CVE-2026-55717 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
Remote attackers can crash the Unbound DNS server by exploiting a NULL pointer dereference in the serve-expired and response-ip CNAME redirect logic.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55717?
CVE-2026-55717 is a medium-severity vulnerability in Nlnetlabs Unbound, published on 22 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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 'alias_rrset' but not 'partial_rep'. 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.
Weakness: CWE-476
Which versions are affected by CVE-2026-55717?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-55717?
Upgrade to a fixed version.
Is CVE-2026-55717 being exploited?
There is no known exploitation of CVE-2026-55717 in the wild and no public exploit code.
Other vulnerabilities in Nlnetlabs Unbound
- CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
- CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation
- CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
- CVE-2026-42959 Crash during DNSSEC validation of malicious content
- CVE-2026-82717 CNAME synthesis could lead to heap corruption
Sources
Also known as ALPINE-CVE-2026-55717, AZL-92946, DEBIAN-CVE-2026-55717, ECHO-cac0-04fd-234b, GHSA-9vcf-h83p-jghm, UBUNTU-CVE-2026-55717.