CVE-2023-42117
Exim ≤ 4.96.2
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
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, ranked in the top 6% 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
- 🇪🇺 ENISA EUVD: EUVD-2023-46576
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)
Related Threats
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Input validation enforces neutralization or rejection of special syntactic elements before they reach a downstream interpreter.
Security engineering principles include requirements for safe input handling and neutralization of control characters.
Output filtering can neutralize special elements before they are interpreted by a receiving component.
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.8.25 mandates secure SDLC practices including secure coding guidelines, security requirements, checkpoints, testing (scans/penetration), and developer training that reach improper neutralization (CWE-138) in first-party code, but leaves a bounded remainder (e.g. third-party/supplied components, incomplete enforcement of guidelines, or outsourced development without full assurance).
Application security requirements explicitly call for controls against injection and malformed input that cause CWE-138.
Secure system architecture principles reduce attack surface but do not prescribe the specific neutralization techniques needed for CWE-138.
Secure coding standards require proper escaping and neutralization of special elements, directly eliminating CWE-138.
Security testing can detect instances of CWE-138 but does not itself implement the neutralization controls.
Hardening callouts derived
DISA STIG baselines are authored against specific weaknesses. Where this CVE cites a broad weakness class, the mapping is not asserted at that abstraction (owner ruling, 2026-09-25: grade only at Base and below), so no rule is shown here rather than a rule shown against the wrong grain.
- No fine-grained STIG row for CWE-138, a class-level weakness, and none of its children carries one either.