CVE-2026-40691 Vulnerability Analysis & Exploit Details

CVE-2026-40691
Vulnerability Scoring

7.5

/10

Very High Risk

Highly exploitable, CVE-2026-40691 poses a critical security risk that could lead to severe breaches.

Attack Complexity Details

Attack Complexity: Low
Exploits can be performed without significant complexity or special conditions.
Attack Vector: Network
Vulnerability is exploitable over a network without physical access.
Privileges Required: None
No privileges are required for exploitation.
Scope: Unchanged
Exploit remains within the originally vulnerable component.
User Interaction: None
No user interaction is necessary for exploitation.

CVE-2026-40691 Details

Status: Analyzed

Last updated: 🕜 24 Jul 2026, 13:57 UTC
Originally published on: 🕑 22 Jul 2026, 14:17 UTC

Time between publication and last update: 1 days

CVSS Release: version 3

CVSS3 Source

sep@nlnetlabs.nl

CVSS3 Type

Secondary

CVSS3 Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVE-2026-40691 Vulnerability Summary

CVE-2026-40691: 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.

Assessing the Risk of CVE-2026-40691

Access Complexity Graph

The exploitability of CVE-2026-40691 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).

Exploitability Analysis for CVE-2026-40691

With low attack complexity and no required privileges, CVE-2026-40691 is an easy target for cybercriminals. Organizations should prioritize immediate mitigation measures to prevent unauthorized access and data breaches.

Understanding AC and PR

A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.

Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.

Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.

CVSS Score Breakdown Chart

Above is the CVSS Sub-score Breakdown for CVE-2026-40691, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.

CIA Impact Analysis

Below is the Impact Analysis for CVE-2026-40691, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.

Confidentiality: None
CVE-2026-40691 has no significant impact on data confidentiality.
Integrity: None
CVE-2026-40691 poses no threat to data integrity.
Availability: High
CVE-2026-40691 can disrupt system operations, potentially causing complete denial of service (DoS).

CVE-2026-40691 References

External References

https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt

CWE Common Weakness Enumeration

CWE-787

Vulnerable Configurations

cpe:2.3:a:nlnetlabs:unbound:1.9.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.2:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.2:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.2:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.2:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.2:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.2:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.3:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.3:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.3:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.3:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.3:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.3:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.4:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.4:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.5:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.5:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.6:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.6:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.6:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.9.6:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.0:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.0:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.10.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.11.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.11.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.11.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.11.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.12.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.12.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.12.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.12.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc3:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc3:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc4:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.0:rc4:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.1:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.1:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.1:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.1:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.1:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.1:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.2:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.2:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.2:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.13.2:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.14.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.14.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.14.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.14.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.15.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.15.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.15.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.15.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.1:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.1:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.1:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.1:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.2:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.2:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.3:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.16.3:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.1:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.1:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.1:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.1:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.1:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.17.1:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.18.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.18.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.18.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.18.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.2:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.2:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.3:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.3:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.3:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.3:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.3:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.19.3:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.20.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.20.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.20.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.20.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.21.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.21.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.21.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.21.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.21.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.21.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.22.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.22.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.22.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.22.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.0:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.0:rc2:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.23.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.2:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.24.2:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.25.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.25.0:-:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.25.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.25.0:rc1:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.25.1:*:*:*:*:*:*:*
cpe:2.3:a:nlnetlabs:unbound:1.25.1:*:*:*:*:*:*:*

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