# CVE-2018-6557: Insecure temporary file use in base-files

> A local attacker can exploit insecure temporary file handling in the MOTD script to cause denial of service or escalate privileges if symlink restrictions are disabled.

- **Severity:** High 7.0 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 Aug 2018
- **Updated:** 16 Sep 2024
- **Also known as:** GHSA-vfrm-87wr-xvjf, UBUNTU-CVE-2018-6557, USN-3748-1

## What is CVE-2018-6557?

CVE-2018-6557 is a high-severity path traversal vulnerability in Canonical Ubuntu Linux, published on 21 Aug 2018. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-59 (Improper Link Resolution Before File Access)

## Which versions are affected by CVE-2018-6557?

It affects 2 products.


Products: Base-Files Project Base-Files, Canonical Ubuntu Linux.

## How do I fix CVE-2018-6557?

Apply the vendor's patch.

## Is CVE-2018-6557 being exploited?

There is no known exploitation of CVE-2018-6557 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2018-6557 and fix it if it is affected.

What it is: CVE-2018-6557 is a high-severity path traversal vulnerability in Canonical Ubuntu Linux, published on 21 Aug 2018. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2018-6557 in the wild and no public exploit code.

Affected product: Canonical Ubuntu Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2018-6557/ (Markdown: https://insights.sbomify.com/CVE-2018-6557/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2018-6557
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2018-6557
- OSV: https://osv.dev/vulnerability/CVE-2018-6557

---
From sbomify Insights: https://insights.sbomify.com/CVE-2018-6557/
Summaries are generated and may contain mistakes; the official description above is the record.