Cyber Resilience

CVE-2026-40684

Exim ≤ 4.99.2

Published
30 April 2026
Modified
01 May 2026
Patch / advisory
View advisory
CVSS Score v3.1 5.9
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0036 29th percentile
Risk Priority 45 floored blend · peak EPSS

Summary

CVE-2026-40684 is a medium-severity Incorrect Provision of Specified Functionality (CWE-684) vulnerability in Exim Exim. Its CVSS base score is 5.9 (Medium).

Operationally, ranked at the 29th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2026-40684 affects Exim versions before 4.99.2 on systems using musl libc, excluding those with glibc. The vulnerability stems from an oddity in the dn_expand function during octal printing, enabling an attacker to crash a connection instance when malformed DNS data appears in PTR records. Published on 2026-04-30, it carries a CVSS v3.1 base score of 5.9 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H) and is linked to CWE-684.

A remote attacker with network access can exploit this without privileges or user interaction, though it demands high attack complexity. Exploitation crashes the targeted connection instance, resulting in a denial-of-service impact limited to availability disruption for that specific session.

Exim advisories recommend upgrading to version 4.99.2 or later for mitigation, with the fix implemented in commit 628bbaca7672748d941a12e7cd5f0122a4e18c81 available at https://code.exim.org/exim/exim/commit/628bbaca7672748d941a12e7cd5f0122a4e18c81. Further details appear in https://exim.org/static/doc/security/CVE-2026-40684.txt and https://exim.org/static/doc/security/cve-2026-04.1/CVE2026-40684.assessment, alongside announcements on oss-security lists at https://www.openwall.com/lists/oss-security/2026/04/30/21 and http://www.openwall.com/lists/oss-security/2026/05/01/11.

EU & UK References

Vulnerability Data

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.

CWE(s)
CWE-684

Related Threats

CVEs Like This One

CVE-2026-40685Same product: Exim Exim
CVE-2023-42114Same product: Exim Exim
CVE-2023-42115Same product: Exim Exim
CVE-2023-42119Same product: Exim Exim
CVE-2026-45185Same product: Exim Exim
CVE-2025-30232Same product: Exim Exim
CVE-2023-42117Same product: Exim Exim
CVE-2026-40686Same product: Exim Exim
CVE-2020-28018Same product: Exim Exim
CVE-2025-26794Same product: Exim Exim

Affected Assets

exim
exim
≤ 4.99.2

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)
  • 1 hardening rule · 1 OS baseline
Validate
Prove the fix (OWASP ASVS)

—

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation activities are performed precisely to discover when delivered functionality fails to match published specifications.

SA-15 Development Process, Standards, and Tools partial match

Requiring a documented development process and supporting tools reduces the chance that specified functionality will be implemented incorrectly.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly enforce spec-compliant functionality and testing, while eliminating this weakness fulfills only part of the control's broader monitoring intent.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development and acceptance validates that implemented functionality conforms to specifications before release.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle includes requirements and verification activities that directly reduce the risk of delivering functionality that deviates from published specifications.

A.8.26 Application security requirements partial match
prevents

Explicit application security requirements and acceptance criteria help ensure the delivered software matches its intended specifications.

A.8.32 Change management partial match
prevents

Change management processes reduce the chance of unintended functional deviations during updates, though they do not directly address initial specification conformance.

Hardening callouts derived

Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).

RHEL 9 (1 rule)
  • V-258078 RHEL 9 must use a Linux Security Module configured to enforce limits on system services. prevents CWE-684

References