The CVE number for this vulnerability is CVE-2020-28935.

== Summary
Unbound and NSD when writing the PID file would not check if an existing file
was a symlink. This could allow for a local symlink attack if an attacker has
access to the user Unbound/NSD runs as.

== Affected products
Unbound up to and including 1.12.0.
NSD up to and including 4.3.3.

== Description
When writing the PID file, Unbound and NSD create the file if it is not
there, or open an existing file for writing. In case the file was already
present, they would follow symlinks if the file happened to be a symlink
instead of a regular file.

An additional chown of the file would then take place after it was written,
making the user Unbound/NSD is supposed to run as the new owner of the file.

If an attacker has local access to the user Unbound/NSD runs as, she could
create a symlink in place of the PID file pointing to a file that she would
like to erase. If then Unbound/NSD is killed and the PID file is not cleared,
upon restarting with root privileges, Unbound/NSD will rewrite any file pointed
at by the symlink.

This is a local vulnerability that could create a Denial of Service of the
system Unbound/NSD is running on. It requires an attacker having access to the
limited permission user Unbound/NSD runs as and point through the symlink to a
critical file on the system.

== Solution
Download patched version of Unbound (1.13.0) and/or NSD (4.3.4), or apply the
patches manually.

+ Unbound
Unbound 1.13.0 is going to be released including the patch.

The patch is:
https://nlnetlabs.nl/downloads/unbound/patch_cve-2020-28935_unbound.diff
and can be applied to versions 1.6.6 up until 1.12.0.

Apply the patch containing the fix using:
'patch -p1 < patch_cve-2020-28935_unbound.diff'
then run 'make install' to install Unbound.

+ NSD
NSD 4.3.4 is going to be released including the patch.

The patch is:
https://nlnetlabs.nl/downloads/nsd/patch_cve-2020-28935_nsd.diff
and can be applied to versions 4.2.3 up until 4.3.3.

Apply the patch containing the fix using:
'patch -p1 < patch_cve-2020-28935_nsd.diff'
then run 'make install' to install NSD.

== Acknowledgements
We would like to thank Mason Loring Bliss for bringing the issue to our attention.