CVE-2026-29140: S/MIME Signature Vulnerability in SEPPmail

The core impact of CVE-2026-29140 lies in the potential for man-in-the-middle (MITM) attacks and compromised communication confidentiality. An attacker can craft malicious S/MIME signatures containing certificates they control. When SEPPmail processes these signatures, it may inadvertently add these attacker-controlled certificates to its trusted certificate store. Subsequently, any encrypted communication relying on S/MIME will use these compromised certificates, allowing the attacker to decrypt and potentially modify messages. This could lead to data breaches, phishing attacks, and other malicious activities, particularly if SEPPmail is used to handle sensitive email communications.

1. Reserved2026-03-04
2. Published2026-04-02
3. EPSS updated2026-04-09

The primary mitigation for CVE-2026-29140 is to immediately upgrade SEPPmail Secure Email Gateway to version 15.0.3 or later. If upgrading is not immediately feasible, consider implementing strict input validation on S/MIME signatures to prevent the inclusion of untrusted certificates. Review and audit existing S/MIME configurations to identify and remove any suspicious or unauthorized certificates. Monitor email traffic for unusual certificate usage patterns that could indicate exploitation. While a WAF is unlikely to directly address this vulnerability, it can help detect and block malicious S/MIME signatures based on known patterns.