking server or user is not permitted to knock on the room, such as when the
diff --git a/data/api/server-server/leaving-v1.yaml b/data/api/server-server/leaving-v1.yaml
index a630f6d7..b36ac6ac 100644
--- a/data/api/server-server/leaving-v1.yaml
+++ b/data/api/server-server/leaving-v1.yaml
@@ -36,7 +36,7 @@ paths:
             type: string
         - in: path
           name: userId
-          description: The user ID the leave event will be for.
+          description: The user ID the leave event will be for. This MUST be a user ID on the origin server.
           required: true
           example: "@someone:example.org"
           schema:
@@ -249,6 +249,27 @@ paths:
                     200,
                     {}
                   ]
+        "400":
+          description: |-
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The leave event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `leave`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not equal to the `sender`.
+
+            Servers MUST apply the validation above to the leave event.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_INVALID_PARAM",
+                    "error": "Not a leave event."
+                  }
 servers:
   - url: "{protocol}://{hostname}{basePath}"
     variables:
diff --git a/data/api/server-server/leaving-v2.yaml b/data/api/server-server/leaving-v2.yaml
index 0db16cbe..019ff1b3 100644
--- a/data/api/server-server/leaving-v2.yaml
+++ b/data/api/server-server/leaving-v2.yaml
@@ -134,6 +134,27 @@ paths:
               examples:
                 response:
                   value: {}
+        "400":
+          description: |-
+            The `M_INVALID_PARAM` error code is used to indicate one or more of the following:
+
+            * The leave event fails a [signature check](/server-server-api/#validating-hashes-and-signatures-on-received-events).
+            * The event type is not `m.room.member`.
+            * The `membership` field inside the event content is not `leave`.
+            * The event sender is not a user ID on the origin server.
+            * The `state_key` is not equal to the `sender`.
+
+            Servers MUST apply the validation above to the leave event.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_INVALID_PARAM",
+                    "error": "Not a leave event."
+                  }
 servers:
   - url: "{protocol}://{hostname}{basePath}"
     variables: