CVE-2026-55717: 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
Other sources
In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'aliasrrset' but not 'partialrep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 1.25.2-1 - Upgrade
Upgrade
NLnet Labs Unboundto a version that resolves this vulnerability.Fixed in 1.25.2 - Configuration
To avoid the vulnerable path, do not set 'serve-expired: yes' together with a 'response-ip: <net> redirect' / 'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent) on vulnerable versions 1.10.0 through 1.25.1.
NLnet Labs Unbound serve-expired = no
Event History
Frequently Asked Questions
What is the severity of CVE-2026-55717?
CVE-2026-55717 has a medium severity score of 5.9.
How do I fix CVE-2026-55717?
To mitigate CVE-2026-55717, update Unbound to a version higher than 1.25.1.
What systems are affected by CVE-2026-55717?
CVE-2026-55717 affects NLnet Labs Unbound versions 1.10.0 through 1.25.1.
What can exploit CVE-2026-55717?
CVE-2026-55717 can be exploited by a remote client controlling any delegated domain.
What type of vulnerability is CVE-2026-55717 classified as?
CVE-2026-55717 is classified as a Null Pointer Dereference vulnerability.