What is CVE-2026-55991?

CVE-2026-55991 is a medium-severity vulnerability in Nlnet Labs Unbound, classified under CWE-195. CVSS score: 5.9/10. Published 2026-07-22.

How severe is CVE-2026-55991?

Medium severity. CVSS v3 base score is 5.9 out of 10.

Vulnerability in Nlnet Labs Unbound

CVE-2026-55991

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connect…

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.

Affected products

Nlnet Labs Unbound — versions 1.22.0

Weakness classification (CWE)

CWE-195

References

sep@nlnetlabs.nl (vendor-advisory)

Frequently asked questions

What is CVE-2026-55991?: CVE-2026-55991 is a medium-severity vulnerability in Nlnet Labs Unbound, classified under CWE-195. CVSS score: 5.9/10. Published 2026-07-22.
How severe is CVE-2026-55991?: Medium severity. CVSS v3 base score is 5.9 out of 10.