How to Fix CVE-2026-40686: Exim (Bundle Sibling)
By Sai Kiran Pandrala
Last verified: 2026-05-25
CVE-2026-40686 is a sibling vulnerability in the same vendor advisory as CVE-2026-40684. Applying the patched build named in the primary write-up closes this CVE as well.
| Severity | CVSS 3.7 - Low |
|---|---|
| Actively exploited? | Not currently in CISA KEV |
| Affected | Same as the bundle - see CVE-2026-40684 |
| Fixed in | Same patched build as CVE-2026-40684 (See vendor advisory) |
| Type (CWE) | CWE-125: CWE-125 Out-of-bounds Read |
What's different about CVE-2026-40686?
In Exim before 4.99.2, when utf8 operators are enabled, there is an out-of-bounds read if large UTF-8 trailing characters are present (malformed UTF-8 header data). Information might be divulged within an error message produced during handling of an unrelated e-mail message.
The technical impact and remediation are identical to the primary CVE in the bundle. The same vendor patch closes both.
How to fix CVE-2026-40686
Apply the patched build per the primary write-up: How to Fix CVE-2026-40684.
The patch installation procedure, verification commands, and interim mitigations are documented there. Reusing one runbook keeps the rollout consistent across the bundle.
Frequently asked questions
Is CVE-2026-40686 fixed by the same patch as CVE-2026-40684?
Yes. CVE-2026-40686 ships in the same vendor advisory as CVE-2026-40684. Applying the patched build named in the primary write-up closes both.
What is the CVSS score for CVE-2026-40686?
The CVSS base score is 3.7 (Low).
Is it being exploited?
It is not currently listed in CISA KEV.
References
- Official vendor advisory: https://www.openwall.com/lists/oss-security/2026/04/30/21
- NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-40686
- CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
- Primary write-up: How to Fix CVE-2026-40684
*Part of the Exim bundle. Full procedure at CVE-2026-40684.*