# CVE-2018-18955: Privilege escalation in n/a

> A local attacker with CAP_SYS_ADMIN in a user namespace can bypass access controls to read sensitive files like /etc/shadow due to mishandled ID transformations.

- **Severity:** High 7.0 (`CVSS:3.0/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:** 16 Nov 2018
- **Updated:** 5 Aug 2024
- **Also known as:** DEBIAN-CVE-2018-18955, GHSA-c6xj-3c77-g5rg, UBUNTU-CVE-2018-18955

## What is CVE-2018-18955?

CVE-2018-18955 is a high-severity vulnerability in Canonical Ubuntu Linux, published on 16 Nov 2018. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel 4.15.x through 4.19.x before 4.19.2, map_write() in kernel/user_namespace.c allows privilege escalation because it mishandles nested user namespaces with more than 5 UID or GID ranges. A user who has CAP_SYS_ADMIN in an affected user namespace can bypass access controls on resources outside the namespace, as demonstrated by reading /etc/shadow. This occurs because an ID transformation takes place properly for the namespaced-to-kernel direction but not for the kernel-to-namespaced direction.

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2018-18955?

It affects 2 products.


Products: Canonical Ubuntu Linux, Linux kernel.

## How do I fix CVE-2018-18955?

Apply the vendor's patch.

## Is CVE-2018-18955 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2018-18955 exists.

Public exploit code:

- 💀 Linux local root exploit for CVE-2018-18955 (github_poc): https://github.com/scheatkode/CVE-2018-18955
- Linux Nested User Namespace idmap Limit Local Privilege Escalation (metasploit)
- Linux - Broken uid/gid Mapping for Nested User Namespaces (exploitdb)
- Linux Kernel 4.15.x < 4.19.2 - 'map_write() CAP_SYS_ADMIN' Local Privilege Escalation (cron Method) (exploitdb)
- Linux Kernel 4.15.x < 4.19.2 - 'map_write() CAP_SYS_ADMIN' Local Privilege Escalation (ldpreload Method) (exploitdb)
- Linux - Nested User Namespace idmap Limit Local Privilege Escalation (Metasploit) (exploitdb)

## Other vulnerabilities in Canonical Ubuntu Linux

- [CVE-2020-11651](https://insights.sbomify.com/CVE-2020-11651/): SaltStack Salt Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2020-7247](https://insights.sbomify.com/CVE-2020-7247/): OpenSMTPD Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2019-16928](https://insights.sbomify.com/CVE-2019-16928/): Exim Out-of-bounds Write (critical 9.8, exploited in the wild)
- [CVE-2018-6789](https://insights.sbomify.com/CVE-2018-6789/): Exim Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2015-2590](https://insights.sbomify.com/CVE-2015-2590/): Oracle Java SE and Java SE Embedded Remote Code Execution (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2018-18955 and fix it if it is affected.

What it is: CVE-2018-18955 is a high-severity vulnerability in Canonical Ubuntu Linux, published on 16 Nov 2018. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2018-18955 exists.

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-2018-18955/ (Markdown: https://insights.sbomify.com/CVE-2018-18955/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2018-18955
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2018-18955
- OSV: https://osv.dev/vulnerability/CVE-2018-18955

---
From sbomify Insights: https://insights.sbomify.com/CVE-2018-18955/
Summaries are generated and may contain mistakes; the official description above is the record.