Summary
RabbitMQ Java client: Unvalidated Class.forName in JSON-RPC ProcedureDescription enables arbitrary class loading
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:
- Victim uses
JsonRpcClientto connect to a JSON-RPC service via RabbitMQ - Attacker (co-tenant on shared broker, or MITM) intercepts the
system.describerequest - Attacker responds with crafted
javaReturnTypevalues - Victim's client calls
Class.forName(attackerInput)with defaultinitialize=true - 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.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-63337? CVE-2026-63337 is a high-severity security vulnerability in com.rabbitmq:amqp-client (maven), affecting versions < 5.33.0. It is fixed in 5.33.0.
- Which versions of com.rabbitmq:amqp-client are affected by CVE-2026-63337? com.rabbitmq:amqp-client (maven) versions < 5.33.0 is affected.
- Is there a fix for CVE-2026-63337? Yes. CVE-2026-63337 is fixed in 5.33.0. Upgrade to this version or later.
- Is CVE-2026-63337 exploitable, and should I be worried? Whether CVE-2026-63337 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-63337 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-63337? Upgrade
com.rabbitmq:amqp-clientto 5.33.0 or later.