# CVE-2020-28935: Local symlink attack in Unbound and NSD

> Local attackers can exploit symlink handling in PID file creation to overwrite critical system files, causing 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 Dec 2020
- **Updated:** 16 Sep 2024
- **Also known as:** ALPINE-CVE-2020-28935, AZL-6931, DEBIAN-CVE-2020-28935, GHSA-q92f-7cwj-w4w5, MGASA-2021-0154, OESA-2021-1083, UBUNTU-CVE-2020-28935, openSUSE-SU-2025:15069-1

## What is CVE-2020-28935?

CVE-2020-28935 is a medium-severity path traversal vulnerability in Debian Debian Linux, published on 7 Dec 2020. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

NLnet Labs Unbound, up to and including version 1.12.0, and NLnet Labs NSD, up to and including version 4.3.3, contain a local vulnerability that would allow for a local symlink attack. When writing the PID file, Unbound and NSD create the file if it is not there, or open an existing file for writing. In case the file was already present, they would follow symlinks if the file happened to be a symlink instead of a regular file. An additional chown of the file would then take place after it was written, making the user Unbound/NSD is supposed to run as the new owner of the file. If an attacker has local access to the user Unbound/NSD runs as, she could create a symlink in place of the PID file pointing to a file that she would like to erase. If then Unbound/NSD is killed and the PID file is not cleared, upon restarting with root privileges, Unbound/NSD will rewrite any file pointed at by the symlink. This is a local vulnerability that could create a Denial of Service of the system Unbound/NSD is running on. It requires an attacker having access to the limited permission user Unbound/NSD runs as and point through the symlink to a critical file on the system.

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

## Which versions are affected by CVE-2020-28935?

It affects 3 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `sslh` | openSUSE:Tumbleweed | All versions | 2.2.3-1.1 |

Products: Debian Debian Linux, Nlnetlabs Name Server Daemon, Nlnetlabs Unbound.

## How do I fix CVE-2020-28935?

Upgrade to a fixed version.

## Is CVE-2020-28935 being exploited?

There is no known exploitation of CVE-2020-28935 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-28935 and fix it if it is affected.

What it is: CVE-2020-28935 is a medium-severity path traversal vulnerability in Debian Debian Linux, published on 7 Dec 2020. 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-2020-28935 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-2020-28935/ (Markdown: https://insights.sbomify.com/CVE-2020-28935/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-28935
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-28935
- OSV: https://osv.dev/vulnerability/CVE-2020-28935

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-28935/
Summaries are generated and may contain mistakes; the official description above is the record.