CVE-2026-40691

Packet of death for DNSCrypt over TCP

Published Jul 22, 2026 CVSS 3.1 · 7.5 High EPSS < 1% KEV · Not listed
Description
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.

Remediation

Vendor Solution

This issue is fixed starting with version 1.25.2

Tracking

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Advisories

Source ID Title
Debian DSA Debian DSA DSA-6507-1 unbound security update
Ubuntu USN Ubuntu USN USN-8946-1 Unbound vulnerabilities
Ubuntu USN Ubuntu USN USN-8946-2 Unbound vulnerabilities
History
Type Values Removed Values Added
First Time appeared Nlnetlabs
Nlnetlabs unbound
Vendors & Products Nlnetlabs
Nlnetlabs unbound
Type Values Removed Values Added
First Time appeared Redhat
Redhat hummingbird
CPEs cpe:/a:redhat:hummingbird:1
Vendors & Products Redhat
Redhat hummingbird
References
Metrics threat_severity

None

threat_severity

Important

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Description 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.
Title Packet of death for DNSCrypt over TCP
Weaknesses CWE-122
CWE-787
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
NLnet Labs
Published
Updated
Jul 22, 2026, 2:31 PM
Reserved
Jun 22, 2026, 10:11 AM
Status
Analyzed
Published
Jul 22, 2026, 2:17 PM
Modified
Oct 24, 2026, 1:10 PM
Severity
Important
Public date
Jul 22, 2026, 12:00 AM