CVE-2020-28935: 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 woul...

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.

Metrics

CVSS 3.1

5.5/10

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

0.48%

38.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Nlnetlabs	Name Server Daemon	< 4.3.4
Nlnetlabs	Unbound	< 1.13.0
Debian	Debian Linux	9.0

References

https://lists.debian.org/debian-lts-announce/2021/02/msg00017.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2023/03/msg00024.html
https://security.gentoo.org/glsa/202101-38Third Party Advisory
https://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txtVendor Advisory
https://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txtVendor Advisory
https://lists.debian.org/debian-lts-announce/2021/02/msg00017.htmlMailing List, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2023/03/msg00024.html
https://security.gentoo.org/glsa/202101-38Third Party Advisory
https://www.nlnetlabs.nl/downloads/nsd/CVE-2020-28935.txtVendor Advisory
https://www.nlnetlabs.nl/downloads/unbound/CVE-2020-28935.txtVendor Advisory

Timeline

Published: Dec 7, 2020
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-28935?

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.

How severe is CVE-2020-28935?

CVE-2020-28935 has a CVSS score of 5.5/10 (MEDIUM severity). The EPSS model estimates a 0.48% probability of exploitation in the next 30 days.

How do I fix CVE-2020-28935?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.