CVE-2021-26720

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.

Published: 2021-02-17 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-26720 is rated Moderate Risk (42.4/100): CVSS High severity, with low exploitation likelihood (EPSS 0.40%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-26720

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.07% 0.40% +0.33%
2 2025-12-07 0.03% 0.07% +0.03%
3 2025-11-21 — 0.03% —

Full EPSS history (13 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-26720

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.8 3.1 HIGH
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:L)
They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 1.8 5.9 [email protected]
4.6 2.0 MEDIUM
AV:L/AC:L/Au:N/C:P/I:P/A:P Click to expand
Access vector (AV:L)
Requires local access to the target system.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
 3.9 6.4 [email protected]

Weakness enumeration for CVE-2021-26720

OS Trackers for CVE-2021-26720

vendor priority summary link
alpine high CVE-2021-26720: 1 source package rows (avahi); 62 state rows across 11 repos (3.10-main, 3.11-main, 3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, 3.23-main, edge-main); fixed 62, open 0. https://security.alpinelinux.org/vuln/CVE-2021-26720
debian not yet assigned CVE-2021-26720 not yet assigned priority: Debian including 1 source packages (avahi), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-26720
suse high CVE-2021-26720 severity important: SUSE including 504 source package names (5.2.2-5.24:libavahi-client3-0.7-3.6.1, 5.2.2-5.24:libavahi-common3-0.7-3.6.1, …), 1175 product×package rows across 132 product lines (Container suse/pcp, Container suse/sle-micro-rancher/5.2, … (132 product lines)): Fixed 628, Known Not Affected 417, Known Affected 130. https://www.suse.com/security/cve/CVE-2021-26720/
ubuntu low CVE-2021-26720 low priority: Ubuntu including 1 source packages (avahi), 16 status rows across 16 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 13, deferred 3. https://ubuntu.com/security/CVE-2021-26720

Affected software / configurations for CVE-2021-26720

Vendor Product Version Raw CPE
avahi avahi <= 0.8-4 cpe:2.3:a:avahi:avahi:*:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*

References for CVE-2021-26720

URL Tags
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=982796 Mailing List Third Party Advisory
https://bugs.launchpad.net/ubuntu/+source/avahi/+bug/1870824 Issue Tracking Third Party Advisory
https://bugzilla.suse.com/show_bug.cgi?id=1180827 Issue Tracking Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/06/msg00009.html Mailing List Third Party Advisory
https://metadata.ftp-master.debian.org/changelogs/main/a/avahi/avahi_0.8-4_changelog Broken Link Third Party Advisory
https://packages.debian.org/bullseye/avahi-daemon Product Third Party Advisory
https://packages.debian.org/buster/avahi-daemon Product Third Party Advisory
https://packages.debian.org/sid/avahi-daemon Product Third Party Advisory
https://security-tracker.debian.org/tracker/CVE-2021-26720 Third Party Advisory
https://www.openwall.com/lists/oss-security/2021/02/15/2 Mailing List Third Party Advisory
cvelogic Threat Intelligence