CVE-2026-75516

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.DEFAULT_FRAME_MAX 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.MAX_VALUE 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.

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.DEFAULT_FRAME_MAX 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.MAX_VALUE 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.

Vulnerability

In AMQConnection.java (line 435-436), after Connection.Tune negotiation, the frame-max limit is set via:

_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.MAX_VALUE (line 77-79 of Utils.java):

static int framePayloadLimit(int frameMax) {
    if (frameMax <= 0) {
      return Integer.MAX_VALUE;
    }
    // ...
}

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.DEFAULT_FRAME_MAX, 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.MAX_VALUE — 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.MAX_VALUE 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.MAX_VALUE
• Frame.java:135 — new byte[frameSize] allocation site
• ConnectionFactory.java:82 — DEFAULT_FRAME_MAX = 0

Suggested Fix

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".

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.DEFAULT_FRAME_MAX 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.MAX_VALUE 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.

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.DEFAULT_FRAME_MAX 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.MAX_VALUE 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.

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.DEFAULT_FRAME_MAX 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.MAX_VALUE 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.

Vulnerability

In AMQConnection.java (line 435-436), after Connection.Tune negotiation, the frame-max limit is set via:

_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.MAX_VALUE (line 77-79 of Utils.java):

static int framePayloadLimit(int frameMax) {
    if (frameMax <= 0) {
      return Integer.MAX_VALUE;
    }
    // ...
}

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.DEFAULT_FRAME_MAX, 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.MAX_VALUE — 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.MAX_VALUE 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.MAX_VALUE
• Frame.java:135 — new byte[frameSize] allocation site
• ConnectionFactory.java:82 — DEFAULT_FRAME_MAX = 0

Suggested Fix

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".

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.DEFAULT_FRAME_MAX 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.MAX_VALUE 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.

Root has patched CVE-2026-75516 in the com.rabbitmq:amqp-client package for Root:Maven. Multiple fixed versions available.