CVE-2026-40685

Description

In Exim before 4.99.2, when JSON lookup is enabled, an out-of-bounds heap write can occur when a JSON operator encounters malformed JSON in an untrusted header, because of an incorrect implementation of \ skipping.

CVSS v3.1 Score

6.5

MEDIUM

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

EPSS — Exploit Prediction

0.0012

Probability of exploitation

0.30%

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

CWE-787 Out-of-bounds Write

Affected Products

Vendor	Product	Versions
exim	exim	< 4.99.2

References

Advisories & Patches

https://code.exim.org/exim/exim/commit/9fdc057e71b87c87a0d3d2288b2810a0efaaba57 https://exim.org/static/doc/security/cve-2026-04.1/CVE2026-40685.assessment

Other References

https://exim.org/static/doc/security/CVE-2026-40685.txt https://www.openwall.com/lists/oss-security/2026/04/30/21

Frequently Asked Questions

What is CVE-2026-40685? +

In Exim before 4.99.2, when JSON lookup is enabled, an out-of-bounds heap write can occur when a JSON operator encounters malformed JSON in an untrusted header, because of an incorrect implementation of \ skipping. It has a CVSS v3.1 base score of 6.5 (MEDIUM).

How severe is CVE-2026-40685? +

CVE-2026-40685 has a CVSS v3.1 score of 6.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0012, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-40685? +

CVE-2026-40685 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-2026-40685? +

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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