Medium [CVE-2026-55990] Denial of Service via faulty DNSCrypt configuration

In NLnet Labs Unbound 1.7.0 up to and including 1.25.1, when the 'dnscrypt:' clause lists more 'dnscrypt-provider-cert:' files than there are matching 'dnscrypt-secret-key:' files, Unbound fills only the matched prefix and leaves the tail slots at the '0xdb' fill that libsodium's allocator writes into every allocation.

Unbound would then iterate over the number of cert files, not the actual slots, so it walks into a slot with garbage data filled with '0xdb' bytes. Any unauthenticated client that sends one UDP datagram of ≥ 68 bytes whose first 8 bytes are '0xdb' to 'dnscrypt-port' will use that garbage entry which leads to a garbage dereference killing the server.

This is a silent faulty configuration that goes unnoticed until triggered with the right client query. Unbound needs to be compiled with DNSCrypt support ('--enable-dnscrypt').

An unauthenticated remote attacker could exploit a faulty configuration, where an imbalance between DNSCrypt provider certificate and secret key files leads to memory corruption. By sending a specially crafted network request, the attacker can cause a garbage dereference, leading to a server crash and a denial of service (DoS).

This Moderate flaw in Unbound can lead to a denial of service if the DNSCrypt feature is enabled and misconfigured. An unauthenticated attacker could then send a crafted UDP packet to trigger a server crash.