Low [CVE-2026-56444] Denial of Service due to incorrect client reply accounting with specific serve-expired configuration
This low-severity Red Hat Linux advisory covers CVE-2026-56444 affecting Red Hat Hardened Images, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9.
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Summary
In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query.
Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service.
A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. A Denial of Service (DoS) flaw exists in Unbound under specific serve-expired and discard-timeout configurations.
By sending crafted queries, a remote attacker can exhaust an internal counter, causing the server to drop new legitimate client queries and degrade resolution services. This Low-impact flaw in Unbound can lead to a denial of service.
While a remote attacker could exhaust the reply address counter with specially crafted queries, Red Hat products are not affected by default.
- 1.20.0
- 1.25.1
Official advisory · high-confidence parse· fetched 7 hours ago·verify at source
- unbound-main-1.25.2-0.1.hum1
- RHSA-2026:43588
Official advisory · high-confidence parse· fetched 7 hours ago·verify at source
Mitigation checklist
- To mitigate this issue, ensure that Unbound is configured with `serve-expired-client-timeout` not greater than `discard-timeout` when `serve-expired: yes` is enabled. Alternatively, disable the `serve-expired` feature if it is not required. Red Hat's default configurations for Unbound prevent this vulnerability from being exploited.
Official advisory · high-confidence parse· fetched 7 hours ago·verify at source
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