AI agents use mqttPublish to create or update resources in MQTTX SSE Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MQTTX SSE Server environment.
mqttPublish sends a message to an MQTT topic, which is a write/create operation. It can trigger downstream subscribers and external systems, but the act itself is publishing data rather than executing code or deleting data. Severity is medium because a misused publish could trigger unintended actions in IoT or automation systems subscribed to those topics.
From the tool's definition Publishes a message to the specified MQTT topic
Documented attack patterns abuse exactly the kind of access mqttPublish gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and MQTTX SSE Server, and nothing reaches the server without passing your rules. This is the rule we recommend for mqttPublish:
{
"version": "1",
"default": "deny",
"tools": {
"mqttPublish": {
"limits": [
{
"counter": "mqttpublish_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}mqttPublish stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Publishes a message to the specified MQTT topic. It is categorised as a Write tool in the MQTTX SSE Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the MQTTX SSE Server MCP server in PolicyLayer and add a rule for mqttPublish: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches MQTTX SSE Server. Nothing to install.
mqttPublish is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the mqttPublish rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for mqttPublish. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
mqttPublish is provided by the MQTTX SSE Server MCP server (ysfscream/mqttx-mcp-sse-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from MQTTX SSE Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
4 MQTTX SSE Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.