diff --git a/sachet/server/files/views.py b/sachet/server/files/views.py
index c617b73..e17d8e8 100644
--- a/sachet/server/files/views.py
+++ b/sachet/server/files/views.py
@@ -117,6 +117,17 @@ class FileContentAPI(ModelAPI):
                 jsonify({"status": "fail", "message": "This share does not exist."})
             ), 404
 
+        if auth_user != share.owner:
+            return (
+                jsonify(
+                    {
+                        "status": "fail",
+                        "message": "Share must be initialized by its owner.",
+                    }
+                ),
+                403,
+            )
+
         if not share.initialized:
             return (
                 jsonify(
diff --git a/tests/test_files.py b/tests/test_files.py
index 4649203..7306f20 100644
--- a/tests/test_files.py
+++ b/tests/test_files.py
@@ -182,6 +182,18 @@ class TestSuite:
         )
         assert resp.status_code == 201
 
+        # test other user being unable to modify this share
+        resp = client.put(
+            url + "/content",
+            headers=auth("dave"),
+            data={
+                "upload": FileStorage(stream=BytesIO(upload_data), filename="upload")
+            },
+            content_type="multipart/form-data",
+        )
+        assert resp.status_code == 403
+
+
         # test not allowing re-upload
         resp = client.post(
             url + "/content",