The CVE number for this vulnerability is CVE-2026-55990.


== Summary
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').

Unbound 1.25.2 includes a fix to fail fast on such faulty configuration and an
in-depth-fix to zero out the certificate array and skip NULL keypairs.


== Affected products
Unbound 1.7.0 up to and including version 1.25.1.


== Mitigation

=== Downloading patched version
Unbound 1.25.2 is released with the patch
https://nlnetlabs.nl/downloads/unbound/unbound-1.25.2.tar.gz

=== Applying the patch manually
For Unbound 1.25.1 the patch is:
https://nlnetlabs.nl/downloads/unbound/patch_CVE-2026-55990.diff

Apply the patch on the Unbound source directory with:
    patch -p1 < patch_CVE-2026-55990.diff
then run 'make install' to install Unbound.

The patch is tested to work on Unbound 1.25.1.


== Acknowledgments
We would like to thank Qifan Zhang from Palo Alto Networks for discovering and
responsibly disclosing the vulnerability.