What is CVE-2026-40691?

CVE-2026-40691 is a high-severity vulnerability in Nlnet Labs Unbound, classified under Heap-based Buffer Overflow. CVSS score: 7.5/10. Published 2026-07-22.

How severe is CVE-2026-40691?

High severity. CVSS v3 base score is 7.5 out of 10.

Buffer overflow in Nlnet Labs Unbound

CVE-2026-40691

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP…

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

Nlnet Labs Unbound — versions 1.9.0

Weakness classification (CWE)

CWE-122 — Heap-based Buffer Overflow
CWE-787 — Out-of-bounds Write

References

sep@nlnetlabs.nl (vendor-advisory)

Frequently asked questions

What is CVE-2026-40691?: CVE-2026-40691 is a high-severity vulnerability in Nlnet Labs Unbound, classified under Heap-based Buffer Overflow. CVSS score: 7.5/10. Published 2026-07-22.
How severe is CVE-2026-40691?: High severity. CVSS v3 base score is 7.5 out of 10.