CVE-2026-63337 in Java Client
Summary
by MITRE • 08/18/2026
The RabbitMQ Java client library allows Java and JVM-based applications to connect to and interact with RabbitMQ nodes. Prior to 5.33.0, com.rabbitmq.tools.jsonrpc.ProcedureDescription receives a javaReturnType value in an untrusted system.describe response and passes it through JSONUtil.tryFill, setJavaReturnType, and computeReturnTypeAsJavaClass to Class.forName(javaReturnType) with initialization enabled. An attacker able to answer the JsonRpcClient request through a shared broker or network interception can select a class already present in the victim JVM and trigger its static initializer, while JsonRpcClient.java later passes getReturnType output to mapper.parse and may also create type confusion. Successful exploitation can affect confidentiality, integrity, and availability in the client process. This issue is fixed in version 5.33.0.
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Analysis
by VulDB Data Team • 08/18/2026
The RabbitMQ Java client library serves as a critical component for enabling Java and JVM-based applications to establish connections with RabbitMQ message broker nodes. A significant security vulnerability was identified within this library, specifically affecting versions prior to 5.33.0. The flaw resides in the com.rabbitmq.tools.jsonrpc.ProcedureDescription class, which handles the processing of responses from system.describe requests via JSON-RPC protocols. This component is responsible for interpreting return types provided by the broker and mapping them to corresponding Java classes within the client application's runtime environment.
The core technical deficiency involves an insecure deserialization pattern coupled with unvalidated input handling. When a JsonRpcClient receives a response, it extracts a javaReturnType value from what should be considered an untrusted source, such as a shared broker or network traffic subject to interception. This raw string is then passed through several internal methods including JSONUtil.tryFill and setJavaReturnType before ultimately reaching computeReturnTypeAsJavaClass. The critical failure occurs when this method invokes Class.forName(javaReturnType) with initialization enabled. By allowing the caller to specify an arbitrary class name, the vulnerability permits an attacker to force the JVM to load and initialize any class that is already present in the victim's classpath.
This mechanism allows for a sophisticated attack vector where an adversary who can intercept or manipulate traffic between the client and broker can inject malicious class names into the response payload. Upon receiving this manipulated data, the vulnerable code attempts to resolve and instantiate the specified class. If the attacker selects a class with a static initializer block that performs harmful operations, such as executing arbitrary commands, modifying system state, or exfiltrating sensitive data, those actions will be executed within the context of the victim's JVM process. This represents a classic case of insecure deserialization where user-controlled input directly influences reflective API calls without adequate validation against an allowlist of expected types.
The operational impact of this vulnerability is severe and multifaceted, affecting confidentiality, integrity, and availability of the client application. From a confidentiality standpoint, malicious static initializers could access sensitive in-memory data or credentials stored within the JVM heap. Integrity risks arise if the attacker can modify internal state variables or trigger logic paths that alter the behavior of the messaging system. Availability is compromised through potential denial-of-service conditions caused by resource exhaustion during class loading or execution of destructive code sequences. Furthermore, because JsonRpcClient.java passes the getReturnType output to mapper.parse, there is an additional risk of type confusion if the parsed data does not match the expected structure, potentially leading to runtime exceptions or further exploitation opportunities depending on how the parser handles mismatched types.
This vulnerability aligns with Common Weakness Enumeration (CWE) identifiers such as CWE-502 Deserialization of Untrusted Data and CWE-470 Use of Externally-Controlled Input to Select Classes or Code Paths. In terms of the MITRE ATT&CK framework, this exploit technique corresponds to T1610 Role-Based Access Control Bypass if it allows privilege escalation within the application logic, but more accurately maps to T1203 Exploitation for Client Execution and potentially T1574 Hijack Execution Flow through static initializer manipulation. The attack requires network access or broker compromise, placing it in the context of lateral movement or remote code execution via trusted software components.
Mitigation strategies primarily involve upgrading the RabbitMQ Java client library to version 5.33.0 or later, where this issue has been resolved by implementing stricter validation on class names and restricting reflective operations to a predefined set of safe classes. For environments unable to upgrade immediately, network-level controls such as TLS encryption can prevent attackers from intercepting and modifying the JSON-RPC responses containing malicious javaReturnType values. Additionally, deploying Java Security Managers or using custom ClassLoaders with restricted permissions can limit the damage caused by arbitrary class loading, although these are less practical than patching the library directly. Organizations should also audit their dependencies for similar patterns of unvalidated reflective calls to prevent analogous vulnerabilities in other components.