# CVE-2020-28018: Use after free in n/a

> A use-after-free vulnerability in Exim's smtp_reset function allows attackers to potentially execute arbitrary code or cause a denial of service.

- **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:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 May 2021
- **Updated:** 4 Aug 2024
- **Also known as:** CLEANSTART-2025-XY17493, DEBIAN-CVE-2020-28018, GHSA-5hqr-vx33-mfmh, UBUNTU-CVE-2020-28018

## What is CVE-2020-28018?

CVE-2020-28018 is a critical use after free vulnerability in Exim Exim, published on 6 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Exim 4 before 4.94.2 allows Use After Free in smtp_reset in certain situations that may be common for builds with OpenSSL.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2020-28018?

It affects 1 product.


Products: Exim Exim.

## How do I fix CVE-2020-28018?

Upgrade to a fixed version.

## Is CVE-2020-28018 being exploited?

There is no known exploitation of CVE-2020-28018 in the wild and no public exploit code.

Public exploit code:

- exim use after free exploit and detection (github_poc): https://github.com/dorkerdevil/CVE-2020-28018

## Other vulnerabilities in Exim Exim

- [CVE-2026-45185](https://insights.sbomify.com/CVE-2026-45185/): Use after free in Exim (critical 9.8)
- [CVE-2023-42115](https://insights.sbomify.com/CVE-2023-42115/): Exim AUTH Out-Of-Bounds Write Remote Code Execution Vulnerability (critical 9.8)
- [CVE-2020-28017](https://insights.sbomify.com/CVE-2020-28017/): Integer overflow in n/a (critical 9.8)
- [CVE-2020-28020](https://insights.sbomify.com/CVE-2020-28020/): Integer overflow in n/a (critical 9.8)
- [CVE-2020-28022](https://insights.sbomify.com/CVE-2020-28022/): Memory corruption in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-28018 and fix it if it is affected.

What it is: CVE-2020-28018 is a critical use after free vulnerability in Exim Exim, published on 6 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2020-28018 in the wild and no public exploit code.

Affected product: Exim Exim. 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-2020-28018/ (Markdown: https://insights.sbomify.com/CVE-2020-28018/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-28018
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-28018
- OSV: https://osv.dev/vulnerability/CVE-2020-28018

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-28018/
Summaries are generated and may contain mistakes; the official description above is the record.