NLnet Labs Unbound up to 1.25.0 Hash Calculation algorithmic complexity

Summaryinfo

A vulnerability labeled as problematic has been found in NLnet Labs Unbound up to 1.25.0. This affects an unknown part of the component Hash Calculation Handler. Such manipulation leads to algorithmic complexity. This vulnerability is referenced as CVE-2026-42923. It is possible to launch the attack remotely. No exploit is available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in NLnet Labs Unbound up to 1.25.0. It has been rated as problematic. Affected by this issue is an unknown part of the component Hash Calculation Handler. The manipulation with an unknown input leads to a algorithmic complexity vulnerability. Using CWE to declare the problem leads to CWE-407. An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached. Impacted is availability. CVE summarizes:

NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability in the DNSSEC validator where the code path to consult the negative cache for DS records does not take into account the limit on NSEC3 hash calculations introduced in 1.19.1. This leads to degradation of service during the attack. An adversary that controls a DNSSEC signed zone can exploit this by signing NSEC3 records with acceptably high iterations for child delegations and querying a vulnerable Unbound. Unbound will keep performing the allowed hash calculations on the NSEC3 records and will not limit the work by the mitigation introduced in 1.19.1. As a side effect, a global lock for the negative cache will be held for the duration of the hashing, blocking other threads that need to consult the negative cache. Coordinated attacks could raise the vulnerability to denial of service. Unbound 1.25.1 contains a patch with a fix to bound the vulnerable code path with the existing limit for NSEC3 hash calculations.

The weakness was published by Qifan Zhang. The advisory is available at nlnetlabs.nl. This vulnerability is handled as CVE-2026-42923 since 05/07/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. The technical details are unknown and an exploit is not available. This vulnerability is assigned to T1499 by the MITRE ATT&CK project.

The vulnerability scanner Nessus provides a plugin with the ID 315757 (Linux Distros Unpatched Vulnerability : CVE-2026-42923), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.25.1 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (315757) and CERT Bund (WID-SEC-2026-1599). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Affected

• Red Hat Enterprise Linux
• Fedora Linux
• Ubuntu Linux
• Open Source Unbound
• cPanel cPanel/WHM

Productinfo

Vendor

• NLnet Labs

Name

• Unbound

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8
• 1.9
• 1.10
• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20
• 1.21
• 1.22
• 1.23
• 1.24
• 1.25.0

CPE 2.3info

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CPE 2.2info

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Entryinfo

If you want to get best quality of vulnerability data, you may have to visit VulDB.

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