Resource exhaustion in Unbound - CVE-2026-42923

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Main Vulnerability Database Vulnerabilities #VU131928

Resource exhaustion in Unbound - CVE-2026-42923

Published: May 20, 2026


Vulnerability identifier: #VU131928
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-2026-42923
CWE-ID: CWE-400
Exploitation vector: Remote access
Exploit availability: No public exploit available

Vulnerability details

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

The vulnerability exists due to improper resource management in Unbound's DNSSEC validator negative cache handling for DS records when processing DNSSEC-signed zones with NSEC3 records using high iteration counts for child delegations. A remote attacker can control a DNSSEC-signed zone and query a vulnerable Unbound resolver to cause a denial of service.

A global lock for the negative cache may be held for the duration of the hashing, blocking other threads that need to consult the negative cache.


Affected software

Unbound
SUSE Linux Enterprise Server 15 SP6
SUSE Linux Enterprise Server 15 SP5
SUSE Linux Enterprise Server 15 SP4
Debian Linux
SUSE Linux Enterprise Server for SAP Applications 15
SUSE Linux Enterprise High Performance Computing 15
SUSE Linux Enterprise Server 15
SUSE Linux Enterprise High Performance Computing ESPOS 15
SUSE Linux Enterprise High Performance Computing LTSS 15
SUSE Linux Enterprise Desktop 15
SUSE Linux Enterprise Real Time 15
SUSE Linux Enterprise Micro
FreeBSD
Ubuntu
SUSE Package Hub 15
Basesystem Module
openSUSE Leap
openEuler
Anolis OS
Fedora
LANTIME Operating System Firmware (LTOS)
unbound (Ubuntu package)
unbound-help
unbound-devel
unbound-debugsource
unbound-debuginfo
python3-unbound
unbound
unbound-libs
unbound-anchor
unbound-utils
unbound-anchor-debuginfo
libunbound8-debuginfo
libunbound8
libunbound-devel-mini-debugsource
unbound-munin
libunbound-devel-mini-debuginfo
unbound-python
libunbound-devel-mini
unbound-python-debuginfo
unbound (Debian package)

How to mitigate CVE-2026-42923

Install security update from vendor's website.

Unbound - update to 1.25.1
LANTIME Operating System Firmware (LTOS) - update to 7.10.012
unbound (Ubuntu package) - addressed in versions 1.4.22-1ubuntu4.14.04.3+esm3, 1.5.8-1ubuntu1.1+esm3, 1.6.7-1ubuntu2.6+esm4, 1.9.4-2ubuntu1.11+esm1, 1.13.1-1ubuntu5.15, 1.19.2-1ubuntu3.8, 1.22.0-2ubuntu2.3, 1.24.2-1ubuntu2.1
unbound-help - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
unbound-devel - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
unbound-debugsource - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
unbound-debuginfo - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
python3-unbound - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
unbound - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
unbound-libs - addressed in versions 1.11.0-23, 1.13.2-23, 1.17.1-17
unbound-anchor - addressed in versions 1.13.2-23, 1.17.1-17
python3-unbound - update to 1.17.1-11
unbound - update to 1.17.1-11
unbound-utils - update to 1.17.1-11
unbound-libs - update to 1.17.1-11
unbound-devel - update to 1.17.1-11
unbound-anchor - update to 1.17.1-11
unbound-utils - update to 1.17.1-17
unbound-anchor-debuginfo - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
unbound-devel - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
libunbound8-debuginfo - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
unbound-anchor - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
unbound-debugsource - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
libunbound8 - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
unbound-debuginfo - addressed in versions 1.20.0-150100.10.25.1, 1.20.0-150600.23.16.1
libunbound-devel-mini-debugsource - update to 1.20.0-150600.23.16.1
unbound-munin - update to 1.20.0-150600.23.16.1
unbound - update to 1.20.0-150600.23.16.1
libunbound-devel-mini-debuginfo - update to 1.20.0-150600.23.16.1
unbound-python - update to 1.20.0-150600.23.16.1
libunbound-devel-mini - update to 1.20.0-150600.23.16.1
unbound-python-debuginfo - update to 1.20.0-150600.23.16.1
unbound (Debian package) - update to 1.22.0-2+deb13u3
unbound - addressed in versions 1.25.1-1.fc43, 1.25.1-1.fc44

External References

Related Security Bulletins