What’s Vulnerable › Vulnerabilities
CVE-2026-55717: 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
- Severity: medium (CVSS 5.9 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.2% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-07-22 · updated 2026-10-24
- Weakness: CWE-476
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.
Affected products
- NLnet Labs Unbound: from 1.10.0 before 1.25.2 — fixed in 1.25.2
How to fix
Upgrade to a fixed version: NLnet Labs Unbound 1.25.2. Check the vendor advisory for mitigations.
References
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