CVE-2021-26720

Publication date 17 February 2021

Last updated 26 August 2025

Ubuntu priority

Cvss 3 Severity Score

Description

avahi-daemon-check-dns.sh in the Debian avahi package through 0.8-4 is executed as root via /etc/network/if-up.d/avahi-daemon, and allows a local attacker to cause a denial of service or create arbitrary empty files via a symlink attack on files under /run/avahi-daemon. NOTE: this only affects the packaging for Debian GNU/Linux (used indirectly by SUSE), not the upstream Avahi product.

Read the notes from the security team

Status

Show unmaintained releases

Package	Ubuntu Release	Status
avahi	25.10 questing	Not affected
	25.04 plucky	Not affected
	24.10 oracular	Not affected
	24.04 LTS noble	Not affected
	23.10 mantic	Not affected
	23.04 lunar	Not affected
	22.10 kinetic	Not affected
	22.04 LTS jammy	Not affected
	21.10 impish	Not affected
	21.04 hirsute	Not affected
	20.10 groovy	Not affected
	20.04 LTS focal	Not affected
	18.04 LTS bionic	Vulnerable, fix deferred
	16.04 LTS xenial	Vulnerable, fix deferred
	14.04 LTS trusty	Vulnerable, fix deferred

Notes

mdeslaur

the script was removed in the 0.7-4ubuntu7 package in focal. for bionic and earlier, we can't simply remove the script as libnss-mdns is too old there.

litios

there is no upstream patch available to fix this issue, as the proposed fix was to delete the script file. marking as deferred

Severity score breakdown

Parameter	Value
Base score	7.8 · High
Attack vector	Local
Attack complexity	Low
Privileges required	Low
User interaction	None
Scope	Unchanged
Confidentiality	High
Integrity impact	High
Availability impact	High
Vector	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H