# CVE-2026-40684: Denial of service in Exim

> An attacker can crash Exim connections via malformed DNS PTR records on musl libc systems, requiring an upgrade to mitigate.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/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:** 30 Apr 2026
- **Updated:** 1 May 2026
- **Also known as:** CLSA-2026-1778174719, DEBIAN-CVE-2026-40684, GHSA-q496-q274-jgh9

## What is CVE-2026-40684?

CVE-2026-40684 is a medium-severity denial of service vulnerability in Exim Exim, published on 30 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In Exim before 4.99.2, on systems using musl libc (not glibc), an attacker can crash the connection instance when malformed DNS data is present in PTR records. This is caused by a dn_expand oddity in octal printing.

Weakness: CWE-684 (Incorrect Provision of Specified Functionality)

## Which versions are affected by CVE-2026-40684?

It affects 1 product.


Products: Exim Exim.

## How do I fix CVE-2026-40684?

Upgrade to a fixed version.

## Is CVE-2026-40684 being exploited?

There is no known exploitation of CVE-2026-40684 in the wild and no public exploit code.

## 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-2020-28018](https://insights.sbomify.com/CVE-2020-28018/): Use after free in n/a (critical 9.8)
- [CVE-2020-28020](https://insights.sbomify.com/CVE-2020-28020/): Integer overflow in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40684 and fix it if it is affected.

What it is: CVE-2026-40684 is a medium-severity denial of service vulnerability in Exim Exim, published on 30 Apr 2026. 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-2026-40684 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-2026-40684/ (Markdown: https://insights.sbomify.com/CVE-2026-40684/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40684
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40684
- OSV: https://osv.dev/vulnerability/CVE-2026-40684

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40684/
Summaries are generated and may contain mistakes; the official description above is the record.