RabbitMQ amqp091-go: Consumer Message Flooding via Signed-to-Unsigned Integer Casting in Qos Configuration

Summary

A logic and resource exhaustion vulnerability exists in the AMQP client's Quality of Service (Qos) configuration method. The Qos function accepts signed integers (int) for the prefetchCount and prefetchSize parameters but casts them directly to unsigned integers (uint16 and uint32, respectively) when formatting the wire-level frame.

If a developer passes a negative integer (such as -1) to these parameters—frequently intended as a sentinel value meaning "no change" or "no limit"—the application performs an implicit signed-to-unsigned conversion. This wraps the values to their absolute maximum limit ($65535$ and $4294967295$). Consequently, a consumer expecting restricted message delivery rates is suddenly flooded with an unlimited volume of messages, potentially exhausting memory resources and crashing the application.

Vulnerability Details

Mechanism

The bug manifests during the structural assignment inside the channel's Qos method:

// channel.go:795-796
PrefetchCount: uint16(prefetchCount),  // -1 wraps to 65535
PrefetchSize:  uint32(prefetchSize),   // -1 wraps to 4294967295

In Go, converting a negative signed integer to an unsigned integer shifts the value via two's complement arithmetic. Because no boundary validation or signedness check occurs prior to the cast:

• Passing -1 for prefetchCount yields a wire value of 65535.
• Passing -1 for prefetchSize yields a wire value of 4294967295.

Impact

The AMQP broker interprets a prefetch-count of 65535 as an instruction to dispatch messages to the consumer with virtually no concurrency limits.

If the application is processing heavy payloads or relies on strict rate-limiting to maintain stability, this unexpected flood will cause rapid heap memory growth, unmanageable processing queues, and an eventual Out-Of-Memory (OOM) termination.

Attack Vector

An attacker who can manipulate configuration files, environmental variables, or API inputs that dictate client Qos settings can trigger an application-layer Denial of Service:

1. Malicious Input: An attacker sets a service's prefetch configuration parameter to -1.
2. Implicit Overflow: The application initializes the channel, executes Qos(-1, ...), and transmits an unintended maximum-capacity request to the RabbitMQ/AMQP broker.
3. Consumer Exhaustion: The broker flushes the entire contents of the queue directly into the client consumer's network buffer, bypassing expected application concurrency barriers and inducing a crash.