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.

Weakness

The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.

Affected Software

Potential Mitigations

• Follow the principle of least privilege when assigning access rights to entities in a software system.
• Denying access to a file can prevent an attacker from replacing that file with a link to a sensitive file. Ensure good compartmentalization in the system to provide protected areas that can be trusted.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/132.html
• https://cwe.mitre.org/data/definitions/17.html
• https://cwe.mitre.org/data/definitions/35.html
• https://cwe.mitre.org/data/definitions/76.html

References

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