Avahi CVE-2021-26720

HIGH

Improper Link Resolution Before File Access (CWE-59)

2021-02-17 cve@mitre.org

Denial Of Service Debian Avahi Debian Linux

7.8

CVSS 3.1 · NVD

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Vulnerability Assessment

Weakness	CWE CWE-59 OWASP Top 10:2025 A01:2025 - Broken Access Control ASVS 5.0 V5 - File Handling Symlink resolution safety
Impact	CVSS score of 7.8 (HIGH). Attack characteristics: local attack vector, low complexity, low privileges required, no user interaction not required.
Exploit ScenarioDerived	Successful exploitation could compromise the high-severity affected system.

This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).

No patch listed yet 0.4% exploitation probability (EPSS)

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AnalysisAI

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. Rated high severity (CVSS 7.8), this vulnerability is low attack complexity. No vendor patch available.

RemediationAI

No vendor patch is available at time of analysis. Monitor vendor advisories for updates. Apply vendor patches when available. Implement network segmentation and monitoring as interim mitigations.

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Technical ContextAI · show details

This vulnerability is classified under CWE-59. 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. Affected products include: Avahi, Debian Debian Linux. Version information: through 0.8.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

NVD PRIMARY 7.8 HIGH

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

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector

Local

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

DescriptionNVD

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.

Threat Intelligence Overview

EPSS Score

0.4%

Top 68% exploitable

Data Quality

2 sources

Confidence: Low

CVE.org

EPSS

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity HIGH

CISA KEV No

Public POC No

EPSS 0.4%

Patch available No

Active threat No

Deciding rule

HIGH severity (CVSS 7.0-8.9)

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived

Defer

Exploitation· derived

None

System Exposure· derived

Small

Automatable· derived

No

Human Impact· derived

Medium

Priority Score

39

Low Medium High Critical

KEV: 0

EPSS: 0

CVSS: +39

PoC: 0

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

Denial of Service MEDIUM

Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.

Affected Products (CPE)NVD

cpe:2.3:a:avahi:avahi:*:*:*:*:*:*:*:*

cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*

cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*

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Same weakness CWE-59 – Improper Link Resolution Before File Access

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References

Advisories & patches

REF https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=982796 REF https://bugs.launchpad.net/ubuntu/+source/avahi/+bug/1870824 REF https://bugzilla.suse.com/show_bug.cgi?id=1180827 REF https://lists.debian.org/debian-lts-announce/2022/06/msg00009.html REF https://metadata.ftp-master.debian.org/changelogs/main/a/avahi/avahi_0.8-4_changelog REF https://packages.debian.org/bullseye/avahi-daemon REF https://packages.debian.org/buster/avahi-daemon REF https://packages.debian.org/sid/avahi-daemon REF https://security-tracker.debian.org/tracker/CVE-2021-26720 REF https://www.openwall.com/lists/oss-security/2021/02/15/2

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