CVE-2026-63337

RabbitMQ Java client: Unvalidated Class.forName in JSON-RPC ProcedureDescription enables arbitrary class loading

7.5 HighFix available

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.

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: Low
Privileges needed: Low
User interaction: Passive

What an attacker can affect

Confidentiality: High
Integrity: High
Availability: High

Exploit likelihood

0.56%Up 0.2 points since Aug 19, 2026

What each source reports

CVE List Score used 7.5

RabbitMQ Java client: Unvalidated Class.forName in JSON-RPC ProcedureDescription enables arbitrary class loading

Record: CVE-2026-63337
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L
Weakness: CWE-470
Updated: 2026-11-02

EUVDEUVD-2026-61043 7.5

Record: EUVD-2026-61043
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L
Updated: 2026-11-02

GitHub AdvisoriesGHSA-6g32-pxv4-2wfj 7.5

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 JsonRpcClient to connect to a JSON-RPC service via RabbitMQ
Attacker (co-tenant on shared broker, or MITM) intercepts the system.describe request
Attacker responds with crafted javaReturnType values
Victim's client calls Class.forName(attackerInput) with default initialize=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.

Record: GHSA-6g32-pxv4-2wfj
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L
Weakness: CWE-470
Updated: 2026-11-02

NVD 7.5

Record: CVE-2026-63337
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Weakness: CWE-470
Updated: 2026-11-02

OSV 7.5

RabbitMQ Java client: Unvalidated Class.forName in JSON-RPC ProcedureDescription enables arbitrary class loading

Record: CVE-2026-63337
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L
Weakness: CWE-470
Updated: 2026-11-02

OSVDEBIAN-CVE-2026-63337 7.5

Record: DEBIAN-CVE-2026-63337
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Updated: 2026-11-02

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OSVGHSA-6g32-pxv4-2wfj 7.5

RabbitMQ Java client: Unvalidated Class.forName in JSON-RPC ProcedureDescription enables arbitrary class loading

Attack scenario:

Victim uses JsonRpcClient to connect to a JSON-RPC service via RabbitMQ
Attacker (co-tenant on shared broker, or MITM) intercepts the system.describe request
Attacker responds with crafted javaReturnType values
Victim's client calls Class.forName(attackerInput) with default initialize=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.

Record: GHSA-6g32-pxv4-2wfj
CVSS 4.0: 7.5 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:P/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L
Weakness: CWE-470
Updated: 2026-11-02

OSVRHLW-2026-45242cd5d8021673 6.0

Lightwell Security Advisory: amqp-client 5.25.0.rhlw-00028

Red Hat Lightwell has released com.rabbitmq:amqp-client 5.25.0.rhlw-00028 with backported fixes for CVE-2026-106121, CVE-2026-106122, CVE-2026-106123, CVE-2026-61634, CVE-2026-63335, CVE-2026-63336, CVE-2026-63337, CVE-2026-69219, and CVE-2026-69220.

CVE-2026-106121: RabbitMQ: JSONReader in the default JSON-RPC mapper never terminates on truncated input, causing DoS

CVE-2026-106122: RabbitMQ: Malformed UTF-8 in shortstr properties permanently disables RPC consumers

CVE-2026-106123: RabbitMQ Java client: plaintext broker credentials leaked in exception message from ConnectionFactoryConfigurator.load()

CVE-2026-61634: RabbitMQ Java client accepts broker frames larger than the negotiated AMQP frame_max

CVE-2026-63335: RabbitMQ Java client malformed body frame triggers raw command assembler exception

CVE-2026-63336: RabbitMQ Java client: TrustEverythingTrustManager used by default in useSslProtocol() enables MITM

CVE-2026-63337: RabbitMQ Java client: Unvalidated Class.forName in JSON-RPC ProcedureDescription enables arbitrary class loading

CVE-2026-69219: RabbitMQ Java client ValueReader: Oversized LongString/bytes length triggers OOM via unchecked allocation

CVE-2026-69220: RabbitMQ Java client ValueReader: Unbounded recursive table/array nesting causes StackOverflowError DoS

This patched artifact is a drop-in replacement for com.rabbitmq:amqp-client 5.25.0 from Maven Central.

Record: RHLW-2026-45242cd5d8021673
CVSS 4.0: 6.0 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-06

OSVRHLW-2026-f4441c6bf0c89319 6.0

Lightwell Security Advisory: amqp-client 5.30.0.rhlw-00016

Red Hat Lightwell has released com.rabbitmq:amqp-client 5.30.0.rhlw-00016 with backported fixes for CVE-2026-106121, CVE-2026-106122, CVE-2026-106123, CVE-2026-61634, CVE-2026-63335, CVE-2026-63336, CVE-2026-63337, CVE-2026-69219, and CVE-2026-69220.