CVE-2026-40684

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.

CVSS v3.1 Score

5.9

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS — Exploit Prediction

0.0017

Probability of exploitation

0.38%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-684 CWE-684

Affected Products

Vendor	Product	Versions
exim	exim	< 4.99.2

References

Advisories & Patches

https://code.exim.org/exim/exim/commit/628bbaca7672748d941a12e7cd5f0122a4e18c81 https://exim.org/static/doc/security/cve-2026-04.1/CVE2026-40684.assessment

Other References

https://exim.org/static/doc/security/CVE-2026-40684.txt https://www.openwall.com/lists/oss-security/2026/04/30/21 http://www.openwall.com/lists/oss-security/2026/05/01/11

Frequently Asked Questions

What is CVE-2026-40684? +

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. It has a CVSS v3.1 base score of 5.9 (MEDIUM).

How severe is CVE-2026-40684? +

CVE-2026-40684 has a CVSS v3.1 score of 5.9 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0017, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-40684? +

CVE-2026-40684 affects products from exim, specifically: exim. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-40684? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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