Stack-based buffer overflow in Unbound - CVE-2026-55973
Published: July 23, 2026
Vulnerability identifier: #VU139222
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-2026-55973
CWE-ID: CWE-121
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 a stack-based buffer overflow in find_closest_of_type() when processing a crafted upstream DNS response containing an EDNS Report-Channel option. A remote attacker can send a specially crafted upstream response from a delegated zone they control to cause a denial of service.
The issue is reachable only when 'dns-error-reporting: yes' is enabled.
Affected software
Unbound
Red Hat Enterprise Linux for IBM z Systems
Red Hat Enterprise Linux for Power, little endian
Red Hat Enterprise Linux for ARM 64
Red Hat Enterprise Linux for x86_64
Fedora
unbound-debugsource (Red Hat package)
unbound-devel (Red Hat package)
unbound (Red Hat package)
unbound
Red Hat Enterprise Linux for IBM z Systems
Red Hat Enterprise Linux for Power, little endian
Red Hat Enterprise Linux for ARM 64
Red Hat Enterprise Linux for x86_64
Fedora
unbound-debugsource (Red Hat package)
unbound-devel (Red Hat package)
unbound (Red Hat package)
unbound
How to mitigate CVE-2026-55973
Install security update from vendor's website.
Unbound - update to 1.25.2
unbound-debugsource (Red Hat package) - update to 1.24.2-3.el9_8.4
unbound-devel (Red Hat package) - update to 1.24.2-3.el9_8.4
unbound (Red Hat package) - update to 1.24.2-7.el10_2.4
unbound - update to 1.25.2-1.fc43
unbound-debugsource (Red Hat package) - update to 1.24.2-3.el9_8.4
unbound-devel (Red Hat package) - update to 1.24.2-3.el9_8.4
unbound (Red Hat package) - update to 1.24.2-7.el10_2.4
unbound - update to 1.25.2-1.fc43