CoreDNS Vulnerable to DoQ Memory Exhaustion via Stream Amplification

Summary

A Denial of Service (DoS) vulnerability was discovered in the CoreDNS DNS-over-QUIC (DoQ) server implementation. The server previously created a new goroutine for every incoming QUIC stream without imposing any limits on the number of concurrent streams or goroutines. A remote, unauthenticated attacker could open a large number of streams, leading to uncontrolled memory consumption and eventually causing an Out Of Memory (OOM) crash — especially in containerized or memory-constrained environments.

Impact

• Component: server_quic.go
• Attack Vector: Remote, network-based
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Impact: High availability loss (OOM kill or unresponsiveness)

This issue affects deployments with quic:// enabled in the Corefile. A single attacker can cause the CoreDNS instance to become unresponsive using minimal bandwidth and CPU.

Patches

The patch introduces two key mitigation mechanisms:

• max_streams: Caps the number of concurrent QUIC streams per connection. Default: 256.
• worker_pool_size: Introduces a server-wide, bounded worker pool to process incoming streams. Default: 1024.

This eliminates the 1:1 stream-to-goroutine model and ensures that CoreDNS remains resilient under high concurrency. The new configuration options are exposed through the quic Corefile block:

quic {
    max_streams 256
    worker_pool_size 1024
}

These defaults are generous and aligned with typical DNS-over-QUIC client behavior.

Workarounds

If you're unable to upgrade immediately, you can:

• Disable QUIC support by removing or commenting out the quic:// block in your Corefile
• Use container runtime resource limits to detect and isolate excessive memory usage
• Monitor QUIC connection patterns and alert on anomalies

References

• RFC 9250 - DNS over Dedicated QUIC Connections
• quic-go GitHub project
• QUIC stream exhaustion class of vulnerabilities (related)

Credit

Thanks to @thevilledev for disclovering this vulnerability and contributing a high-quality fix.

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