In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can distinguish whether a candidate port matches the current upstream source port by checking whether an existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to probe the upstream source port. Although the attack is not feasible on default configurations and the suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the attacker can keep a concurrent query window alive, the attacker can first filter the source port and then perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be
    exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-
    like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does
    not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count
    reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache
    clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can
    distinguish whether a candidate port matches the current upstream source port by checking whether an
    existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to
    probe the upstream source port. Although the attack is not feasible on default configurations and the
    suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the
    attacker can keep a concurrent query window alive, the attacker can first filter the source port and then
    perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.
    (CVE-2026-109125)

The version of FreeBSD installed on the remote host is prior to tested version. It is, therefore, affected by multiple vulnerabilities as referenced in the 0be28489-cd2f-11f1-8135-8447094a420f advisory.

    NLNetLabs reports:
    This release consolidates security fixes for issues reported over     a period of time.

Tenable has extracted the preceding description block directly from the FreeBSD security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - In NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be     exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-     like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does     not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count     reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache     clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can     distinguish whether a candidate port matches the current upstream source port by checking whether an     existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to     probe the upstream source port. Although the attack is not feasible on default configurations and the     suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the     attacker can keep a concurrent query window alive, the attacker can first filter the source port and then     perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.
    (CVE-2026-109125)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Type	Severity
477069	Echo: unbound: security update to 1.26.2	Tenable Cloud Security	Echo Local Security Checks	Local	medium
477069	Echo: unbound: security update to 1.26.2	Tenable Self-Hosted Container Security	Echo Local Security Checks	Local	medium
370456	FreeBSD : Unbound -- Multiple vulnerabilities (0be28489-cd2f-11f1-8135-8447094a420f)	Nessus	FreeBSD Local Security Checks	Local	medium
370061	Linux Distros Unpatched Vulnerability : CVE-2026-109125	Nessus	Misc.	Local	medium

Detections

Analytics

CVE-2026-109125

medium

Tenable Plugins

medium

medium

medium

medium