CVE-2026-63337

The JSON-RPC tools in `com.rabbitmq.tools.jsonrpc` perform `Class.forName(javaReturnType)` with `initialize=true` on class names received from untrusted AMQP messages, without any validation or allowlist. **Vulnerable code** (`ProcedureDescription.java:101-127`): When a `JsonRpcClient` connects, it calls `system.describe` and receives a service description from the AMQP queue. The response JSON includes `javaReturnType` fields that are reflectively set via `JSONUtil.tryFill()`, triggering `setJavaReturnType()` → `computeReturnTypeAsJavaClass()` → `Class.forName(javaReturnType)`. **Attack scenario:** 1. Victim uses `JsonRpcClient` to connect to a JSON-RPC service via RabbitMQ 2. Attacker (co-tenant on shared broker, or MITM) intercepts the `system.describe` request 3. Attacker responds with crafted `javaReturnType` values 4. Victim's client calls `Class.forName(attackerInput)` with default `initialize=true` 5. Static initializers of attacker-specified classes execute in victim's JVM Additionally, the loaded class from `getReturnType()` is passed to `mapper.parse(replyStr, expectedType)` at `JsonRpcClient.java:168`, potentially enabling type-confusion. **Recommended fix:** Use `Class.forName(javaReturnType, false, classLoader)` to prevent static initializer execution, or add an allowlist of permitted return types. **CWE:** CWE-470 --- **Reply from reporter (2026-06-29):** Thanks for the quick turnaround. Fix looks good. Looking forward to the CVE assignment.