NULL pointer dereference in Nlnet Labs Unbound
CVE-2026-55717
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.002 (13.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 14th percentile — 13.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000
Affected products
- Nlnet Labs Unbound — versions 1.10.0
- Nlnetlabs Unbound
Weakness classification (CWE)
References
- www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt (vendor-advisory, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-55717?
- CVE-2026-55717 is a medium-severity vulnerability in Nlnet Labs Unbound, classified under NULL Pointer Dereference. CVSS score: 5.9/10. Published 2026-07-22.
- How severe is CVE-2026-55717?
- Medium severity. CVSS v3 base score is 5.9 out of 10.