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Unbound CVE-2026-40691

| EUVDEUVD-2026-47664 HIGH
Heap-based Buffer Overflow (CWE-122)
2026-07-22 NLnet Labs GHSA-vxvj-6rhp-3pqq
Buffer Overflow Denial Of Service Heap Overflow Red Hat Suse Unbound Nlnet Labs
7.5
CVSS 3.1 · Vendor: NLnet Labs
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Vulnerability Assessment

Weakness
CWE
CWE-122
ASVS 5.0
V15 - Secure Coding and Architecture Heap buffer bounds checking
MITRE ATT&CK

1 technique across 1 tactic: Execution

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.5% exploitation probability (EPSS)

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AnalysisAIv1 – Jul 22, 2026

Remote denial-of-service in NLnet Labs Unbound 1.9.0 through 1.25.1 allows an unauthenticated attacker to crash the recursive DNS resolver by sending a single malicious encrypted DNSCrypt query over TCP. The TCP in-place encryption routine omits the reply-length clamp that protects the UDP path, so an oversized reply is shifted 48 bytes forward past the end of a heap allocation sized to 'msg-buffer-size' (CWE-122). Only builds compiled with '--enable-dnscrypt' and an active 'dnscrypt:' clause are affected; there is no public exploit identified at time of analysis and the issue is not listed in CISA KEV.

Affected VersionsENISA

Unbound 1.9.0 <1.25.2

Affected Packages

euvd unbound fix: 1.25.2

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in euvd

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

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  • Exploit and PoC references, threat intelligence from 22 sources
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Severity by source

Vendor (NLnet Labs) PRIMARY 7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
SUSE HIGH
Red Hat 7.5 HIGH

Primary rating from Vendor (NLnet Labs).

CVSS VectorVendor: NLnet Labs

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

2
Patch available
Jul 22, 2026 - 16:18 EUVD
CVE Published
Jul 22, 2026 - 13:04 cve.org
HIGH 7.5

DescriptionCVE.org

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.

Executive ViewAI

Why It Matters

NLnet Labs Unbound DNS resolver versions 1.9.0 through 1.25.1 contain a HIGH severity remote denial-of-service vulnerability; no public exploit identified at time of analysis. Organizations running Unbound with DNSCrypt support enabled face DNS availability disruption risk, potentially compromising recursive resolver services for dependent infrastructure.

Recommended Action

Within 24 hours, identify and audit all Unbound deployments compiled with '--enable-dnscrypt' running versions 1.9.0-1.25.1; determine which systems have active 'dnscrypt:' clauses in configuration. Within 7 days, disable DNSCrypt on all systems where operationally feasible, or restrict DNSCrypt-accepting interfaces to trusted resolver clients via firewall rules; monitor NLnet Labs repository and vendor advisories for patch availability. Within 30 days, apply vendor-released patches to all affected systems immediately upon release, or confirm permanent DNSCrypt disablement as an alternative mitigation.

Business Impact: High
Compensating Controls

Disable DNSCrypt support if not operationally required; restrict inbound DNSCrypt traffic (typically UDP/TCP port 443) to trusted sources using network-level firewall rules; implement monitoring and alerting for Unbound process crashes and resource exhaustion; maintain DNS failover capacity using unaffected resolvers; establish regular patching cadence to apply fix immediately when released by NLnet Labs.

Threat Intelligence Overview

EPSS Score
0.5%
Top 63% exploitable
Data Quality
6 sources
Confidence: High
EUVD NVD CVE.org EPSS Vulnrichment MSRC Vendor VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.5%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision
Scheduled
Exploitation
None
System Exposure· derived
Open
Automatable
Yes
Human Impact
High
Technical Impact
Partial

Priority Score

38
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +38
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Buffer Overflow HIGH

A buffer overflow occurs when a program writes more data to a memory buffer than it was allocated to hold, causing the excess data to spill into adjacent memory regions.

Heap Overflow HIGH

A heap overflow occurs when a program writes data beyond the bounds of a buffer allocated in the heap memory region.

Denial of Service MEDIUM

Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.

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Same weakness CWE-122 – Heap-based Buffer Overflow

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
openSUSE Tumbleweed Fixed
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Affected
SUSE Linux Enterprise Micro 5.5 Affected
SUSE Linux Enterprise Module for Basesystem 15 SP7 Affected

References

Advisories & patches
ADVISORY https://access.redhat.com/errata/RHSA-2026:43588 ADVISORY https://vuldb.com/vuln/382320 ADVISORY https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt

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