CVE-2026-40211 PowerDNS CVE debrief

Who should care

Defenders of systems using DNS over HTTP/3, including operators, platform administrators, vulnerability management teams, and security teams, should assess exposure and verify QUIC connection configurations to prevent potential out-of-memory conditions and denial of service attacks.

Why it matters

CVE-2026-40211 is a denial of service vulnerability in DNS over HTTP/3 queries. Defenders should assess exposure and verify QUIC connection configurations to prevent potential out-of-memory conditions.

• Denial of service through out-of-memory condition
• Verification of QUIC connection configurations required
• Monitoring for concurrent DoH3 streams necessary

Technical summary

An attacker can send crafted DNS over HTTP/3 queries, triggering an exception that prevents some buffer from being freed right away. The buffer will be freed at the end of the QUIC connection, but on some setups it might be possible to open enough concurrent DoH3 streams to trigger an out-of-memory condition, resulting in a denial of service. This vulnerability affects systems using DNS over HTTP/3 and has a CVSS score of 5.3, classified as MEDIUM severity. Defenders should assess exposure and verify QUIC connection configurations.

Defensive priority

Assess exposure and verify QUIC connection configurations.

Recommended defensive actions

• Assess exposure to crafted DNS over HTTP/3 queries
• Verify QUIC connection configurations
• Monitor for concurrent DoH3 streams
• Review compensating controls for exposed systems
• Check relevant monitoring, detection, and logs for exposed assets
• Track exceptions and retest remediated assets
• Confirm whether affected product deployments exist in managed environments

Evidence notes

Official CVE Program and NVD records detail a denial of service vulnerability in DNS over HTTP/3 queries. The vulnerability allows attackers to send crafted queries that can trigger an exception, preventing some buffer from being freed immediately. This can lead to an out-of-memory condition on some setups, resulting in a denial of service. Defenders should verify QUIC connection configurations and monitor for concurrent DoH3 streams.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.