CVE-2026-27443
Published: 04 March 2026
Summary
CVE-2026-27443 is a high-severity Improper Input Validation (CWE-20) vulnerability in Seppmail Seppmail. Its CVSS base score is 8.2 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 12.0th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2026-27443 affects SEPPmail Secure Email Gateway versions prior to 15.0.1, where the software fails to properly sanitize headers from S/MIME protected MIME entities. This improper input validation (CWE-20) enables an attacker to manipulate trusted headers within these entities. The vulnerability carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N), indicating high severity due to its network accessibility, low attack complexity, and lack of required privileges or user interaction.
An unauthenticated attacker can exploit this vulnerability remotely by sending a specially crafted email containing S/MIME protected MIME entities with unsanitized headers. Successful exploitation allows the attacker to control trusted headers, resulting in high integrity impact without affecting confidentiality or availability. This could enable header spoofing or injection, potentially bypassing email security controls or misleading recipients and administrators.
The vendor's advisory in the release notes for version 15.0 at https://downloads.seppmail.com/extrelnotes/150/ERN15.0.html#seppmail-vulnerability-disclosure details the vulnerability disclosure and recommends upgrading to SEPPmail Secure Email Gateway 15.0.1 or later to mitigate the issue.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-9379
Vulnerability details
SEPPmail Secure Email Gateway before version 15.0.1 does not properly sanitize the headers from S/MIME protected MIME entities, allowing an attacker to control trusted headers.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
The CVE describes remote exploitation of a public-facing email security gateway via crafted S/MIME emails (directly matching T1190). Successful exploitation enables header manipulation/spoofing that bypasses security controls and misleads recipients (directly matching T1672).
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Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly requires validation of information inputs such as unsanitized headers from S/MIME protected MIME entities to prevent manipulation.
Ensures timely identification, reporting, and patching of the specific input sanitization flaw in SEPPmail Secure Email Gateway prior to version 15.0.1.
Implements spam protection mechanisms tailored for email gateways that can block or detect header spoofing and injection from crafted S/MIME entities.