Details
moquette is reachable by untrusted MQTT clients (anonymous by default), so every byte from any client, including pre-authentication, is untrusted. This is a memory-safe JVM: the ceiling is authorization/ACL bypass + denial of service + cross-session integrity, **not RCE** (I did not find one and do not claim one). Audited at commit `da7f719a6bab9829d520b5838e13ea7b1f9be3ef`, module `broker/`.
## What a connecting client can do
1. (Critical) Bypass `pattern`-based ACLs across tenants. In `AuthorizationsCollector.canDoOperation` (AuthorizationsCollector.java:116-131, esp. line 123) the clientId/username is substituted raw into a pattern ACL rule and then wildcard-matched, and the clientId is never validated for MQTT wildcard characters +/# at CONNECT (MQTTConnection.processConnect):
Topic substitutedTopic = new Topic(auth.topic.toString().replace("%c", client).replace("%u", username));
if (topic.match(substitutedTopic)) return true;
A client that connects with clientId + turns sensor/%c/# into the filter sensor/+/#, gaining cross-tenant read AND write. (Precondition: pattern ACL rules configured — a common multi-tenant setup.)
2. (High) Crash the whole broker. SessionEventLoop (SessionEventLoop.java:40-54) catches only InterruptedException and is never restarted (SessionEventLoopGroup), so any uncaught exception on it wedges every co-located client. Trivially reachable inputs: malformed $share/grp SUBSCRIBE (SharedSubscriptionUtils.extractShareName -> StringIndexOutOfBoundsException), deeply nested topic (CTrie recursion -> StackOverflowError), and ACL NPE below. Unbounded subscriptions / retained / in-flight / topic-alias / interceptor state (BrokerInterceptor uses an unbounded queue) also allow OOM; durable stores allow disk exhaustion.
3. (High) NPE in ACL sink on clientId # (invalid filter sensor/#/# -> null tokens -> Topic.match NPE at Topic.java:173).
4. (High) Will-message authorization bypass. Last-Will topic is published (PostOffice.publishWill) without canWrite/reserved-topic checks used for normal PUBLISH.
5. (Medium) Cross-session durable corruption. H2PersistentQueue opens queue_"+clientId and queue_"+clientId+"_meta; client id sensor_meta collides with victim sensor metadata map -> corrupts head/tail.
6. (Medium) Fail-open if authenticator/authorizator class fails to load -> PermitAll/AcceptAll (Server.java:483-531).
## Proof of concept
Source-only, no network; PoCs run on JDK 17:
- PoCPatternAcl — clientId + gains cross-tenant read/write; clientId # triggers NPE
- PoCSharedSubCrash — extractShareName("$share/grp") throws StringIndexOutOfBoundsException
- PoCMapCollision — H2 MVStore collision overwrites victim metadata pointer
## Impact
Cross-tenant eavesdropping and injection, whole-broker DoS, unauthorized Will publishes, and cross-session durable corruption.
## Remediation
1. Reject clientId/username containing +/# (and / if structural) at CONNECT; expand %c/%u as literal tokens.
2. Harden SessionEventLoop (catch Throwable + restart supervision) and validate $share filters.
3. Apply authorization to Will publishes like normal PUBLISH.
4. Add resource caps (connections, queues, retained, aliases, interceptor queue) + bounded session expiry.
5. Separate H2 namespaces and fail closed on auth-class load failure.
