CVE-2026-77403: RabbitMQ amqp091-go: Denial of Service via Sub-Spec Frame Size Negotiation

Summary A Denial of Service (DoS) vulnerability exists in the AMQP client's connection negotiation logic. The AMQP specification explicitly mandates a strict minimum frame size of 4096 bytes to prevent pathological packet fragmentation. While the library defines a frameMinSize = 4096 constant, the connection negotiation loop fails to enforce this boundary, blindly accepting whatever maximum frame size (FrameMax) the server advertises during the handshake.

If a client connects to a malicious or compromised AMQP broker that advertises an extremely low FrameMax (such as 1 byte), the negotiation succeeds. Consequently, every subsequent message transmission is forced to splinter into thousands or millions of single-byte frames, causing massive CPU overhead, thread contention, and a near-instantaneous application freeze.

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Vulnerability Details

Mechanism During the connection establishment phase, the client and server negotiate connection parameters—including maximum channel count, heartbeat intervals, and maximum frame sizes. The vulnerability is located where the client accepts the server's tuning parameters:

go // Connection negotiation logic maps server values directly without validation if serverSettings.FrameMax > 0 { // VULNERABILITY: Lacks a floor validation check against frameMinSize (4096) c.config.FrameMax = serverSettings.FrameMax }

Because there is no conditional check asserting that serverSettings.FrameMax >= frameMinSize, a value below the protocol specification floor is successfully registered. When the application later passes data payloads to the frame writer, the chunking algorithm splits the payload using the negotiated FrameMax value as its chunk window divisor.

Impact When FrameMax is set to an absurdly low threshold (e.g., 1 to 10 bytes): A standard 10 KiB message payload requires tens of thousands of individual write operations and frame headers. The system's CPU becomes entirely bound by frame serialization, memory allocation for frame structures, and context switching within the network output loops. This results in an application-layer Denial of Service (DoS) affecting not just the specific AMQP connection, but potentially the entire host system due to CPU resource starvation.

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Attack Vector An attacker who compromises an upstream AMQP broker, performs a Man-in-the-Middle (MitM) interception, or tricks an application into connecting to an unauthorized external rogue broker can trigger this vulnerability:

1. Rogue Handshake: The client application connects to an AMQP endpoint controlled by the attacker. 2. Malicious Parameter Tuning: During the connection.tune phase, the rogue server returns a FrameMax value of 1. 3. Resource Exhaustion Trigger: The client completes the handshake successfully. As soon as the client attempts to publish a message or process traffic, the internal loop fragments the data into single-byte frames, spiking the host's CPU usage to 100% and disabling the application thread.