High [CVE-2026-40691] Denial of Service via crafted DNSCrypt query

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size.

The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A single malicious encrypted query crashes the resolver and lead to denial of service.

This vulnerability needs Unbound to be compiled with DNSCrypt support ('--enable-dnscrypt') and the 'dnscrypt:' clause to be configured and enabled for the listening interfaces. A flaw was found in Unbound, a domain name system (DNS) resolver.

When Unbound is configured with DNSCrypt support, a remote attacker could send a specially crafted DNSCrypt query over TCP. This malicious query could cause a memory error, leading to a server crash and a Denial of Service (DoS).

This means legitimate users would be unable to access services relying on the Unbound server. This is an Important denial of service vulnerability affecting Unbound when built with DNSCrypt support.

This vulnerability doesn't impact any supported the versions of unbound as shipped with Red Hat Enterprise Linux as it is not compiled with DNSCrypt support.