Cyber Resilience

CVE-2023-42117

Exim ≤ 4.96.2

Published
03 May 2024
Modified
03 November 2025
CVSS Score v3.1 9.8
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS Score 0.057 92th percentile
Risk Priority 80 floored blend · peak EPSS

Summary

CVE-2023-42117 is a critical-severity Improper Neutralization of Special Elements (CWE-138) vulnerability in Exim Exim. Its CVSS base score is 9.8 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 8% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) and SA-8 (Security and Privacy Engineering Principles) — 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-2023-42117 is an improper neutralization of special elements vulnerability in Exim that leads to remote code execution. It affects the SMTP service listening on TCP port 25 and stems from insufficient validation of user-supplied data, resulting in a memory corruption condition that allows arbitrary code execution in the context of the Exim process. The issue was originally reported as ZDI-CAN-17554 and carries a CVSS 3.1 base score of 9.8.

Remote, unauthenticated attackers can exploit the flaw over the network by sending crafted input to the SMTP service, achieving code execution without any user interaction or privileges. The vulnerability impacts any exposed Exim installation that has not been patched.

Advisories from the Zero Day Initiative (ZDI-23-1471) and Debian LTS detail the issue and direct administrators to updated Exim packages.

The EPSS score rose from a low baseline to a peak of 0.1090 on 2025-12-11 before receding, indicating that exploitation interest increased after public disclosure.

EU & UK References

Vulnerability Data

Exim Improper Neutralization of Special Elements Remote Code Execution Vulnerability. This vulnerability allows remote attackers to execute arbitrary code on affected installations of Exim. Authentication is not required to exploit this vulnerability. The specific flaw exists within the smtp service,…

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which listens on TCP port 25 by default. The issue results from the lack of proper validation of user-supplied data, which can result in a memory corruption condition. An attacker can leverage this vulnerability to execute code in the context of the current process. Was ZDI-CAN-17554.

CWE(s)
CWE-138

Related Threats

MITRE ATT&CK Enterprise Techniques

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 →
T1059 Command and Scripting Interpreter Execution
Adversaries may abuse command and script interpreters to execute commands, scripts, or binaries.
attack.mitre.org →
T1059.001 PowerShell Execution
Adversaries may abuse PowerShell commands and scripts for execution.
attack.mitre.org →
T1059.003 Windows Command Shell Execution
Adversaries may abuse the Windows command shell for execution.
attack.mitre.org →
T1059.004 Unix Shell Execution
Adversaries may abuse Unix shell commands and scripts for execution.
attack.mitre.org →
T1505 Server Software Component Persistence
Adversaries may abuse legitimate extensible development features of servers to establish persistent access to systems.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2024-39929Same product: Exim Exim
CVE-2025-26794Same product: Exim Exim
CVE-2026-45185Same product: Exim Exim
CVE-2025-30232Same product: Exim Exim
CVE-2020-28018Same product: Exim Exim
CVE-2023-42116Same product: Exim Exim
CVE-2025-67896Same product: Exim Exim
CVE-2023-42114Same product: Exim Exim
CVE-2023-42119Same product: Exim Exim
CVE-2026-40686Same product: Exim Exim

Affected Assets

exim
exim
≤ 4.96.2

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.3.12

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

Input validation enforces neutralization or rejection of special syntactic elements before they reach a downstream interpreter.

SA-8 Security and Privacy Engineering Principles partial match

Security engineering principles include requirements for safe input handling and neutralization of control characters.

SI-15 Information Output Filtering partial match

Output filtering can neutralize special elements before they are interpreted by a receiving component.

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 require proper neutralization of inputs to prevent injection 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 can detect instances of CWE-138 but does not itself implement the neutralization controls.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates input validation and sanitization that directly prevents improper neutralization of special elements.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly call for controls against injection and malformed input that cause CWE-138.

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

Secure system architecture principles reduce attack surface but do not prescribe the specific neutralization techniques needed for CWE-138.

A.8.28 Secure coding partial match
prevents

Secure coding standards require proper escaping and neutralization of special elements, directly eliminating CWE-138.

References