High [CVE-2026-55973] Denial of Service via malformed EDNS Report-Channel option

In NLnet Labs Unbound 1.23.0 up to and including 1.25.1, when 'dns-error-reporting: yes' is set, the EDNS Report-Channel option (code 18) from the last upstream response is read and uses the option's length as the length of the agent domain.

When a domain name check is performed on the agent domain, the returned lenght is not used and if the agent domain is followed by garbage, those bytes are moved onto the tail of the synthetic '_er.' report query name.

That query name is later used in the iterator via a subquery to send out the DNS Error Report and when Unbound tries to walk that query name during 'find_closest_of_type()', it strips labels using the query name length rather than stopping at the embedded root, walks one byte past it, and feeds the first garbage byte to 'dname_query_hash()' as a label length writing over the stack variable 'labuf'.

One ordinary upstream response from a delegated zone the attacker controls is sufficient to terminate the daemon. A flaw was found in Unbound.

When the 'dns-error-reporting: yes' option is enabled, a remote attacker can send a specially crafted DNS response containing a malformed EDNS Report-Channel option from a delegated zone they control. This can lead to a stack variable overwrite, causing the Unbound daemon to terminate.

This vulnerability results in a denial of service.