Resource exhaustion in Unbound - CVE-2026-42923
Published: May 20, 2026
Vulnerability identifier: #VU131928
CSH Severity: Medium
CVSS v4.0: 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/E:U/U:Green
CVE-ID: CVE-2026-42923
CWE-ID: CWE-400
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 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.
How to mitigate CVE-2026-42923
Install security update from vendor's website.