CoreDNS Vulnerability Allows Attackers to Exhaust Server Memory via Amplification Attack

A high-severity vulnerability (CVE-2025-47950) in CoreDNS’s DNS-over-QUIC (DoQ) implementation enables remote attackers to crash DNS servers through stream amplification attacks.

Patched in v1.21.2, this flaw highlights risks in modern protocol adoption for cloud-native systems

Goroutine Proliferation in DoQ Implementation

The vulnerability stems from CoreDNS’s handling of QUIC streams in its server_quic.go component.

For every incoming QUIC stream, the server spawned a new goroutine without concurrency limits, creating a 1:1 stream-to-goroutine mapping.

This design allowed attackers to:

• Open unlimited streams via single/multiple QUIC connections
• Trigger OOM crashes in memory-constrained environments (common in Kubernetes/container setups)
• Bypass authentication using standard DNS-over-QUIC handshakes

go// Vulnerable pre-1.21.2 code snippet from server_quic.go  
func (s *Server) handleStream(stream quic.Stream) {  
    go s.processStream(stream) // Uncontrolled goroutine creation  
}  

The attack requires minimal bandwidth (~1 MB/s for 10k streams) and no privileges, earning a CVSSv3 score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

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Mitigation Strategies: Concurrent Stream Throttling

The patch introduces two concurrency controls via new Corefile directives:

Parameter	Default	Function
max_streams	256	Limits streams per QUIC connection
worker_pool_size	1024	Sets global worker pool for stream processing

bashquic {  
    max_streams 256          # Prevents single-connection attacks  
    worker_pool_size 1024    # Global stream processing cap  
}  

For unpatched systems, administrators can:

• Disable DoQ by removing quic:// from Corefile
• Implement Kubernetes resources.limits for memory/cpu
• Monitor QUIC traffic for abnormal stream counts (>500/sec)

Cloud DNS Infrastructure Exposure

Risk Factor	Severity Level	Details
Attack Surface	Critical	78% of Kubernetes clusters use CoreDNS
Exploit Effort	Low	Requires standard QUIC client tools
Impact	High	Cluster-wide DNS outage in 90 seconds
Patching Cadence	Moderate	40% of enterprises update DNS >30 days

Red Hat’s modified CVSS score (5.3) underestimates containerized environments where OOM kills cascade to pod rescheduling storms.

The vulnerability particularly threatens:

• Multi-tenant SaaS platforms using shared CoreDNS instances
• Edge computing nodes with limited RAM (e.g., <4GB)
• CI/CD pipelines relying on DNS-based service discovery

Security teams should prioritize upgrades using go get github.com/coredns/coredns@v1.21.2 and audit QUIC usage via:

bashkubectl get cm -n kube-system coredns -o yaml | grep "quic://"  

This incident underscores the tension between adopting newer protocols (QUIC) and maintaining backward-compatible security models – a challenge likely to intensify as DoQ adoption grows 300% YoY.

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