An attacker who can influence the response to a system.describe call, such as through a compromised or malicious broker, can force the client to load arbitrary classes present on the classpath. Because the initialize parameter is set to true, any static initializers defined within those classes are executed immediately upon loading. This flaw facilitates potential remote code execution (RCE) in the context of the victim's application, in addition to potential type-confusion attacks during subsequent data parsing.

Attack Chain

1. The victim application initializes a JsonRpcClient to interact with a JSON-RPC service over a RabbitMQ broker.
2. The client sends a system.describe request via the AMQP queue.
3. The attacker intercepts the communication or acts as a malicious broker and provides a crafted JSON response.
4. The attacker sets the javaReturnType field in the JSON response to a target malicious or sensitive class name present in the application's classpath.
5. The JsonRpcClient receives the response and processes the javaReturnType field via JSONUtil.fill().
6. The computeReturnTypeAsJavaClass() method is invoked, triggering Class.forName() with the attacker-supplied class name and initialize=true.
7. The JVM loads the specified class and triggers its static initializer block.
8. Arbitrary code defined in the static initializer executes within the victim's process, achieving RCE or other local impact.

Impact

The vulnerability allows for arbitrary class loading and static initializer execution, which poses a significant risk of remote code execution for any application utilizing the com.rabbitmq.tools.jsonrpc package. Affected versions include all versions of the RabbitMQ Java client prior to 5.33.0. Successful exploitation requires an attacker to be positioned as a broker or intercepting party between the client and the JSON-RPC endpoint.

Recommendation

Prioritized actions for addressing CVE-2026-63337:

• Upgrade the RabbitMQ Java client (amqp-client) to version 5.33.0 or later immediately to patch the insecure Class.forName calls.
• Review applications using com.rabbitmq.tools.jsonrpc for exposure to untrusted AMQP brokers.
• If immediate patching is not possible, implement an application-side filter to intercept and validate the javaReturnType string against an strict allowlist of expected classes before the library processes the JSON response.