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CVE-2021-26720

avahi-daemon-check-dns.sh in the Debian avahi package through 0.8-4 is executed as root via /etc...

7.8 HighFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Local
Complexity
Low
Privileges needed
Low
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
High
Availability
High

Exploit likelihood

0.40%Down 1.0 points since Apr 14, 2021
90%50%10%1%

What each source reports

NVD Score used 7.8

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.

Record
CVE-2021-26720
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CVSS 2.0
4.6 MediumAV:L/AC:L/Au:N/C:P/I:P/A:P
Weakness
CWE-59
Updated
2026-11-02
CVE List No score

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.

Record
CVE-2021-26720
Updated
2026-11-02
EUVDEUVD-2021-13508 No score

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.

Record
EUVD-2021-13508
Updated
2026-11-03
GitHub AdvisoriesGHSA-jhxr-wvf5-hrp6 7.8

avahi-daemon-check-dns.sh in the Debian avahi package through 0.8-4 is executed as root via /etc...

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.

Record
GHSA-jhxr-wvf5-hrp6
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Weakness
CWE-59
Updated
2026-11-02
OSVALPINE-CVE-2021-26720 7.8

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.

Record
ALPINE-CVE-2021-26720
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-02
OSVAZL-34547 7.8

CVE-2021-26720 affecting package avahi for versions less than 0.8-1

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.

Record
AZL-34547
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-05
Show 12 moreShow less
OSVAZL-6323 7.8

CVE-2021-26720 affecting package avahi for versions less than 0.8-1

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.

Record
AZL-6323
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-02
OSVBELL-CVE-2021-26720 7.8

CVE-2021-26720 does not affect BellSoft software

Record
BELL-CVE-2021-26720
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-02
OSV 7.8

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.

Record
CVE-2021-26720
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-02
OSVDEBIAN-CVE-2021-26720 7.8

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.

Record
DEBIAN-CVE-2021-26720
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-02
OSVUBUNTU-CVE-2021-26720 7.8

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.

Record
UBUNTU-CVE-2021-26720
CVSS 3.1
7.8 HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Updated
2026-11-02
OSVCLSA-2026-1787932412 No score

Fix CVE(s): CVE-2021-26720

  • SECURITY UPDATE: local symlink attack through the unicast .local DNS check: /etc/network/if-up.d/avahi-daemon exec's /usr/lib/avahi/avahi-daemon-check-dns.sh as root, and the script writes into /run/avahi-daemon (the checked_nameservers[.$$] cache and the disabled-for-unicast-local tag) without protecting against symlinks, so a local attacker able to place symlinks in that avahi-owned directory can make root create arbitrary empty files or cause a denial of service.
    • debian/avahi-daemon.if-up: run the check script under "sudo -u avahi -g avahi" instead of exec'ing it as root, so the writes are performed by the same unprivileged account that owns /run/avahi-daemon. Backported verbatim from Debian stretch 0.6.32-2+deb9u1 (DLA-3047-1); the alternative vendor fix, dropping the check-dns mechanism outright (Debian buster 0.7-4+deb10u1, Ubuntu focal 0.7-4ubuntu7), is not applicable here because bionic ships libnss-mdns 0.10-8ubuntu1, the same generation as stretch.
    • debian/control: add sudo to the avahi-daemon Depends, required by the privilege drop.
    • CVE-2021-26720
Record
CLSA-2026-1787932412
Updated
2026-11-02
OSVopenSUSE-SU-2021:0370-1 No score

Security update for avahi

This update for avahi fixes the following issues:

  • CVE-2021-26720: drop privileges when invoking avahi-daemon-check-dns.sh (bsc#1180827)
  • Update avahi-daemon-check-dns.sh from Debian. Our previous version relied on ifconfig, route, and init.d.
  • Add sudo to requires: used to drop privileges.

This update was imported from the SUSE:SLE-15-SP1:Update update project.

