Fluentd's Monitor Agent plugin (in_monitor_agent) exposes internal metrics and plugin information via a REST API. It was discovered that the API response (/api/plugins.json and related endpoints) unintentionally includes internal instance variables of loaded plugins.

If any plugins store sensitive information—such as database passwords, API keys, or cloud credentials—in its instance variables, this information may be exposed in plain text to any user or system that has HTTP access to the Monitor Agent API.

Impact

This vulnerability allows for unauthorized information disclosure. An attacker who can reach the Monitor Agent API port (default: 24220) can potentially extract sensitive credentials used by other Fluentd plugins. The impact severity depends highly on the network configuration (whether the Monitor Agent port is exposed to untrusted networks) and the specific plugins configured in the Fluentd instance.