CVE-2020-28935

NLnet Labs Unbound, up to and including version 1.12.0, and NLnet Labs NSD,
up to and including version 4.3.3, contain a local vulnerability that would
allow for a local symlink attack. 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.

Bugs

• <https://github.com/NLnetLabs/unbound/issues/303&gt;