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. --- ## 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