A ZoomEye query reported 1,160,264 Internet-reachable services matching its Mosquitto fingerprint at 2026-09-29 02:47 UTC. That is a scanner’s service-identification count—not a count of vulnerable brokers, anonymous logins, exposed messages, or systems an attacker can control. Each of those outcomes requires separate checks.
What a Mosquitto fingerprint tells you
Eclipse Mosquitto is an open-source broker for MQTT, a publish/subscribe messaging protocol used by small devices as well as full servers. A publishing client sends a message to the broker; the broker routes it to clients subscribed to the relevant topic, subject to the broker’s access rules.
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The figure above comes from the query app="Mosquitto", which Jeffrey Ciend reported running against ZoomEye. A match means the index identified an Internet-reachable service as Mosquitto. It does not establish the software version, patch status, authentication settings, or permissions. Nor does it prove that every match represents a unique vulnerable system.
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An Internet-facing listener means a service can be reached over the network. What a client can do after reaching it depends on authentication and authorization configuration. Treat these as separate questions:
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- Can a client connect? A reachable broker may still require valid credentials.
- Can an unauthenticated client subscribe or read? This depends on the broker’s authentication choices and topic-level access controls.
- Can a client publish or write? Read permission does not imply permission to publish. Censys explicitly cautions that subscribing to or reading a topic does not necessarily grant publishing rights.
- Are messages actually delivered? Permission to subscribe does not prove that any message arrived during a particular observation.
So an open port—often associated with MQTT—does not by itself answer who can connect, read, or publish. Each permission must be verified separately on systems you own or are authorized to assess.
What Censys observed across MQTT services
Censys’s 2026 measurements describe MQTT services broadly, not Mosquitto alone. They are scan-specific observations, not estimates that can be applied directly to the ZoomEye fingerprint matches.
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| Censys observation | Reported result | How to interpret it |
|---|---|---|
| Hosts exposing one or more MQTT services | More than 650,000 | Broad MQTT exposure observed by Censys; not a Mosquitto-only count. |
| MQTT hosts in the United States | 36,035 | The population for the U.S. figures below. |
| U.S. hosts accepting unauthenticated connections | 9,649 (26.8%) | Connection acceptance is distinct from permission to read or publish. |
| U.S. hosts allowing unauthenticated subscriptions | 7,756 (21.5%) | Subscription access does not establish write access or prove messages were received. |
| Hosts that emitted messages | 28,595 | Censys subscribed and waited up to one minute; it reported this as about 4.4% of exposed MQTT hosts or about 7.0% of those allowing unauthenticated subscriptions. The result depends on that scan and observation window. |
These figures illustrate why “exposed” is not a single security condition. A connection may be unauthenticated without permitting subscriptions; a subscription may be possible without publishing; and a permitted subscription may receive no message during a brief wait.
Why Mosquitto version and listener configuration matter
Mosquitto’s documented behavior differs at the 2.0 boundary. Version 2.0 and later requires an explicit authentication choice before clients can connect; earlier versions defaulted to allowing unauthenticated clients. The default alone does not tell you how a deployed broker is configured, so check the installed version and its effective configuration rather than inferring access from a fingerprint.
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Review which listeners are bound to network interfaces and which authentication and topic-level access controls apply to each one. The Mosquitto manual documents listener behavior and changes between 1.6.x and 2.0, making the exact deployed version relevant. A broad or public binding does not necessarily mean anonymous access, but it does make the listener’s controls important to verify.
How to audit a Mosquitto broker you operate
- Inventory public-facing assets. Identify the hosts and listeners that should be reachable from outside your network. Compare that inventory with an exposure assessment of assets you own.
- Verify the deployed version and configuration. Check the installed Mosquitto version, listener bindings, and the settings actually used by the running service. Do not rely on version defaults as proof of current permissions.
- Require authentication where the broker is reachable. Confirm that anonymous connections are not unintentionally allowed. The Mosquitto project says anonymous access is not advised when a broker is publicly available.
- Review topic-level access controls. Limit which authenticated clients can subscribe to or publish on each topic; validate read and write permissions separately.
- Protect traffic in transit where appropriate. Use TLS when it fits the deployment and threat model, and verify that clients connect using the intended secure listener.
- Patch supported releases and monitor access. Keep the broker current, change default passwords, use monitored access methods where appropriate, and watch traffic for unexpected activity. These measures align with CISA’s guidance for public-facing assets.
- Reassess exposure routinely. Repeat checks after configuration changes and as part of regular exposure reviews, using scans only against systems you are authorized to assess.
How to read the headline safely
The reported total is useful as a snapshot of the Internet-visible Mosquitto surface identified by one scanner at one time. It is not evidence that every matching service is misconfigured or compromised. For an operator, the actionable question is narrower: which of my brokers are reachable, and what can an unauthenticated or authenticated client actually do there?
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