Arch Linux Security Advisory ASA-202012-18
=========================================
Severity: Low
Date    : 2020-12-09
CVE-ID  : CVE-2020-28935
Package : nsd
Type    : denial of service
Remote  : No
Link    : https://security.archlinux.org/AVG-1327

Summary
======
The package nsd before version 4.3.4-1 is vulnerable to denial of
service.

Resolution
=========
Upgrade to 4.3.4-1.

# pacman -Syu "nsd>=4.3.4-1"

The problem has been fixed upstream in version 4.3.4.

Workaround
=========
None.

Description
==========
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.

Impact
=====
A local attacker might be able to cause a denial of service of the
host.

References
=========
https://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txt
https://github.com/NLnetLabs/unbound/issues/303
https://nlnetlabs.nl/downloads/unbound/patch_cve-2020-28935_unbound.diff
https://github.com/NLnetLabs/unbound/commit/ad387832979b6ce4c93f64fe706301cd7d034e87
https://github.com/NLnetLabs/unbound/commit/19f8f4d9f99a44906ab9dcc46d44da299fde3506
https://nlnetlabs.nl/downloads/nsd/patch_cve-2020-28935_nsd.diff
https://github.com/NLnetLabs/nsd/commit/a4caec3137a1bc9eca05d38d66e2bce572ca9bd3
https://security.archlinux.org/CVE-2020-28935