CVE-2026-40691Network attack vector

Resolver crash via unchecked TCP DNSCrypt reply

Published Jul 22, 2026 · Updated Jul 22, 2026

Heap-based buffer overflow in NLnet Labs Unbound 1.9.0 through 1.25.1 allows remote attackers to crash the resolver via DNSCrypt over TCP. The reply-encryption routine shifts replies larger than 65504 bytes forward by 48 bytes without checking the TCP destination buffer's capacity, writing beyond its heap allocation. Exploitation requires a build compiled with --enable-dnscrypt and an enabled dnscrypt clause on a listening interface; the demonstrated consequence is a resolver-process crash.

CVSS severity7.5
High
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap-based buffer overflow in NLnet Labs Unbound 1.9.0 through 1.25.1 allows remote attackers to crash the resolver via DNSCrypt over TCP. The reply-encryption routine shifts replies larger than 65504 bytes forward by 48 bytes without checking the TCP destination buffer's capacity, writing beyond its heap allocation. Exploitation requires a build compiled with --enable-dnscrypt and an enabled dnscrypt clause on a listening interface; the demonstrated consequence is a resolver-process crash.

The record

CVE
CVE-2026-40691
Published
Jul 22, 2026
Updated
Jul 22, 2026
Vendor
NLnet Labs
Product
Unbound
Classifications
CWE-787, CWE-122, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 22, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 22, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=1.9.0 <1.25.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

NLnet Labs · Unboundversion=1.9.0 <1.25.2

Published CVSS scores

7.5NLnet LabsCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-787Out-of-bounds Write
CWE-122Heap-based Buffer Overflow
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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