diff --git a/src/aleph/web/controllers/p2p.py b/src/aleph/web/controllers/p2p.py
index 5e7f09b00..519cac9af 100644
--- a/src/aleph/web/controllers/p2p.py
+++ b/src/aleph/web/controllers/p2p.py
@@ -62,11 +62,21 @@ def _validate_request_data(config: Config, request_data: Dict) -> None:
 
     topic = request_data.get("topic")
 
-    # Currently, we only check validate messages
+    # Only accept publishing on the message topic.
     message_topic = config.aleph.queue_topic.value
-    if topic == message_topic:
+    if topic != message_topic:
+        raise web.HTTPForbidden(reason=f"Unauthorized P2P topic: {topic}. Use {message_topic}.")
+
+    data = request_data.get("data")
+    if not isinstance(data, str):
+        raise web.HTTPUnprocessableEntity(reason="'data': expected a serialized JSON string.")
+
+    try:
         message_dict = json.loads(cast(str, request_data.get("data")))
-        _validate_message_dict(message_dict)
+    except ValueError:
+        raise web.HTTPUnprocessableEntity(reason="'data': must be deserializable as JSON.")
+
+    _validate_message_dict(message_dict)
 
 
 async def _pub_on_p2p_topics(
diff --git a/tests/api/test_p2p.py b/tests/api/test_p2p.py
new file mode 100644
index 000000000..1ea4121bb
--- /dev/null
+++ b/tests/api/test_p2p.py
@@ -0,0 +1,65 @@
+import copy
+import json
+
+import pytest
+from configmanager import Config
+
+P2P_PUB_URI = "/api/v0/p2p/pubsub/pub"
+
+MESSAGE_DICT = {
+    "chain": "NULS2",
+    "item_hash": "4bbcfe7c4775492c2e602d322d68f558891468927b5e0d6cb89ff880134f323e",
+    "sender": "NULSd6Hgbhr42Dm5nEgf6foEUT5bgwHesZQJB",
+    "type": "STORE",
+    "channel": "MYALEPH",
+    "item_content": '{"address":"NULSd6Hgbhr42Dm5nEgf6foEUT5bgwHesZQJB","item_type":"ipfs","item_hash":"QmUDS8mpQmpPyptyUEedHxHMkxo7ueRRiAvrpgvJMpjXwW","time":1577325086.513}',
+    "item_type": "inline",
+    "signature": "G7/xlWoMjjOr1NBN4SiZ8USYYVM9Q3JHXChR9hPw9/YSItfAplshWysqYDkvmBZiwbICG0IVB3ilMPJ/ZVgPNlk=",
+    "time": 1608297193.717,
+}
+
+
+@pytest.mark.asyncio
+async def test_pubsub_pub_valid_message(ccn_api_client, mock_config: Config):
+    message_topic = mock_config.aleph.queue_topic.value
+
+    response = await ccn_api_client.post(
+        P2P_PUB_URI, json={"topic": message_topic, "data": json.dumps(MESSAGE_DICT)}
+    )
+    assert response.status == 200, await response.text()
+    response_json = await response.json()
+
+    assert response_json["status"] == "success"
+
+
+@pytest.mark.asyncio
+async def test_pubsub_pub_errors(ccn_api_client, mock_config: Config):
+    # Invalid topic
+    serialized_message_dict = json.dumps(MESSAGE_DICT)
+    response = await ccn_api_client.post(
+        P2P_PUB_URI, json={"topic": "random-topic", "data": serialized_message_dict}
+    )
+    assert response.status == 403, await response.text()
+
+    message_topic = mock_config.aleph.queue_topic.value
+
+    # Do not serialize the message
+    response = await ccn_api_client.post(
+        P2P_PUB_URI, json={"topic": message_topic, "data": MESSAGE_DICT}
+    )
+    assert response.status == 422, await response.text()
+
+    # Invalid JSON
+    response = await ccn_api_client.post(
+        P2P_PUB_URI, json={"topic": message_topic, "data": serialized_message_dict[:-2]}
+    )
+    assert response.status == 422, await response.text()
+
+    # Invalid message
+    message_dict = copy.deepcopy(MESSAGE_DICT)
+    del message_dict["item_content"]
+
+    response = await ccn_api_client.post(
+        P2P_PUB_URI, json={"topic": message_topic, "data": json.dumps(message_dict)}
+    )
+    assert response.status == 422, await response.text()