The CVE number for this vulnerability is CVE-2026-42944.


== Summary
A vulnerability was found in Unbound that results in heap overflow when
encoding multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options in
the reply packet.
The relevant options ('nsid', 'answer-cookie', 'pad-responses' (default)) need
to be enabled for the vulnerability to be exploited.

Unbound 1.25.1 includes a fix to de-duplicate the EDNS options and a fix to
prevent truncation of the EDNS field size calculation that also contributes to
the heap overflow.


== Affected products
Unbound 1.14.0 up to and including version 1.25.0.


== Description
An adversary who can query Unbound can exploit the vulnerability by attaching
multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options to the query.

A flaw in the size calculation of the EDNS field truncates the correct value
which allows the encoder to overflow the available space when writing.

Those two combined lead to a heap overflow write of Unbound controlled data and
eventually a crash.

Unbound 1.25.1 includes a fix to de-duplicate the EDNS options and a fix to
prevent truncation of the EDNS field size calculation.

== Mitigation

=== Downloading patched version
Unbound 1.25.1 is released with the patch
https://nlnetlabs.nl/downloads/unbound/unbound-1.25.1.tar.gz

=== Applying the patch manually
For Unbound 1.25.0 the patch is:
https://nlnetlabs.nl/downloads/unbound/patch_CVE-2026-42944_with.diff

Apply the patch on the Unbound source directory with:
    patch -p1 < patch_CVE-2026-42944_with.diff
then run 'make install' to install Unbound.

A minimal patch that only addresses the de-duplication is:
https://nlnetlabs.nl/downloads/unbound/patch_CVE-2026-42944.diff
and can be used INSTEAD for easier patching.

Both patches mitigate the vulnerability and are tested to work on Unbound 1.25.0.


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