CVE-2021-3502 Denial of service in avahi

What is CVE-2021-3502?

CVE-2021-3502 is a medium-severity denial of service vulnerability in avahi, published on 7 May 2021. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

A flaw was found in avahi 0.8-5. A reachable assertion is present in avahi_s_host_name_resolver_start function allowing a local attacker to crash the avahi service by requesting hostname resolutions through the avahi socket or dbus methods for invalid hostnames. The highest threat from this vulnerability is to the service availability.

Weakness: CWE-617

Which versions are affected by CVE-2021-3502?

It affects 1 product.

Products include Avahi Avahi.

How do I fix CVE-2021-3502?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2021-3502 being exploited?

There is no known exploitation of CVE-2021-3502 in the wild and no public exploit code.

Other vulnerabilities in avahi

• CVE-2017-6519 Denial of service in n/a Critical 9.1
• CVE-2021-26720 Path traversal in n/a High 7.8
• CVE-2026-24401 Avahi has Uncontrolled Recursion in lookup_handle_cname function Medium 6.5
• CVE-2025-68468 Avahi has a reachable assertion in lookup_multicast_callback Medium 6.5
• CVE-2025-68471 Avahi has a reachable assertion in lookup_start Medium 6.5

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as ALPINE-CVE-2021-3502, AZL-40943, CVE-2021-36217, DEBIAN-CVE-2021-3502, GHSA-mw7q-3wxj-rqfx, MGASA-2021-0339 and 1 more.