Nlnet · Unbound · CVE-2026-55717
**Name of the Vulnerable Software and Affected Versions**
NLnet Labs Unbound versions 1.10.0 through 1.25.1
**Description**
A remote client controlling a delegated domain can cause the daemon to crash, leading to a denial of service. This occurs when the `serve-expired: yes` setting is enabled alongside a `response-ip: <net> redirect` or `response-ip-data: <net> CNAME <target>` rule, or the equivalent `rpz-cname-override`. The issue stems from the `serve-expired-client-timeout()` function, which executes a two-pass loop to follow a CNAME alias generated by response-ip. During the second pass, the function resets the `alias rrset` variable but fails to reset `partial rep`, resulting in a NULL pointer dereference. An attacker can trigger this by controlling a zone that returns an A/AAAA record within the configured response-ip/rpz subnet and delaying the response after the previous record has expired.
**Recommendations**
Update NLnet Labs Unbound to a version later than 1.25.1.
As a temporary mitigation, disable the `serve-expired: yes` setting or restrict the use of `response-ip` and `rpz-cname-override` rules.