CVE-2026-40684
MEDIUMDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Low risk: more likely to be exploited than 29% of all known CVEs.
References
Related Vulnerabilities
Other CWE-684 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2023-5363 | High | 7.5 | 3.3% | - | Fix |
| CVE-2025-47227 | High | 7.5 | 2.0% | - | - |
| CVE-2020-11054 | Low | 3.5 | 1.5% | - | Fix |
| CVE-2026-34478 | High | 7.5 | 0.8% | - | Fix |
| CVE-2023-24845 | Critical | 9.1 | 0.7% | - | - |
| CVE-2024-50357 | Critical | 9.8 | 0.6% | - | - |
Common questions
How do I fix CVE-2026-40684?
Upgrade exim4 on debian 12 to 4.96-15+deb12u9 or later.
Is CVE-2026-40684 being actively exploited?
Not that we know of. CVE-2026-40684 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.36% is the estimated probability that it will be exploited in the next 30 days. That is higher than 29% of all scored CVEs.
How severe is CVE-2026-40684?
CVE-2026-40684 has a CVSS v3 base score of 5.9, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-40684 affect?
Published advisories record a fix for exim4 (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
Embed a live status badge for CVE-2026-40684
Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-05-01.