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CVE-2026-56444 is a denial-of-service vulnerability in NLnet Labs Unbound affecting versions 1.20.0 through 1.25.1. When Unbound is configured with serve-expired enabled and with a non-default timeout relationship where serve-expired-client-timeout is greater than discard-timeout and discard-timeout is greater than 0, the discard-timeout branch in the serve-expired logic can drop an aged client reply without correctly decrementing the internal counter that tracks reply addresses for a duplicate in-flight query. Equivalent branches outside the serve-expired path perform the required decrement, but this branch does not. As a result, the counter can accumulate until it reaches its maximum limit, after which additional clients associated with duplicate in-flight queries are silently dropped. An attacker can trigger this condition by issuing queries through the resolver for a client-controlled authoritative zone that responds slowly on demand, driving the counter to exhaustion and degrading resolver availability.
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No public exploit code observed for this vulnerability.
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Cross-references every affected SKU, including bundled OEM variants.
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