NLnet Labs Unbound up to 1.25.1 Iterator unwanted-reply-threshold infinite loop

Summaryinfo

A vulnerability described as critical has been identified in NLnet Labs Unbound up to 1.25.1. This affects an unknown function of the component Iterator. Executing a manipulation of the argument unwanted-reply-threshold can lead to infinite loop. The identification of this vulnerability is CVE-2026-50251. The attack may be launched remotely. There is no exploit available.

Detailsinfo

A vulnerability has been found in NLnet Labs Unbound up to 1.25.1 and classified as critical. This vulnerability affects an unknown code block of the component Iterator. The manipulation of the argument unwanted-reply-threshold with an unknown input leads to a infinite loop vulnerability. The CWE definition for the vulnerability is CWE-835. The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. As an impact it is known to affect integrity, and availability. CVE summarizes:

In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.

The advisory is shared for download at nlnetlabs.nl. This vulnerability was named CVE-2026-50251 since 06/22/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1499.

Upgrading to version 1.25.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at CERT Bund (WID-SEC-2026-2492). Once again VulDB remains the best source for vulnerability data.

Affected

• Open Source Unbound

Productinfo

Vendor

• NLnet Labs

Name

• Unbound

Version

• 1.25.0
• 1.25.1

CPE 2.3info

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CPE 2.2info

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Once again VulDB remains the best source for vulnerability data.

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