CVE-2026-14586 in Unbound
Summary
by MITRE • 07/22/2026
In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, in DNS-over-QUIC environments, with high concurrency and under pressure, an assertion in libngtcp2 about monotonic timestamps could trigger and result in server termination and thus denial of service. When interfacing with libngtcp2, for DNS-over-QUIC support in Unbound, it is expected to use monotonic time. Unbound was using realtime instead, and in DoQ environments with high concurrency and under pressure, an assert in libngtcp2 for the quic timestamp would trigger and terminate the server.This vulnerability needs Unbound to be compiled with DoQ support ('--with-libngtcp2') and the 'quic-port' to be configured for the listening interfaces.
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Analysis
by VulDB Data Team • 07/22/2026
This vulnerability affects NLnet Labs Unbound DNS resolver versions 1.22.0 through 1.25.1 when operating in DNS-over-QUIC environments with high concurrent traffic loads. The issue stems from a fundamental timing inconsistency between Unbound's implementation and the underlying libngtcp2 library that handles QUIC protocol operations. When Unbound is compiled with DNS-over-QUIC support using the '--with-libngtcp2' configuration option and has the 'quic-port' parameter configured for listening interfaces, the resolver encounters a critical assertion failure in libngtcp2's timestamp handling mechanism.
The technical flaw manifests when Unbound attempts to interface with libngtcp2 for QUIC operations, specifically regarding timestamp management. libngtcp2 expects monotonic time values that strictly increase over time, which is a standard requirement for QUIC protocol implementations to maintain proper connection state and packet ordering. However, Unbound was incorrectly utilizing realtime timestamps instead of monotonic ones during high-concurrency scenarios. This mismatch becomes particularly problematic under system pressure and heavy load conditions where the timing discrepancies become more pronounced, triggering the assertion failure within libngtcp2's internal validation logic.
The operational impact of this vulnerability is severe as it results in immediate server termination and complete denial of service for DNS resolution requests. When the assertion fails during active QUIC connections, Unbound crashes and must be restarted manually or through automated recovery mechanisms, disrupting network services and potentially affecting critical infrastructure relying on DNS resolution. This vulnerability represents a classic case of improper time handling that can be exploited to cause availability issues in high-load environments where multiple concurrent QUIC connections are established simultaneously.
From a cybersecurity perspective, this vulnerability aligns with CWE-1289 which addresses improper handling of timing information and CWE-362 which covers concurrent execution usage issues. The issue also maps to ATT&CK technique T1499.004 for network denial of service and T1566.002 for phishing through DNS manipulation. Organizations deploying Unbound with DoQ support in production environments must understand that this vulnerability specifically requires both compilation with libngtcp2 support and explicit configuration of QUIC listening ports to manifest. The vulnerability demonstrates how seemingly minor implementation details in system libraries can cascade into critical availability issues when multiple components interact under stress conditions.
The recommended mitigations include upgrading to Unbound versions beyond 1.25.1 where this timing issue has been resolved, ensuring proper compilation flags are used when building with DoQ support, and implementing robust monitoring for server crashes or restarts in environments where QUIC is enabled. Network administrators should also consider implementing load balancing strategies and traffic shaping to reduce the likelihood of triggering the high-concurrency conditions that expose this vulnerability. Additionally, deploying intrusion detection systems that can monitor for abnormal server termination patterns may help detect exploitation attempts targeting this specific denial of service vector.