Debian LinuxOperating system · Debian
CVE-2021-26720
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedavahi-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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe avahi-daemon-check-dns.sh script in Debian's avahi package (0.8-4) is executed as root from /etc/network/if-up.d/ and creates files under /run/avahi-daemon without proper symlink checks. A local attacker can exploit this symlink attack to cause denial of service or create arbitrary empty files on the system.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Debian LinuxOperating system
= 9.0= 10.0<= 0.8-4CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Local
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Confirm avahi package is installedRun 'dpkg -l | grep -i avahi' or 'dpkg -s avahi-daemon' to list installed avahi packagesAffected if Any avahi package version is installed on Debian 9 or 10
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Check avahi-daemon package versionRun 'dpkg -l avahi-daemon' or 'apt show avahi-daemon' to retrieve the installed version numberAffected if The installed version is 0.8-4 or lower
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Verify the vulnerable script existsRun 'ls -la /etc/network/if-up.d/avahi-daemon-check-dns.sh' to confirm the script is presentAffected if The file /etc/network/if-up.d/avahi-daemon-check-dns.sh exists on the system
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Confirm script is executable by root on network upCheck that the script has executable permissions (ls -la shows -rwxr-xrx) and is invoked by /etc/network/if-up.d/ when network interfaces are brought upAffected if The script is executable and will run as root during network interface initialization
A system is affected if it runs Debian 9 or 10 with avahi-daemon version 0.8-4 or lower installed, and the /etc/network/if-up.d/avahi-daemon-check-dns.sh script is present and executable.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataApply Debian security patches to fix the symlink vulnerability in the packaging, or disable/remove the avahi-daemon if-up.d script as a workaround until the patch is available.
Fix this in Debian Linux
Scoped from the published advisory
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-26720 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-26720 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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