What is CVE-2026-41637?

CVE-2026-41637 is a low-severity vulnerability in Nlnet Labs Unbound, classified under Missing Release of Resource after Effective Lifetime. CVSS score: 3.7/10. Published 2026-07-22.

How severe is CVE-2026-41637?

Low severity. CVSS v3 base score is 3.7 out of 10.

Vulnerability in Nlnet Labs Unbound

CVE-2026-41637

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, client terminated DNS-over-QUIC (DoQ) queries are not accounted properly by Unbound resulting in low-cost inflation of the waiting number of replies for already in-flight resolution queries. This results in degradation of resolution service for new clients for already in-flight queries. A malicious actor can exploit the vulnerability by issuing DoQ queries for query names that need resolution and proceeding on immediately terminating the query by one of STOP_SENDING/RESET_STREAM/CONNECTION_CLOSE QUIC frames. Those terminated DoQ queries are not properly counted for and keep inflating the number of waiting replies for in-flight queries. When the maximum is reached, it results in silent query drops for new clients needing resolution for already in-flight queries. This vulnerability needs Unbound to be compiled with DoQ support ('--with-libngtcp2') and the 'quic-port' to be configured for the listening interfaces. Additionally, a malicious actor needs access to multiple source IPs to bypass the by-default configured 'wait-limit' option.

Published 2026-07-22 · last modified 2026-07-24.

CVSS v3 metric

CVSS v3 base score 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L.

EPSS exploit prediction

EPSS: 0.004 (29.3th percentile), scored 2026-10-21.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 29.3% 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 1.22.0
Nlnetlabs Unbound

Weakness classification (CWE)

CWE-772 — Missing Release of Resource after Effective Lifetime

References

www.nlnetlabs.nl/downloads/unbound/CVE-2026-41637.txt (vendor-advisory, Vendor Advisory)

Frequently asked questions

What is CVE-2026-41637?: CVE-2026-41637 is a low-severity vulnerability in Nlnet Labs Unbound, classified under Missing Release of Resource after Effective Lifetime. CVSS score: 3.7/10. Published 2026-07-22.
How severe is CVE-2026-41637?: Low severity. CVSS v3 base score is 3.7 out of 10.