# CVE-2018-6789: Exim Buffer Overflow

> Remote attackers can execute arbitrary code by sending a crafted message to the Exim SMTP listener, triggering a buffer overflow in the base64d function.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Yes, since 3 Nov 2021 (CISA KEV), used in ransomware
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Feb 2018
- **Updated:** 21 Oct 2025
- **Also known as:** CLEANSTART-2025-AY16629, DEBIAN-CVE-2018-6789, DLA-1274-1, DSA-4110-1, GHSA-868p-wr6f-7jfr, UBUNTU-CVE-2018-6789, USN-3565-1

## What is CVE-2018-6789?

CVE-2018-6789 is a critical memory corruption vulnerability in Canonical Ubuntu Linux, published on 8 Feb 2018. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.

Weakness: CWE-120 (Buffer Copy without Checking Size of Input)

## Which versions are affected by CVE-2018-6789?

It affects 3 products.


Products: Canonical Ubuntu Linux, Debian Debian Linux, Exim Exim.

## How do I fix CVE-2018-6789?

Upgrade to a fixed version. US federal agencies must fix it by 3 May 2022.

## Is CVE-2018-6789 being exploited?

Yes. Attackers have exploited CVE-2018-6789 in the wild since at least 3 Nov 2021, including in ransomware campaigns.

Public exploit code:

- This repository provides a learning environment to understand how an Exim RCE exploit for CVE-2018-6789 works. (github_poc): https://github.com/martinclauss/exim-rce-cve-2018-6789
- PoC materials to exploit CVE-2018-6789 (github_poc): https://github.com/synacktiv/Exim-CVE-2018-6789
- https://github.com/beraphin/CVE-2018-6789 (github_poc): https://github.com/beraphin/CVE-2018-6789
- Exim < 4.90.1 RCE Vulnerability remake for Python3 with arguments passed from CLI (github_poc): https://github.com/thistehneisen/CVE-2018-6789-Python3
- CVE-2018-6789 (github_poc): https://github.com/c0llision/exim-vuln-poc
- Exim < 4.90.1 - 'base64d' Remote Code Execution (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-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)
- [CVE-2013-0422](https://insights.sbomify.com/CVE-2013-0422/): Oracle JRE Remote Code Execution (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2018-6789 and fix it if it is affected.

What it is: CVE-2018-6789 is a critical memory corruption vulnerability in Canonical Ubuntu Linux, published on 8 Feb 2018. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2018-6789 in the wild since at least 3 Nov 2021, including in ransomware campaigns.

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-2018-6789/ (Markdown: https://insights.sbomify.com/CVE-2018-6789/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2018-6789
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2018-6789
- OSV: https://osv.dev/vulnerability/CVE-2018-6789

---
From sbomify Insights: https://insights.sbomify.com/CVE-2018-6789/
Summaries are generated and may contain mistakes; the official description above is the record.