NULL pointer dereference in Unbound - CVE-2026-55717
Published: July 23, 2026
Vulnerability identifier: #VU139223
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-55717
CWE-ID: CWE-476
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 cause a denial of service.
The vulnerability exists due to a NULL pointer dereference in the serve-expired-client-timeout callback when processing delayed responses involving a respip-generated CNAME alias. A remote attacker can control a delegated domain and delay an answer so the vulnerable code path is reached to cause a denial of service.
The issue occurs when 'serve-expired: yes' is used together with a 'response-ip: redirect' and 'response-ip-data: CNAME' rule, or the equivalent RPZ cname override configuration.
How to mitigate CVE-2026-55717
Install security update from vendor's website.