Record
openSUSE-SU-2021:0370-1
Updated
2026-11-02
OSVopenSUSE-SU-2021:1845-1 No score

Security update for avahi

This update for avahi fixes the following issues:

  • CVE-2021-3468: avoid infinite loop by handling HUP event in client_work (bsc#1184521).
  • CVE-2021-26720: drop privileges when invoking avahi-daemon-check-dns.sh (bsc#1180827)
  • Update avahi-daemon-check-dns.sh from Debian. Our previous version relied on ifconfig, route, and init.d.
  • Add sudo to requires: used to drop privileges.
Record
openSUSE-SU-2021:1845-1
Updated
2026-11-02
OSVopenSUSE-SU-2024:10643-1 No score

avahi-0.8-7.2 on GA media

These are all security issues fixed in the avahi-0.8-7.2 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2024:10643-1
Updated
2026-11-02
OSVSUSE-SU-2021:0551-1 No score

Security update for avahi

This update for avahi fixes the following issues:

  • CVE-2021-26720: drop privileges when invoking avahi-daemon-check-dns.sh (bsc#1180827)
  • Update avahi-daemon-check-dns.sh from Debian. Our previous version relied on ifconfig, route, and init.d.
  • Add sudo to requires: used to drop privileges.
Record
SUSE-SU-2021:0551-1
Updated
2026-11-02
OSVSUSE-SU-2021:0563-1 No score

Security update for avahi

This update for avahi fixes the following issues:

  • CVE-2021-26720: drop privileges when invoking avahi-daemon-check-dns.sh (bsc#1180827)
  • Add sudo to requires: used to drop privileges.
Record
SUSE-SU-2021:0563-1
Updated
2026-11-02
OSVSUSE-SU-2021:1845-1 No score

Security update for avahi

This update for avahi fixes the following issues:

