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NVD / CVE Program· CNA NLnet Labs

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

Noroxi assessment

Priority: Monitor (action score 22/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.49% (FIRST EPSS).
  • Someone with local access to the system can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: nsd (Alpine:v3.12) 4.3.4-r0; unbound (Debian:10) 1.9.0-2+deb10u3; nsd (Debian:12) 4.3.4-1.
  • Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to nsd (Alpine:v3.12) 4.3.4-r0; unbound (Debian:10) 1.9.0-2+deb10u3; nsd (Debian:12) 4.3.4-1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 5.5 · v3.1—No exploit Fix available
Published
Dec 7, 2020
Updated
Jun 16, 2026
EPSS
0.4% · 42th percentile

Report tools

JSON

Action score

22

Monitor

Low priority for now.

CVSS
22 / 40 · 5.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.4%

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  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
nlnetlabsname server daemon
nlnetlabsunbound
debiandebian linux

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • debian debian linux9.0
  • nlnetlabs name server daemonbefore 4.3.4
  • nlnetlabs unboundbefore 1.13.0

Versions reported by the vendor

Affected version ranges reported by the assigning authority (NLnet Labs). Independent of NVD's CPE analysis and usually ahead of it.

  • NLnet Labs NSD

    • <= 4.3.3affected

  • NLnet Labs Unbound

    • <= 1.12.0affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Alpine:v3.12nsdbefore 4.3.4-r04.3.4-r0
Debian:10unboundbefore 1.9.0-2+deb10u31.9.0-2+deb10u3
Debian:12nsdbefore 4.3.4-14.3.4-1
Debian:12unboundbefore 1.13.0-11.13.0-1
Debian:9unbound1.9before 1.9.0-2+deb10u2~deb9u11.9.0-2+deb10u2~deb9u1
openSUSE:Leap 15.1nsdbefore 4.3.4-bp152.2.3.14.3.4-bp152.2.3.1
openSUSE:Leap 15.3unboundbefore 1.6.8-10.6.11.6.8-10.6.1
openSUSE:Tumbleweednsdbefore 4.3.7-1.24.3.7-1.2
openSUSE:Tumbleweedsslhbefore 2.2.3-1.12.2.3-1.1
openSUSE:Tumbleweedunboundbefore 1.13.2-1.21.13.2-1.2
Red Hat:enterprise_linux:8::appstreampython3-unboundbefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreampython3-unbound-debuginfobefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreamunboundbefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreamunbound-debuginfobefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreamunbound-debugsourcebefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreamunbound-develbefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreamunbound-libsbefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:enterprise_linux:8::appstreamunbound-libs-debuginfobefore 0:1.7.3-15.el80:1.7.3-15.el8
Red Hat:rhel_eus:8.2::appstreampython3-unboundbefore 0:1.7.3-12.el8_20:1.7.3-12.el8_2
Red Hat:rhel_eus:8.2::appstreampython3-unbound-debuginfobefore 0:1.7.3-12.el8_20:1.7.3-12.el8_2
Red Hat:rhel_eus:8.2::appstreamunboundbefore 0:1.7.3-12.el8_20:1.7.3-12.el8_2
Red Hat:rhel_eus:8.2::appstreamunbound-debuginfobefore 0:1.7.3-12.el8_20:1.7.3-12.el8_2
Red Hat:rhel_eus:8.2::appstreamunbound-debugsourcebefore 0:1.7.3-12.el8_20:1.7.3-12.el8_2
Red Hat:rhel_eus:8.2::appstreamunbound-develbefore 0:1.7.3-12.el8_20:1.7.3-12.el8_2

+6

Same product

nlnetlabs: all records

Other highest-scoring records for the same primary product.

  • CVE-2019-13207nsd-checkzone in NLnet Labs NSD 4.2.0 has a Stack-based Buffer Overflow in the dname_concatenate() function in dname.c.
    40Plan

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
alpine:nsd4.3.4-r0 · Alpine:v3.12Package registry (OSV)
debian:nsd4.3.4-1 · Debian:12Package registry (OSV)
debian:unbound1.13.0-1 · Debian:12Package registry (OSV)
debian:unbound1.91.9.0-2+deb10u2~deb9u1 · Debian:9Package registry (OSV)
opensuse:nsd4.3.4-bp152.2.3.1 · openSUSE:Leap 15.1Package registry (OSV)
opensuse:sslh2.2.3-1.1 · openSUSE:TumbleweedPackage registry (OSV)
opensuse:unbound1.6.8-10.6.1 · openSUSE:Leap 15.3Package registry (OSV)
red hat:python3-unbound0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:python3-unbound-debuginfo0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:unbound0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:unbound-debuginfo0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:unbound-debugsource0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:unbound-devel0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:unbound-libs0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:unbound-libs-debuginfo0:1.7.3-15.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
rocky linux:unbound0:1.7.3-15.el8 · Rocky Linux:8Package registry (OSV)
suse:nsd4.3.4-bp152.2.3.1 · SUSE:Package Hub 12Package registry (OSV)
suse:unbound1.6.8-10.6.1 · SUSE:Linux Enterprise Module for Basesystem 15 SP3Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

All national notices →

Technical details

Attack conditions

  • Someone with local access to the system can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
none
Availability
high · the service can be disrupted
Attack vector
Local
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
None
Availability impact
High

Weakness class (CWE)

CWE-59 · Improper Link Resolution Before File Access ('Link Following')

CVSS vector

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

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (4 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records