====================================================================

                             CERT-Renater

                 Note d'Information No. 2020/VULN677
_____________________________________________________________________

DATE                : 10/12/2020

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running Unbound versions prior to 1.13.0,
                            NSD Unbound versions prior to 4.3.4.

=====================================================================
https://nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txt
_____________________________________________________________________

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.


=========================================================
+ CERT-RENATER       |    tel : 01-53-94-20-44          +
+ 23/25 Rue Daviel   |    fax : 01-53-94-20-41          +
+ 75013 Paris        |    email:cert@support.renater.fr +
=========================================================