  • CVE-2021-3468: avoid infinite loop by handling HUP event in client_work (bsc#1184521).
  • CVE-2021-26720: drop privileges when invoking avahi-daemon-check-dns.sh (bsc#1180827)
  • Update avahi-daemon-check-dns.sh from Debian. Our previous version relied on ifconfig, route, and init.d.
  • Add sudo to requires: used to drop privileges.
Record
SUSE-SU-2021:1845-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
avahi
Azure Linux:3
 before 0.8-1
0.8-1
avahi
Azure Linux:2
 before 0.8-1
0.8-1
avahi-autoipd
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
avahi-daemon
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
avahi-discover
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
avahi-dnsconfd
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
avahi-ui-utils
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
avahi-utils
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
gir1.2-avahi-0.6
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-client-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
Show 49 moreShow less
Affected packages
PackageAffectedFixed
libavahi-client3
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-common-data
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-common-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-common3
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-compat-libdnssd-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-compat-libdnssd1
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-core-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-core7
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-glib-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-glib1
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-gobject-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-gobject0
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-ui-gtk3-0
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
libavahi-ui-gtk3-dev
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
python-avahi
TuxCare:Ubuntu:18.04
 before 0.7-3.1ubuntu1.3+tuxcare.els1
0.7-3.1ubuntu1.3+tuxcare.els1
avahi
Debian:12
 before 0.8-4
0.8-4
avahi
Debian:13
 before 0.8-4
0.8-4
avahi
Debian:14
 before 0.8-4
0.8-4
avahi
Debian:9
From an advisory covering 2 CVEs
 before 0.6.32-2+deb9u1, plus listed versions
0.6.32-2
0.6.32-2+deb9u1
avahi
SUSE:Linux Enterprise Module for Basesystem 15 SP2
 before 0.7-3.6.1
0.7-3.6.1
avahi-glib2
SUSE:Linux Enterprise Module for Basesystem 15 SP2
 before 0.7-3.6.1
0.7-3.6.1
avahi
SUSE:Linux Enterprise Module for Desktop Applications 15 SP2
 before 0.7-3.6.1
0.7-3.6.1
avahi-glib2
SUSE:Linux Enterprise Module for Desktop Applications 15 SP2
 before 0.7-3.6.1
0.7-3.6.1
avahi
SUSE:Linux Enterprise Module for Package Hub 15 SP2
 before 0.7-3.6.1
0.7-3.6.1
avahi
SUSE:Linux Enterprise Software Development Kit 12 SP5
 before 0.6.32-32.12.2
0.6.32-32.12.2
avahi-glib2
SUSE:Linux Enterprise Software Development Kit 12 SP5
 before 0.6.32-32.12.3
0.6.32-32.12.3
avahi
SUSE:Linux Enterprise Server 12 SP5
 before 0.6.32-32.12.2
0.6.32-32.12.2
avahi-glib2
SUSE:Linux Enterprise Server 12 SP5
 before 0.6.32-32.12.3
0.6.32-32.12.3
avahi
SUSE:Linux Enterprise Server for SAP Applications 12 SP5
 before 0.6.32-32.12.2
0.6.32-32.12.2
avahi-glib2
SUSE:Linux Enterprise Server for SAP Applications 12 SP5
 before 0.6.32-32.12.3
0.6.32-32.12.3
avahi-glib2
SUSE:Linux Enterprise Workstation Extension 12 SP5
 before 0.6.32-32.12.3
0.6.32-32.12.3
avahi
SUSE:Linux Enterprise High Performance Computing 15-ESPOS
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi-glib2
SUSE:Linux Enterprise High Performance Computing 15-ESPOS
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi
SUSE:Linux Enterprise High Performance Computing 15-LTSS
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi-glib2
SUSE:Linux Enterprise High Performance Computing 15-LTSS
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi
SUSE:Linux Enterprise Server 15-LTSS
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi-glib2
SUSE:Linux Enterprise Server 15-LTSS
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi
SUSE:Linux Enterprise Server for SAP Applications 15
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi-glib2
SUSE:Linux Enterprise Server for SAP Applications 15
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi
Ubuntu:Pro:14.04:LTS
 All versions, plus listed versions
0.6.31-2ubuntu40.6.31-2ubuntu50.6.31-3ubuntu10.6.31-4ubuntu10.6.31-4ubuntu1.1
Show 7 moreShow less
0.6.31-4ubuntu1.20.6.31-4ubuntu1.30.6.31-4ubuntu1.3+esm10.6.31-4ubuntu1.3+esm20.6.31-4ubuntu1.3+esm30.6.31-4ubuntu1.3+esm40.6.31-4ubuntu1.3+esm5
 None known
avahi
Ubuntu:Pro:16.04:LTS
 All versions, plus listed versions
0.6.31-4ubuntu40.6.32~rc+dfsg-1ubuntu20.6.32~rc+dfsg-1ubuntu2.10.6.32~rc+dfsg-1ubuntu2.20.6.32~rc+dfsg-1ubuntu2.3
Show 5 moreShow less
0.6.32~rc+dfsg-1ubuntu2.3+esm10.6.32~rc+dfsg-1ubuntu2.3+esm20.6.32~rc+dfsg-1ubuntu2.3+esm30.6.32~rc+dfsg-1ubuntu2.3+esm40.6.32~rc+dfsg-1ubuntu2.3+esm5
 None known
avahi
Ubuntu:Pro:18.04:LTS
 All versions, plus listed versions
0.6.32-1ubuntu10.7-3ubuntu10.7-3ubuntu20.7-3.1ubuntu10.7-3.1ubuntu1.1
Show 6 moreShow less
0.7-3.1ubuntu1.20.7-3.1ubuntu1.30.7-3.1ubuntu1.3+esm10.7-3.1ubuntu1.3+esm20.7-3.1ubuntu1.3+esm30.7-3.1ubuntu1.3+esm4
 None known
avahi
openSUSE:Leap 15.2
 before 0.7-lp152.3.6.1
0.7-lp152.3.6.1
avahi-glib2
openSUSE:Leap 15.2
 before 0.7-lp152.3.6.1
0.7-lp152.3.6.1
avahi-mono
openSUSE:Leap 15.2
 before 0.7-lp152.3.6.1
0.7-lp152.3.6.1
avahi-qt4
openSUSE:Leap 15.2
 before 0.7-lp152.3.6.1
0.7-lp152.3.6.1
avahi
openSUSE:Leap 15.3
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi-glib2
openSUSE:Leap 15.3
From an advisory covering 2 CVEs
 before 0.6.32-5.13.1
0.6.32-5.13.1
avahi
openSUSE:Tumbleweed
From an advisory covering 8 CVEs
 before 0.8-7.2
0.8-7.2

History

History
RecordedChangedSources
affected
factsOSV
affected
aliasesEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-26720 is a high vulnerability (CVSS 7.8) in Avahi, published February 17, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.40%, the 33rd percentile. Fixed in avahi 0.8-1.

References

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