# CVE-2021-3502: Denial of service in avahi

> A local attacker can crash the Avahi service by requesting hostname resolutions for invalid hostnames, causing a denial of service.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** ALPINE-CVE-2021-3502, AZL-40943, CVE-2021-36217, DEBIAN-CVE-2021-3502, GHSA-mw7q-3wxj-rqfx, MGASA-2021-0339, UBUNTU-CVE-2021-3502

## 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 (Reachable Assertion)

## Which versions are affected by CVE-2021-3502?

It affects 1 product.


Products: Avahi Avahi.

## How do I fix CVE-2021-3502?

Apply the vendor's patch.

## 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](https://insights.sbomify.com/CVE-2017-6519/): Denial of service in n/a (critical 9.1)
- [CVE-2021-26720](https://insights.sbomify.com/CVE-2021-26720/): Path traversal in n/a (high 7.8)
- [CVE-2026-24401](https://insights.sbomify.com/CVE-2026-24401/): Avahi has Uncontrolled Recursion in lookup_handle_cname function (medium 6.5)
- [CVE-2025-68468](https://insights.sbomify.com/CVE-2025-68468/): Avahi has a reachable assertion in lookup_multicast_callback (medium 6.5)
- [CVE-2025-68471](https://insights.sbomify.com/CVE-2025-68471/): Avahi has a reachable assertion in lookup_start (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3502 and fix it if it is affected.

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

Affected product: avahi. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-3502/ (Markdown: https://insights.sbomify.com/CVE-2021-3502/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3502
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3502
- OSV: https://osv.dev/vulnerability/CVE-2021-3502

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3502/
Summaries are generated and may contain mistakes; the official description above is the record.