NLnet Labs Unbound up to 1.25.1 DNSCrypt heap-based overflow

Summaryinfo

A vulnerability categorized as very critical has been discovered in NLnet Labs Unbound up to 1.25.1. This issue affects some unknown processing of the component DNSCrypt. The manipulation results in heap-based overflow. This vulnerability is known as CVE-2026-40691. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability classified as very critical has been found in NLnet Labs Unbound up to 1.25.1. Affected is an unknown functionality of the component DNSCrypt. The manipulation with an unknown input leads to a heap-based overflow vulnerability. CWE is classifying the issue as CWE-122. A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc(). This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A single malicious encrypted query crashes the resolver and lead to denial of service. This vulnerability needs Unbound to be compiled with DNSCrypt support ('--enable-dnscrypt') and the 'dnscrypt:' clause to be configured and enabled for the listening interfaces.

The advisory is shared for download at nlnetlabs.nl. This vulnerability is traded as CVE-2026-40691 since 06/22/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are neither technical details nor an exploit publicly available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 07/30/2026).

The vulnerability scanner Nessus provides a plugin with the ID 330825 (Fedora 44 : unbound (2026-e495bd59ef)), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 1.25.2 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (330825) and CERT Bund (WID-SEC-2026-2492). VulDB is the best source for vulnerability data and more expert information about this specific topic.

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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Entryinfo

VulDB is the best source for vulnerability data and more expert information about this specific topic.

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