# CVE-2021-26720: Path traversal in n/a

> Local attackers can cause denial of service or create arbitrary files via symlink attacks in the Debian Avahi package script.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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:** 17 Feb 2021
- **Updated:** 3 Aug 2024
- **Also known as:** AZL-34547, AZL-6323, CLSA-2026-1787932412, DEBIAN-CVE-2021-26720, GHSA-jhxr-wvf5-hrp6, SUSE-SU-2021:0551-1, SUSE-SU-2021:0563-1, UBUNTU-CVE-2021-26720, openSUSE-SU-2021:0370-1

## What is CVE-2021-26720?

CVE-2021-26720 is a high-severity path traversal vulnerability in Debian Debian Linux, published on 17 Feb 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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: CWE-59 (Improper Link Resolution Before File Access)

## Which versions are affected by CVE-2021-26720?

It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `avahi` | SUSE:Linux Enterprise Module for Basesystem 15 SP2 | All versions | 0.7-3.6.1 |
| `avahi-glib2` | SUSE:Linux Enterprise Module for Basesystem 15 SP2 | All versions | 0.7-3.6.1 |
| `avahi` | SUSE:Linux Enterprise Module for Desktop Applications 15 SP2 | All versions | 0.7-3.6.1 |
| `avahi-glib2` | SUSE:Linux Enterprise Module for Desktop Applications 15 SP2 | All versions | 0.7-3.6.1 |
| `avahi` | SUSE:Linux Enterprise Module for Package Hub 15 SP2 | All versions | 0.7-3.6.1 |
| `avahi` | SUSE:Linux Enterprise Server 12 SP5 | All versions | 0.6.32-32.12.2 |
| `avahi-glib2` | SUSE:Linux Enterprise Server 12 SP5 | All versions | 0.6.32-32.12.3 |
| `avahi` | SUSE:Linux Enterprise Server for SAP Applications 12 SP5 | All versions | 0.6.32-32.12.2 |
| `avahi-glib2` | SUSE:Linux Enterprise Server for SAP Applications 12 SP5 | All versions | 0.6.32-32.12.3 |
| `avahi` | SUSE:Linux Enterprise Software Development Kit 12 SP5 | All versions | 0.6.32-32.12.2 |
| `avahi-glib2` | SUSE:Linux Enterprise Software Development Kit 12 SP5 | All versions | 0.6.32-32.12.3 |
| `avahi-glib2` | SUSE:Linux Enterprise Workstation Extension 12 SP5 | All versions | 0.6.32-32.12.3 |
| `avahi` | openSUSE:Leap 15.2 | All versions | 0.7-lp152.3.6.1 |
| `avahi-glib2` | openSUSE:Leap 15.2 | All versions | 0.7-lp152.3.6.1 |
| `avahi-mono` | openSUSE:Leap 15.2 | All versions | 0.7-lp152.3.6.1 |
| `avahi-qt4` | openSUSE:Leap 15.2 | All versions | 0.7-lp152.3.6.1 |

Products: Avahi Avahi, Debian Debian Linux.

## How do I fix CVE-2021-26720?

Apply the vendor's patch.

## Is CVE-2021-26720 being exploited?

There is no known exploitation of CVE-2021-26720 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2017-6519](https://insights.sbomify.com/CVE-2017-6519/): Denial of service in n/a (critical 9.1)
- [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)
- [CVE-2023-38469](https://insights.sbomify.com/CVE-2023-38469/): Reachable assertion in avahi_dns_packet_append_record (medium 6.2)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-26720 and fix it if it is affected.

What it is: CVE-2021-26720 is a high-severity path traversal vulnerability in Debian Debian Linux, published on 17 Feb 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-2021-26720 in the wild and no public exploit code.

Affected product: Debian Debian Linux. 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-2021-26720/ (Markdown: https://insights.sbomify.com/CVE-2021-26720/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-26720
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-26720
- OSV: https://osv.dev/vulnerability/CVE-2021-26720

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-26720/
Summaries are generated and may contain mistakes; the official description above is the record.