CVE-2026-75516: RabbitMQ Java client: Frame-level OOM: Math.min(maxInboundMessageBodySize, 0) defeats frame size enforcement
Vulnerability
In AMQConnection.java (line 435-436), after Connection.Tune negotiation, the frame-max limit is set via:
java frameHandler.setFrameMax( Math.min(this.maxInboundMessageBodySize, frameMax));
When frameMax = 0 (meaning "unlimited" per AMQP spec), Math.min(67108864, 0) = 0. This value is then passed to Utils.framePayloadLimit(0) which returns Integer.MAXVALUE (line 77-79 of Utils.java):
java static int framePayloadLimit(int frameMax) { if (frameMax <= 0) { return Integer.MAXVALUE; } // ... }
This completely defeats the maxInboundMessageBodySize protection (default 64MB) at the frame level.
Attack Scenario
A malicious AMQP server (or MITM) sends Connection.Tune with frameMax=0:
1. Client defaults: requestedFrameMax = 0 (ConnectionFactory.DEFAULTFRAMEMAX, line 82) 2. negotiatedMaxValue(0, 0) = Math.max(0, 0) = 0 (line 673-676) 3. Math.min(maxInboundMessageBodySize, 0) = 0 — 64MB cap defeated 4. framePayloadLimit(0) = Integer.MAXVALUE — no frame size enforcement 5. Attacker sends a single frame with frameSize = 0x1FFFFFFF (~500MB) 6. Frame.readFrom() (line 135) executes new byte[frameSize] — OOM crash
The frame does not need to be a body frame — method frames, header frames, or heartbeat frames with a crafted size field all trigger the allocation before any content-level check fires.
Root Cause
The AMQP spec uses frameMax=0 to mean "unlimited", but Math.min treats it as the integer value zero. The intent of line 435-436 was to take the smaller of the two limits, but when one limit uses 0-means-unlimited semantics, Math.min always selects the zero, disabling the other limit.
Impact
- Default configuration is vulnerable: Both requestedFrameMax (client) and legitimate servers' frameMax in Tune may be 0 - Single-frame OOM: One malicious frame triggers up to ~2GB allocation (Integer.MAXVALUE bytes) - Bypasses existing protection: maxInboundMessageBodySize (introduced to cap allocations at 64MB) is entirely defeated at the frame level - Different from ValueReader OOM: This is a frame-layer allocation in Frame.readFrom(), not a value-layer allocation in ValueReader.readBytes()
Affected Code
- AMQConnection.java:435-436 — Math.min with 0-means-unlimited - Utils.java:77-79 — framePayloadLimit(0) returns Integer.MAXVALUE - Frame.java:135 — new byte[frameSize] allocation site - ConnectionFactory.java:82 — DEFAULTFRAMEMAX = 0
Suggested Fix
java int effectiveFrameMax = (frameMax == 0) ? this.maxInboundMessageBodySize : Math.min(this.maxInboundMessageBodySize, frameMax); frameHandler.setFrameMax(effectiveFrameMax);
This treats frameMax=0 as "use maxInboundMessageBodySize as the cap" instead of "zero".
Other sources
The RabbitMQ Java client library allows Java and JVM-based applications to connect to and interact with RabbitMQ nodes. Prior to 5.34.0, AMQConnection.start() applies Math.min(maxInboundMessageBodySize, frameMax) after Connection.Tune negotiation even though AMQP defines frameMax value zero as unlimited and ConnectionFactory.DEFAULTFRAMEMAX is zero. When the client default and server-negotiated value are both zero, the result is passed to Utils.framePayloadLimit(int), which interprets zero as Integer.MAXVALUE and disables the configured maxInboundMessageBodySize cap. A malicious AMQP server, or a man-in-the-middle attacker able to modify Connection.Tune and inject frames into the connection, can then send an oversized frame of any frame type, causing Frame.readFrom() to allocate a large byte array before content-level validation and potentially terminate the client process through memory exhaustion. This issue is fixed in version 5.34.0.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
maven/com.rabbitmq:amqp-clientto a version that resolves this vulnerability.Fixed in 5.34.0 - Upgrade
Upgrade
RabbitMQ Java clientto a version that resolves this vulnerability.Fixed in 5.34.0 - Configuration
Change the frame-max effective limit logic after Connection.Tune negotiation from Math.min(maxInboundMessageBodySize, frameMax) so that when frameMax negotiated value is 0 (AMQP meaning "unlimited"), the effective frame cap uses maxInboundMessageBodySize instead of resulting in 0. (Issue is fixed in client version 5.34.0).
RabbitMQ Java client (frame-max limit handling) Math.min(this.maxInboundMessageBodySize, frameMax) = Treat frameMax=0 as unlimited should select maxInboundMessageBodySize cap instead of 0
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Java and other JVM-based applications using RabbitMQ Java client versions before 5.34.0 are affected when the effective negotiated frameMax is zero. This includes the default client frame-max setting, because ConnectionFactory.DEFAULT_FRAME_MAX is zero.
What must an attacker be able to do?
An attacker must control a malicious AMQP server, or be able to act as a man-in-the-middle who modifies Connection.Tune and injects frames into the connection. They can then send an oversized AMQP frame and trigger large memory allocation in the client.
What happens if exploitation succeeds?
The client can allocate a large byte array while reading the frame, before content-level validation occurs. This can exhaust memory and terminate the client process.
What is the remediation?
Upgrade the RabbitMQ Java client to version 5.34.0, which fixes the frame-size enforcement issue.