CVE-2026-40211 in DNSdist
Summary
by MITRE • 06/25/2026
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.
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Analysis
by VulDB Data Team • 06/25/2026
This vulnerability represents a denial of service weakness in DNS over HTTP/3 implementations that stems from improper memory management during query processing. The flaw occurs when crafted DNS queries are transmitted through the HTTP/3 protocol layer, specifically within QUIC connections where the underlying transport mechanism handles multiple concurrent streams. When these maliciously constructed queries are processed, they trigger an exception that prevents immediate buffer deallocation, creating a memory leak scenario where allocated resources remain in use beyond their intended lifecycle.
The technical implementation involves the QUIC protocol's stream management system where each DNS query operates as an independent stream within the connection. Under normal circumstances, buffers allocated for processing these queries should be freed immediately upon completion of the query handling routine. However, when exception conditions occur during DNS processing, the memory cleanup routine fails to execute properly, leaving allocated buffers in a dormant state. This memory retention becomes problematic when multiple concurrent DoH3 streams are opened simultaneously, as each stream maintains its own set of buffers that may not be freed promptly.
The operational impact of this vulnerability manifests as a progressive memory exhaustion condition that can ultimately lead to system unresponsiveness or complete service disruption. Attackers can exploit this weakness by opening numerous concurrent DoH3 streams with maliciously crafted DNS queries, effectively consuming available memory resources faster than they can be reclaimed through normal garbage collection mechanisms. The vulnerability is particularly concerning in high-traffic environments where DNS over HTTP/3 servers handle hundreds or thousands of concurrent connections, as the memory consumption can escalate rapidly and potentially overwhelm system resources.
This weakness aligns with CWE-401, which specifically addresses improper resource cleanup or release, and relates to the broader category of memory management errors that can lead to denial of service conditions. From an ATT&CK perspective, this vulnerability could be categorized under T1499.004 for network denial of service attacks, where adversaries leverage protocol implementation flaws to consume system resources. The attack vector specifically involves protocol-level manipulation through HTTP/3 and QUIC implementations, making it particularly relevant to the T1595.002 technique of using network protocol manipulation for exploitation purposes.
Mitigation strategies should focus on implementing robust exception handling routines that ensure buffer cleanup occurs regardless of processing outcomes, while also establishing connection limits and stream throttling mechanisms to prevent excessive concurrent resource consumption. Memory monitoring systems should be deployed to detect unusual memory usage patterns that could indicate exploitation attempts. Additionally, implementing proper resource pool management with timeouts and automatic cleanup procedures can help prevent the accumulation of unreleased buffers. Regular protocol updates and patches addressing QUIC and HTTP/3 implementation issues are essential, along with network-level controls such as rate limiting for DNS over HTTP/3 connections to restrict the number of concurrent streams that can be opened by individual clients or groups of clients.