Improper Initialization in Unbound - CVE-2026-55990
Published: July 23, 2026
Vulnerability identifier: #VU139221
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-2026-55990
CWE-ID: CWE-665
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 initialization in the DNSCrypt certificate array handling in Unbound when processing a UDP datagram sent to dnscrypt-port under a faulty DNSCrypt configuration with more certificate files than matching secret key files. A remote attacker can send a specially crafted UDP datagram whose first 8 bytes are 0xdb to cause a denial of service.
Only installations compiled with DNSCrypt support are affected, and exploitation requires a silent faulty configuration in which the number of dnscrypt-provider-cert files exceeds the number of matching dnscrypt-secret-key files.
How to mitigate CVE-2026-55990
Install security update from vendor's website.