CVE-2026-19401 in NSD
Summary
by MITRE • 08/26/2026
Any remote client can crash a (debugging/non-release build type) NSD serve child by sending it a special crafted message with a specially tuned number of DNS Cookie options (17 when UDP payload size is 512). By continuously crashing the serve childs, the remote client can severely hamper or, when positioned sufficiently close, deny all DNS service.
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Analysis
by VulDB Data Team • 08/26/2026
The vulnerability described involves a denial-of-service condition affecting specific builds of the Name Server Daemon (NSD), particularly those compiled in debugging or non-release modes. This flaw allows any remote client to crash an NSD serve child process by transmitting a specially crafted DNS message containing precisely tuned numbers of DNS Cookie options, specifically seventeen when the UDP payload size is set to 512 bytes. The core technical issue stems from how the application handles resource allocation and memory management during the parsing of these specific option structures within its debug or non-optimized build configurations. Unlike standard release builds which may include bounds checking or optimized error handling routines that prevent such conditions, debugging builds often disable certain optimizations to facilitate easier tracing and logging, inadvertently exposing underlying logic errors in how variable lengths and array indices are managed when processing multiple DNS options of this type.
From a technical perspective, the exploitation mechanism relies on triggering an out-of-bounds access or a memory corruption event within the serve child process. When NSD receives the crafted packet with seventeen Cookie options under the specified UDP size constraint, it attempts to allocate resources or iterate through data structures in a manner that exceeds expected limits for that specific build configuration. This results in a segmentation fault or an unhandled exception that terminates the worker process immediately. The use of DNS Cookies is particularly relevant here as they are designed to mitigate reflection attacks and verify client reachability, but their implementation within NSD contains a logic flaw where the count threshold interacts poorly with the memory allocation strategy for debug builds. This specific interaction highlights how build configurations can significantly alter the attack surface of an application, making vulnerabilities present in development environments potentially exploitable if such binaries are deployed or misconfigured in production-like settings that retain debugging symbols or non-optimized code paths.
The operational impact of this vulnerability is severe, primarily manifesting as a denial-of-service against DNS resolution services provided by affected NSD instances. By continuously sending these crafted packets to crash the serve children, an attacker can effectively halt the server's ability to process legitimate queries. Since each crashed child must be restarted or results in a temporary unavailability of that worker thread, persistent attacks can lead to significant service degradation. If the attacker is positioned sufficiently close to the target network infrastructure, they can saturate the available processing capacity with crash-inducing packets, thereby denying all DNS services to downstream clients relying on this resolver. This not only disrupts normal internet connectivity for users but also potentially impacts internal network operations that depend on reliable name resolution for authentication, service discovery, and application functionality.
This vulnerability aligns with CWE-401, which describes a missing release of memory or other resource after it is no longer needed, although in this context, the immediate crash suggests more severe issues like CWE-787: Out-of-bounds Write or CWE-125: Out-of-bounds Read depending on the exact nature of the memory corruption. Furthermore, from an offensive security framework perspective, such behavior falls under MITRE ATT&CK technique T1499: Endpoint Denial of Service, specifically involving resource exhaustion through application crashes rather than simple network flooding. The ability to remotely trigger a process crash via crafted protocol messages is characteristic of many high-severity vulnerabilities in network daemons, emphasizing the critical importance of rigorous input validation and robust error handling across all build configurations.
To mitigate this risk, organizations running NSD must ensure that they are using release-optimized builds rather than debugging or non-release variants for any production-facing services. Debugging symbols and unoptimized code paths should never be exposed to external networks as they often contain latent logic errors not present in optimized binaries due to compiler transformations and additional safety checks enabled by default optimization flags. Additionally, implementing network-level filtering rules can help mitigate the impact of such attacks by rate-limiting DNS queries from single sources or blocking suspiciously large numbers of Cookie options within a short timeframe. Regular updates to NSD are essential as developers typically patch these types of memory management flaws in subsequent releases. Security teams should also monitor for unusual spikes in process restarts or core dumps associated with the nsd service, which may indicate an ongoing exploitation attempt targeting this specific vulnerability vector.