CVE-2020-28935 | Local symlink attack in Unbound and NSD

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.

Published: 2020-12-07 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2020-28935 is rated Low Risk (29.6/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.08%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2020-28935

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-03-17 0.04% 0.08% +0.04%
2 2023-03-07 0.95% 0.04% -0.91%
3 2022-04-01 — 0.95% —

Full EPSS history (6 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2020-28935

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
5.5 3.1 MEDIUM
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:L)
They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 1.8 3.6 [email protected]
2.1 2.0 LOW
AV:L/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:L)
Requires local access to the target system.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 3.9 2.9 [email protected]

Weakness enumeration for CVE-2020-28935

OS Trackers for CVE-2020-28935

vendor priority summary link
alpine medium CVE-2020-28935: 2 source package rows (nsd, unbound); 26 state rows across 10 repos (3.10-main, 3.11-main, 3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 8, open 18. https://security.alpinelinux.org/vuln/CVE-2020-28935
debian not yet assigned CVE-2020-28935 not yet assigned priority: Debian including 2 source packages (nsd, unbound), 10 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 10. https://security-tracker.debian.org/tracker/CVE-2020-28935
gentoo normal CVE-2020-28935: 1 GLSA(s) (202101-38), 1 atom(s) (net-dns/nsd); latest impact normal. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2020-28935
redhat low — https://access.redhat.com/security/cve/CVE-2020-28935
suse medium CVE-2020-28935 severity moderate: SUSE including 43 source package names (libunbound-devel-mini-1.6.8-10.6.1, libunbound2-1.6.8-10.6.1, …), 122 product×package rows across 42 product lines (SUSE CaaS Platform 4.0, SUSE Enterprise Storage 6, … (42 product lines)): Fixed 122. https://www.suse.com/security/cve/CVE-2020-28935/
ubuntu low CVE-2020-28935 low priority: Ubuntu including 1 source packages (unbound), 16 status rows across 16 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 10, needed 2, released 2, ignored 1, needs-triage 1. https://ubuntu.com/security/CVE-2020-28935

Affected software / configurations for CVE-2020-28935

Vendor Product Version Raw CPE
nlnetlabs name_server_daemon < 4.3.4 cpe:2.3:a:nlnetlabs:name_server_daemon:*:*:*:*:*:*:*:*
nlnetlabs unbound < 1.13.0 cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*

References for CVE-2020-28935

cvelogic Threat Intelligence