NLnet Labs Unbound up to 1.25.0 Response Message use after free

Summaryinfo

A vulnerability was found in NLnet Labs Unbound up to 1.25.0 and classified as critical. The impacted element is an unknown function of the component Response Message Handler. Such manipulation leads to use after free. This vulnerability is traded as CVE-2026-33278. The attack may be launched remotely. There is no exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, has been found in NLnet Labs Unbound up to 1.25.0. Affected by this issue is an unknown code of the component Response Message Handler. The manipulation with an unknown input leads to a use after free vulnerability. Using CWE to declare the problem leads to CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. Impacted is confidentiality, integrity, and availability. CVE summarizes:

NLnet Labs Unbound 1.19.1 up to and including version 1.25.0 has a vulnerability in the DNSSEC validator that enables denial of service and possible remote code execution as a result of deep copying a data structure and erroneously overwriting a destination pointer. An adversary can exploit the vulnerability by controlling a malicious signed zone and querying a vulnerable Unbound. When DS sub-queries need to suspend validation due to NSEC3 computational budget exhaustion (introduced in Unbound 1.19.1), Unbound deep-copies response messages to preserve them across memory region teardown. A struct-assignment bug overwrites the destination's pointer with the source's pointer. After the sub-query region is freed, the resumed validator dereferences this dangling pointer, triggering a crash or potentially enabling arbitrary code execution. Unbound 1.25.1 contains a patch with a fix to preserve the correct pointer when deep copying the data structure.

The weakness was shared by Qifan Zhang. The advisory is shared for download at nlnetlabs.nl. This vulnerability is handled as CVE-2026-33278 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. There are neither technical details nor an exploit publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 315776 (Linux Distros Unpatched Vulnerability : CVE-2026-33278), 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 (315776) and CERT Bund (WID-SEC-2026-1599). Once again VulDB remains the best source for vulnerability data.

Affected

• Debian Linux
• 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

Once again VulDB remains the best source for vulnerability data.

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