diff --git a/README.md b/README.md
index ea5e881..506441d 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,6 @@ query {
 ```
 
 **Set an alarm:**
-*(Note: `command` is optional. If omitted, the API will automatically figure out the correct command to start `wecker.py`)*
 ```graphql
 mutation {
   setAlarm(
@@ -91,6 +90,7 @@ mutation {
   }
 }
 ```
+*(Note: The `command` is managed automatically by the API and cannot be overridden, to prevent command injection into the system crontab.)*
 
 **Delete an alarm:**
 ```graphql
diff --git a/api/schema.py b/api/schema.py
index 976383b..0688dd9 100644
--- a/api/schema.py
+++ b/api/schema.py
@@ -70,12 +70,10 @@ class Mutation:
     def set_alarm(
         self,
         cron_expression: str,
-        command: Optional[str] = None,
         is_enabled: bool = True,
         id: Optional[str] = None,
     ) -> Alarm:
-        if command is None:
-            command = _default_command()
+        command = _default_command()
 
         manager = get_manager()
         new_id = manager.set_alarm(
diff --git a/tests/test_api.py b/tests/test_api.py
index e7b7b5f..80c7069 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -121,6 +121,22 @@ def test_set_alarm_default_command_append():
     client.post("/graphql", json={"query": mutation_delete}, headers=headers)
 
 
+def test_set_alarm_rejects_custom_command():
+    """Custom command argument must be rejected to prevent crontab injection."""
+    headers = {"X-API-Key": "test-secret"}
+    mutation = """
+    mutation {
+      setAlarm(cronExpression: "0 9 * * *", command: "rm -rf /") {
+        id
+      }
+    }
+    """
+    res = client.post("/graphql", json={"query": mutation}, headers=headers)
+    assert res.status_code == 200
+    assert "errors" in res.json()
+    assert "command" in str(res.json()["errors"])
+
+
 def test_graphql_workflow():
     headers = {"X-API-Key": "test-secret"}
 
@@ -139,7 +155,7 @@ def test_graphql_workflow():
     # 2. Set alarm
     mutation_set = """
     mutation {
-      setAlarm(cronExpression: "30 7 * * *", command: "python wecker.py", isEnabled: true) {
+      setAlarm(cronExpression: "30 7 * * *", isEnabled: true) {
         id
         cronExpression
         command
@@ -151,7 +167,7 @@ def test_graphql_workflow():
     assert res.status_code == 200
     alarm = res.json()["data"]["setAlarm"]
     assert alarm["cronExpression"] == "30 7 * * *"
-    assert alarm["command"] == "python wecker.py"
+    assert "wecker.py" in alarm["command"]
     assert alarm["isEnabled"] is True
     alarm_id = alarm["id"]
 
@@ -176,7 +192,7 @@ def test_graphql_workflow():
     # 5. Update alarm
     mutation_update = f"""
     mutation {{
-      setAlarm(id: "{alarm_id}", cronExpression: "0 8 * * *", command: "python wecker.py", isEnabled: false) {{
+      setAlarm(id: "{alarm_id}", cronExpression: "0 8 * * *", isEnabled: false) {{
         id
         cronExpression
         isEnabled