Seppmail EUVD-2026-18160

Remote unauthenticated attacker sends crafted S/MIME-signed email to SEPPmail Secure Email Gateway versions before 15.0.3. Victim receives email with attacker-injected certificates embedded in signature. Gateway accepts malicious certificates for future encryption without validation.

Although CVSS and EPSS scores are unavailable, this vulnerability represents a high real-world risk due to its direct impact on email confidentiality and the certificate-based trust model underpinning S/MIME. The attack vector is network-based (attacker sends a crafted email), does not require authentication or user interaction beyond opening/processing the message, and results in complete loss of confidentiality for future encrypted communications-a severe technical impact. The vulnerability was disclosed by a national security authority (NCSC.ch), indicating it was identified through formal security research rather than opportunistic discovery. The absence of a CVSS vector and public exploit code does not diminish risk; rather, the silent nature of the attack (certificates added to signatures are not obviously visible to users) and its foundational impact on email encryption security justify immediate patching. This is not a low-risk high-CVSS issue but a material confidentiality breach affecting all versions before 15.0.3.

An attacker crafts a signed email containing an embedded certificate matching the victim's identity but actually corresponding to the attacker's private key. The attacker sends this email to the SEPPmail gateway. The gateway processes the S/MIME signature, extracts and stores the attacker's certificate without proper validation, and associates it with the victim's identity in its certificate cache. When the victim or the gateway's user replies to future emails from the attacker with encryption, the gateway uses the attacker's certificate instead of the legitimate recipient certificate, allowing the attacker to decrypt all subsequent encrypted communications intended for the victim. This attack leaves minimal forensic evidence and could persist across multiple encryption sessions.