Buffer overflow in Exim
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H.
EPSS exploit prediction
EPSS: 0.006 (46.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 46th percentile — 46.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.006 · change: +0.003
Affected products
- Exim — versions 0
Weakness classification (CWE)
References
- www.openwall.com/lists/oss-security/2026/04/30/21 (Mailing List, Third Party Advisory)
- exim.org/static/doc/security/cve-2026-04.1/CVE2026-40685.assessment (Vendor Advisory)
- code.exim.org/exim/exim/commit/9fdc057e71b87c87a0d3d2288b2810a0efaaba57 (Patch)
- exim.org/static/doc/security/CVE-2026-40685.txt (Broken Link)
Frequently asked questions
- What is CVE-2026-40685?
- CVE-2026-40685 is a medium-severity vulnerability in Exim, classified under CWE-684. CVSS score: 6.5/10. Published 2026-04-30.
- How severe is CVE-2026-40685?
- Medium severity. CVSS v3 base score is 6.5 out of 10.