High [CVE-2026-32665] Denial of Service via improper validation of DNS-over-QUIC client length
This high-severity Red Hat Linux advisory covers CVE-2026-32665 affecting Red Hat Hardened Images.
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Summary
In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, when downstream DNS-over-QUIC (DoQ) is enabled, the first two bidirectional streams on a new QUIC connection (stream_id 0 and 4) bypass the per-stream 'quic-size' gate entirely, and large input buffers are allocated later, after only the 2-byte length prefix has been received from the initial streams.
As a result, a remote client can make Unbound exceed the configured 'quic-size' limit with low-cost input. Using only one connection and two streams, each sending a declared 65535-byte length prefix and then holding the streams open, a client can already trivially make Unbound roughly allocate double that amount.
This is a remote availability issue / memory-accounting bypass in the downstream DoQ implementation that leads to denial of service for new DoQ clients. This vulnerability needs Unbound to be compiled with DoQ support ('--with-libngtcp2') and the 'quic-port' to be configured for the listening interfaces.
An attacker could exploit improper validation of client-provided data length, leading to excessive memory allocation. This can cause the system to exceed its configured memory limits for QUIC connections, resulting in a Denial of Service (DoS) for new DoQ clients.
This impacts the availability of the DNS resolver for DoQ connections.
Affected product named by the advisory: Red Hat Hardened Images.
- 1.22.0
- 1.25.1
Official advisory · high-confidence parse· fetched 1 day ago·verify at source
- unbound-main-1.25.2-0.1.hum1
- RHSA-2026:43588
Official advisory · high-confidence parse· fetched 1 day ago·verify at source
Mitigation checklist
- To mitigate this issue, disable DNS-over-QUIC (DoQ) in the Unbound configuration if it is not a required service. This can typically be achieved by commenting out or removing DoQ-related configuration options in `unbound.conf`. For example, remove or comment out lines such as `do-quic: yes` or `quic-port: 853`. After modifying the configuration, restart the Unbound service for the changes to take effect: ```bash systemctl restart unbound ``` Warning: Restarting the Unbound service will temporarily interrupt DNS resolution.
Official advisory · high-confidence parse· fetched 1 day ago·verify at source
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