Buffer overflow in Nlnet Labs Unbound
CVE-2026-81634
In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with a large TCP response can lead to a heap buffer overflow during the RRSet canonicalisation routine. This is caused by missing to add the first owner name into the buffer length check. A malicious actor operating a malicious name server or tampering with an incoming response to Unbound (canonicalisation happens before DNSSEC validation), can trigger the vulnerability.
Vulnerability class: Buffer Overflow
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
Affected products
Weakness classification (CWE)
References
- sep@nlnetlabs.nl (vendor-advisory)
Frequently asked questions
- What is CVE-2026-81634?
- CVE-2026-81634 is a high-severity vulnerability in Nlnet Labs Unbound, classified under Heap-based Buffer Overflow. CVSS score: 7.5/10. Published 2026-09-16.
- How severe is CVE-2026-81634?
- High severity. CVSS v3 base score is 7.5 out of 10.