CVE-2020-28935 Information

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.

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Reference

https://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txt https://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txt https://security.gentoo.org/glsa/202101-38 https://lists.debian.org/debian-lts-announce/2021/02/msg00017.html

Attack Complexity

LOW

Privileges Required

LOW

User Interaction Required

LOW

Scope

NONE

Confidentiality Impact

UNCHANGED

Integrity Impact

NONE

Availability Impact

NONE

Base Score

HIGH

Base Severity

5.5