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Ubuntu LinuxOperating system · Canonical

CVE-2018-6557

HIGH · 7.0 CVSS v3.1 Published 2018-08-21
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
72/100
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
The MOTD update script in the base-files package in Ubuntu 18.04 LTS before 10.1ubuntu2.2, and Ubuntu 18.10 before 10.1ubuntu6 incorrectly handled temporary files. A local attacker could use this issue to cause a denial of service, or possibly escalate privileges if kernel symlink restrictions were disabled.

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 · moderate confidence

The MOTD update script in Ubuntu's base-files package contains a time-of-check-to-time-of-use (TOCTOU) vulnerability in its temporary file handling. A local attacker can exploit this by creating malicious symlinks before the script creates its temporary files, potentially causing denial of service or privilege escalation when the script follows the attacker-controlled symlink.

MitigationUpdate base-files to version 10.1ubuntu2.2 (Ubuntu 18.04 LTS) or 10.1ubuntu6 (Ubuntu 18.10). Ensure kernel symlink restrictions are enabled (sysctl -w kernel.protected_symlinks=1). Consider migrating to supported Ubuntu versions if patches are unavailable.

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
Ubuntu LinuxOperating system
Affected:= 18.04= 18.10
Base FilesApplication
Affected:= 10.1ubuntu2.2

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
High
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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 Ubuntu version
    Run 'lsb_release -r' or check /etc/lsb-release
    Affected if The system is running Ubuntu 18.04 or Ubuntu 18.10
  2. Check base-files package version
    Run 'dpkg -l base-files' or 'apt show base-files'
    Affected if The installed version is 10.1ubuntu2.2 or lower for 18.04, or 10.1ubuntu6 or lower for 18.10, matching the affected version ranges
  3. Locate MOTD update script
    Check for the presence of /etc/update-motd.d/ scripts or examine the motd configuration in /etc/default/
    Affected if The system uses the MOTD update script from the base-files package, which is the default on Ubuntu 18.04/18.10

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Update base-files to version 10.1ubuntu2.2 (Ubuntu 18.04 LTS) or 10.1ubuntu6 (Ubuntu 18.10). Ensure kernel symlink restrictions are enabled (sysctl -w kernel.protected_symlinks=1). Consider migrating to supported Ubuntu versions if patches are unavailable.

Fix this in Ubuntu Linux Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
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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-2018-6557 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data