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Debian LinuxOperating system · Debian

CVE-2020-28935

MEDIUM · 5.5 CVSS v3.1 Published 2020-12-07
Fix available
A fix is available. Upgrade to 1.13.0 / 4.3.4 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

NLnet Labs Unbound (up to 1.12.0) and NSD (up to 4.3.3) have a local symlink vulnerability in PID file handling. When writing PID files, these services follow symlinks if the file happens to be a symlink rather than a regular file. An attacker with access to the limited user account can create a symlink pointing to a critical system file; when the service restarts with root privileges, it overwrites the target file, causing potential Denial of Service.

MitigationUpgrade Unbound to version 1.12.1 or later and NSD to version 4.3.4 or later, which include symlink checks before writing PID files. Alternatively, implement manual checks to ensure PID files are regular files before service restart.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Debian LinuxOperating system
Affected:= 9.0
Name Server DaemonApplication
Affected:< 4.3.4
UnboundApplication
Affected:< 1.13.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Unbound version
    Run `unbound -V 2>&1` or `unbound-control status` and locate the version number in the output
    Affected if Version is below 1.13.0
  2. Identify installed NSD version
    Run `nsd -v 2>&1` or `nsd -h` and locate the version number in the output
    Affected if Version is below 4.3.4
  3. Locate PID files for Unbound and NSD
    Check common PID file locations: typically `/var/run/unbound/unbound.pid` for Unbound and `/var/run/nsd/nsd.pid` for NSD. Use `ls -la <pid_file_path>` to see if the file is a symlink
    Affected if The PID file is a symbolic link pointing to another file path
  4. Check PID directory permissions
    Run `ls -la /var/run/` and inspect the unbound and NSD subdirectories. Verify which users have write access to these directories
    Affected if A limited user account has write access to the directory containing PID files
  5. Verify service execution context
    Check if services run as root by examining init scripts, systemd unit files, or running `ps aux | grep -E 'unbound|nsd'`
    Affected if The services execute with root privileges while allowing unprivileged users to modify PID file locations

User is affected if Unbound version is below 1.13.0 or NSD version is below 4.3.4, and a low-privileged user can create symlinks in PID file directories before the service restarts with root privileges.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Upgrade available Upgrade to 1.13.0 / 4.3.4 or later
Fixed in 1.13.04.3.4
Interim mitigation

Upgrade Unbound to version 1.12.1 or later and NSD to version 4.3.4 or later, which include symlink checks before writing PID files. Alternatively, implement manual checks to ensure PID files are regular files before service restart.

Fix this in Debian Linux Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2020-28935 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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