CVE-2020-28935 log

Source
Severity Low
Remote No
Type Denial of service
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.
Group Package Affected Fixed Severity Status Ticket
AVG-1327 nsd 4.3.3-1 4.3.4-1 Low Fixed
AVG-1326 unbound 1.12.0-1 1.13.0-1 Low Fixed
Date Advisory Group Package Severity Type
09 Dec 2020 ASA-202012-18 AVG-1327 nsd Low denial of service
09 Dec 2020 ASA-202012-17 AVG-1326 unbound Low denial of service
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