# CVE-2026-40684: In Exim before 4.99.2, on systems using musl libc (not glibc), an attacker can crash the connection instance when…

Source: https://junglewise.ai/threats/cve-2026-40684-in-exim-before-4-99-2-on-systems-using-musl-libc-not-glibc-an  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-30  
License: CC BY 4.0 with a link back

CVE-2026-40684 · Severity: medium · CVSS 5.9 · Published 2026-04-30

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.

Technologies: [Exim](https://junglewise.ai/threats/technologies/exim). Vendors: [Exim](https://junglewise.ai/threats/vendors/exim).

## Quick answers

**Is CVE-2026-40684 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-40684 being exploited in the wild.

**How severe is CVE-2026-40684?** Medium severity, with a CVSS score of 5.9, published 30 April 2026.

**Which products and versions are affected?** Exim. The advisory does not name versions.

**Is there a fix for CVE-2026-40684?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**Am I affected by CVE-2026-40684?** Check whether you run Exim. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Affected products

- Exim

## Related threats

- [Exim SMTP smuggling in DATA phase rejection handling](https://junglewise.ai/threats/cve-2026-94057-exim-before-4-100-1-allows-smtp-smuggling-in-which-the-received)
- [Exim Proxy Protocol stack memory disclosure](https://junglewise.ai/threats/cve-2026-94056-exim-before-4-100-1-when-proxy-protocol-is-used-with-an-attacker)
- [Exim before 4.100.1, when certain non-default TLS settings are used with GnuTLS, has a use-after-free.](https://junglewise.ai/threats/cve-2026-94055-exim-before-4-100-1-when-certain-non-default-tls-settings-are)
- [Exim out-of-bounds write in Proxy Protocol v1](https://junglewise.ai/threats/cve-2026-94054-exim-before-4-100-1-when-proxy-protocol-is-used-with-an-attacker)
- [Exim privilege escalation in .forward pipe transport force_command](https://junglewise.ai/threats/cve-2026-66141-exim-privilege-escalation-in-forward-pipe-transport-force-command)