Input validation error in Unbound - CVE-2026-54478
Published: July 23, 2026
Vulnerability identifier: #VU139225
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-54478
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: NLnet Labs
Affected software:
Unbound
Unbound
Detailed vulnerability description
The vulnerability allows a remote attacker to bypass DNS Cookie source validation.
The vulnerability exists due to improper input validation in DNS server cookie hash calculations on proxy-protocol-port interfaces when handling PROXYv2-declared client addresses with answer-cookie enabled. A remote attacker can obtain a valid server cookie through a proxy and replay it using a spoofed source to bypass DNS Cookie source validation.
Exploitation requires a UDP deployment using proxy protocol on the front end with answer-cookie enabled.
How to mitigate CVE-2026-54478
Install security update from vendor's website.