Use After Free in Nlnet Labs Unbound
CVE-2026-52863
In NLnet Labs Unbound 1.25.0 up to and including 1.25.1, a fix that makes the 'respip' and 'dns64' modules work together, creates a shallow copy of the view name in effect that could lead to memory corruption if the owner of the original view name is jostled out when Unbound is under pressure. Unbound needs to be configured with one of 'respip'/'rpz' modules, together with a module that can attach subqueries (respip CNAME redirection, dns64, subnetcache) and a configured 'access-control-view' while Unbound is under pressure so that joslte logic kicks in and starts dropping slow queries. The subquery is getting a shallow copy of the view name and if the super query which owns the view name is jostled out, memory corruption can occur. Likelihood of a crash is low, since it relies heavily on the underlying memory allocator and the memory layout. Debug memory builds (e.g., ASAN) that catch the free terminate the server.
Vulnerability class: Use-After-Free
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: 0.004 (28.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 28.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001
Affected products
- Nlnet Labs Unbound — versions 1.25.0
- Nlnetlabs Unbound
Weakness classification (CWE)
References
- www.nlnetlabs.nl/downloads/unbound/CVE-2026-52863.txt (vendor-advisory, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-52863?
- CVE-2026-52863 is a medium-severity vulnerability in Nlnet Labs Unbound, classified under Use After Free. CVSS score: 5.9/10. Published 2026-07-22.
- How severe is CVE-2026-52863?
- Medium severity. CVSS v3 base score is 5.9 out of 10.