Cyber Resilience

CVE-2026-40686

Memory Safety in Exim ≤ 4.99.2

Memory Safety
Published
30 April 2026
Modified
01 May 2026
Patch / advisory
View advisory
CVSS Score v3.1 3.7
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
EPSS Score 0.0025 16th percentile
Risk Priority 31 floored blend · peak EPSS

Summary

CVE-2026-40686 is a low-severity Out-of-bounds Read (CWE-125) vulnerability in Exim Exim. Its CVSS base score is 3.7 (Low).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 16th 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-8 (Security and Privacy Engineering Principles) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

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…

more

message.

CWE(s)
CWE-125

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1211 Exploitation for Stealth Stealth
Adversaries may exploit vulnerabilities to evade detection by hiding activity, suppressing logging, or operating within trusted or unmonitored components.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-42114Same product: Exim Exim
CVE-2023-42119Same product: Exim Exim
CVE-2026-40685Same product: Exim Exim
CVE-2023-42115Same product: Exim Exim
CVE-2023-42116Same product: Exim Exim
CVE-2026-45185Same product: Exim Exim
CVE-2025-30232Same product: Exim Exim
CVE-2025-67896Same product: Exim Exim
CVE-2020-28018Same product: Exim Exim
CVE-2024-39929Same product: Exim Exim

Affected Assets

exim
exim
≤ 4.99.2

Mitigating Controls

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation directly finds out-of-bounds read flaws through static analysis, fuzzing, and dynamic bounds checks.

SA-8 Security and Privacy Engineering Principles partial match

Secure engineering principles require bounds checking and memory-safe constructs that stop out-of-bounds reads from being introduced.

SC-39 Process Isolation partial match

Process isolation confines the effects of an out-of-bounds read to the compromised process.

SI-10 Information Input Validation partial match

Input validation rejects malformed indices or lengths that would otherwise cause reads outside buffer bounds.

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-development practices such as bounds checking and memory-safe languages directly prevent out-of-bounds reads.

ID.RA-01 partial match
prevents

Vulnerability scanning and recording can discover instances of out-of-bounds reads after code is deployed.

PR.PS-02 partial match
prevents

Routine patching replaces vulnerable code containing out-of-bounds read flaws.

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 includes fuzzing and static analysis that detect out-of-bounds read defects before release.

A.8.15 Logging partial match
finds

Logging can record evidence of an out-of-bounds read but does not prevent the weakness itself.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates input validation and bounds checking that directly prevent out-of-bounds reads.

A.8.26 Application security requirements partial match
prevents

Application security requirements include explicit bounds and memory-safety specifications that mitigate buffer over-reads.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure system architecture and engineering principles require memory-safe design patterns and runtime protections against out-of-bounds access.

A.8.28 Secure coding partial match
prevents

Secure coding standards explicitly forbid unsafe pointer arithmetic and mandate bounds-checked reads, eliminating CWE-125.

References