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No exploitation reportedFix available

CVE-2020-28935

No exploitation reported by the sources we read.

  • Published2020-12-07
  • WeaknessCWE-59

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2020-12-0722:15CVE publishedNVD

Versions per branch

NVD
Debian debian linux
BranchAffectedFixed in
9.0= 9.0no fix published
Nlnetlabs name server daemon
BranchAffectedFixed in
4.3< 4.3.44.3.4
Nlnetlabs unbound
BranchAffectedFixed in
1.13< 1.13.01.13.0

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • nsdSUSE:Package Hub 15 SP2all versions · fixed in 4.3.4-bp152.2.3.1
  • nsdSUSE:Package Hub 15 SP1all versions · fixed in 4.3.4-bp152.2.3.1
  • nsdSUSE:Package Hub 12all versions · fixed in 4.3.4-bp152.2.3.1
  • nsdopenSUSE:Tumbleweedall versions · fixed in 4.3.7-1.2
  • nsdopenSUSE:Leap 15.2all versions · fixed in 4.3.4-bp152.2.3.1
  • nsdopenSUSE:Leap 15.1all versions · fixed in 4.3.4-bp152.2.3.1
  • nsdDebian:14all versions · fixed in 4.3.4-1
  • nsdDebian:13all versions · fixed in 4.3.4-1

55 more package ranges on OSV.dev

Vendor advisories

  • RHSA-2022:0632 Red Hat Product Security39 products fixed

    Red Hat Security Advisory: unbound security update

  • RHSA-2021:1853 Red Hat Product Security39 products fixed

    Red Hat Security Advisory: unbound security, bug fix, and enhancement update

Full record on NVD

Fix

  1. Vendor fix Red Hat Product Security RHSA-2022:0632 · 39 productsFor details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258
  2. Workaround from the vendor Red Hat Product Security RHSA-2022:0632 · 39 productsIf SELinux is enabled in Enforcing mode (the default value in Red Hat Enterprise Linux 8), this kind of attack is prevented as unbound would be blocked from accessing the symbolic link file.

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • NVDCVSS 3.15.5read 2026-10-23
  • NVDCVSS 2.02.1read 2026-10-23
  • CNAweakness (CWE)CWE-59known since 2026-10-23
  • NVDweakness (CWE)CWE-59read 2026-10-23
  • FIRST EPSSEPSS score0.5 %2026-11-09
  • OSVaffected packages—
  • GitHub Advisoriesadvisory—

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Published
2020-12-07
Last Modified
2026-06-17
NVD status
Modified
GitHub Advisory
GHSA-q92f-7cwj-w4w5

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.

Is CVE-2020-28935 actively exploited?

Active exploitation of CVE-2020-28935 has not been confirmed. Its EPSS score was 0.5% on 2026-11-09, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2020-28935?

CVE-2020-28935 has a CVSS v3 base score of 5.5 (MEDIUM severity), with vector string CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

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