Heap-based buffer overflow in Unbound - CVE-2026-40691

Vulnerability identifier: #VU139239

CSH Severity: Medium

CVSS v4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-40691

CWE-ID: CWE-122

Exploitation vector: Remote access

Exploit availability: No public exploit available

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to heap-based buffer overflow in the DNSCrypt packet reading and reply encryption procedure when processing a crafted DNSCrypt query over TCP. A remote attacker can send a specially crafted DNSCrypt query to cause a denial of service.

Only installations compiled with DNSCrypt support and configured with the dnscrypt feature enabled for listening interfaces are vulnerable.

Unbound
SUSE Linux Enterprise Server 15 SP6
SUSE Linux Enterprise Server 15 SP5
SUSE Linux Enterprise Server 15 SP4
SUSE Linux Enterprise Server for SAP Applications 15
SUSE Linux Enterprise High Performance Computing 15
SUSE Linux Enterprise High Performance Computing ESPOS 15
SUSE Linux Enterprise High Performance Computing LTSS 15
SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Desktop 15
SUSE Linux Enterprise Real Time 15
SUSE Linux Enterprise Micro
SUSE Package Hub 15
Basesystem Module
openSUSE Leap
openEuler
Fedora
unbound
unbound-utils
unbound-libs
unbound-help
unbound-devel
unbound-debugsource
unbound-debuginfo
unbound-anchor
python3-unbound
libunbound8
libunbound8-debuginfo
unbound-anchor-debuginfo
unbound-munin
unbound-python
unbound-python-debuginfo
libunbound-devel-mini-debuginfo
libunbound-devel-mini-debugsource
libunbound-devel-mini

Install security update from vendor's website.

Unbound - update to 1.25.2
unbound - addressed in versions 1.17.1-19, 1.17.1-20
unbound-utils - addressed in versions 1.17.1-19, 1.17.1-20
unbound-libs - addressed in versions 1.17.1-19, 1.17.1-20
unbound-help - addressed in versions 1.17.1-19, 1.17.1-20
unbound-devel - addressed in versions 1.17.1-19, 1.17.1-20
unbound-debugsource - addressed in versions 1.17.1-19, 1.17.1-20
unbound-debuginfo - addressed in versions 1.17.1-19, 1.17.1-20
unbound-anchor - addressed in versions 1.17.1-19, 1.17.1-20
python3-unbound - addressed in versions 1.17.1-19, 1.17.1-20
unbound - update to 1.25.2-1.fc43
libunbound8 - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
unbound-devel - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
libunbound8-debuginfo - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
unbound-anchor - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
unbound-debugsource - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
unbound-anchor-debuginfo - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
unbound-debuginfo - addressed in versions 1.25.2-150100.10.28.1, 1.25.2-150600.23.19.1
unbound-munin - update to 1.25.2-150600.23.19.1
unbound-python - update to 1.25.2-150600.23.19.1
unbound - update to 1.25.2-150600.23.19.1
unbound-python-debuginfo - update to 1.25.2-150600.23.19.1
libunbound-devel-mini-debuginfo - update to 1.25.2-150600.23.19.1
libunbound-devel-mini-debugsource - update to 1.25.2-150600.23.19.1
libunbound-devel-mini - update to 1.25.2-150600.23.19.1

• https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-40691.txt
• https://nlnetlabs.nl/downloads/unbound/unbound-1.25.2.tar.gz

• Multiple vulnerabilities in Unbound
• Fedora 43 update for unbound
• openEuler 24.03 LTS SP3 update for unbound
• openEuler 24.03 LTS SP1 update for unbound
• openEuler 24.03 LTS SP4 update for unbound
• SUSE update for unbound
• SUSE update for unbound