Copyright © 2012 Red Hat, Inc. All rights reserved. Moderate 2026-09-16T08:30:04 unbound: Unbound: Denial of Service via continuous queries on TCP/DoT connection 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L CWE-1050

In NLnetLabs Unbound up to and including 1.26.0, a degradation of service vulnerability is present in the TCP/DoT reading procedure where there is no limit on consecutive reads. A malicious actor that can stream and sustain a rate of distinct uncached names over the TCP/DoT connection, monopolizes a single worker's entire event loop for as long as its writes stay ahead of the drain.

A flaw was found in Unbound. A malicious actor can exploit a vulnerability in the TCP/DoT reading procedure, which lacks a limit on consecutive reads. By continuously streaming distinct uncached names over a TCP/DoT connection, an attacker can monopolize a worker's event loop. This can lead to a degradation of service, effectively causing a Denial of Service (DoS) for legitimate users.

This Moderate severity flaw in Unbound allows a remote attacker to cause a denial of service by continuously sending distinct, uncached DNS queries over a TCP or DNS-over-TLS (DoT) connection. This can monopolize a single worker's event loop, leading to service degradation for legitimate users. The impact is limited to availability and requires sustained malicious traffic. To mitigate this issue, restrict network access to the Unbound DNS resolver to only trusted clients and networks. This can be achieved by configuring firewall rules to limit inbound connections to the Unbound service ports (e.g., TCP port 53 for DNS over TCP, TCP port 853 for DNS over TLS). For example, using `firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="<TRUSTED_NETWORK>" port port="53" protocol="tcp" accept'` and `firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="<TRUSTED_NETWORK>" port port="853" protocol="tcp" accept'`. After adding rules, apply them with `firewall-cmd --reload`. Replace `<TRUSTED_NETWORK>` with the IP address or network range of trusted clients. Reloading firewall rules may temporarily interrupt network connections. Red Hat Hardened Images 2026-09-17T00:00:00Z RHSA-2026:68590 unbound-main-1.26.1-1.hum1 Red Hat Enterprise Linux 10 Fix deferred unbound Red Hat Enterprise Linux 6 Out of support scope unbound Red Hat Enterprise Linux 7 Fix deferred unbound Red Hat Enterprise Linux 8 Fix deferred unbound Red Hat Enterprise Linux 9 Fix deferred unbound Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-8 Red Hat OpenShift Container Platform 4 Fix deferred openshift/ose-rhel-coreos-9 https://www.cve.org/CVERecord?id=CVE-2026-80225 https://nvd.nist.gov/vuln/detail/CVE-2026-80225 https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-80225.txt