Heap-based buffer overflow in Unbound - CVE-2026-40691
Published: July 23, 2026
Vulnerability identifier: #VU139239
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-2026-40691
CWE-ID: CWE-122
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 heap-based buffer overflow in the DNSCrypt packet reading and reply encryption procedure when processing a crafted DNSCrypt query over TCP. A remote attacker can send a specially crafted DNSCrypt query to cause a denial of service.
Only installations compiled with DNSCrypt support and configured with the dnscrypt feature enabled for listening interfaces are vulnerable.
How to mitigate CVE-2026-40691
Install security update from vendor's website.