CVE-2026-29143 is a vulnerability identified in SEPPmail Secure Email Gateway before version 15.0.3, caused by improper input validation (CWE-20) of the inner message within S/MIME-encrypted MIME entities. SEPPmail Secure Email Gateway is a widely used product for securing email communications, particularly in enterprise and governmental environments. The vulnerability arises because the product fails to properly authenticate the inner message of an S/MIME-encrypted email, allowing an attacker to manipulate trusted headers embedded within the encrypted MIME structure. This manipulation can lead to header spoofing or the insertion of malicious headers that the gateway treats as trustworthy, potentially bypassing security policies or enabling phishing and impersonation attacks. The CVSS 4.0 vector indicates the attack can be performed remotely (AV:N) with low complexity (AC:L), requires no privileges (PR:N), no user interaction (UI:N), and no authentication (AT:N). The vulnerability impacts confidentiality and integrity (VI:L, VC:N), with a limited scope (SC:L) but high impact on system integrity (SI:H). No known exploits have been reported yet, but the vulnerability's nature makes it a critical concern for secure email environments. The patch or fixed version is 15.0.3, which addresses the improper validation issue. Organizations using affected versions should prioritize upgrading to mitigate risks.

The vulnerability allows attackers to control trusted email headers by exploiting improper validation of S/MIME-encrypted messages, which can undermine the integrity and trustworthiness of secure email communications. This can lead to successful phishing, spoofing, or bypass of email security controls, potentially resulting in unauthorized access, data leakage, or delivery of malicious payloads. Since SEPPmail is often deployed in sensitive environments such as government, finance, and healthcare sectors, the impact could be severe, including reputational damage, regulatory penalties, and operational disruption. The lack of required authentication and user interaction increases the risk of automated or large-scale exploitation once an exploit becomes available. The vulnerability primarily affects confidentiality and integrity, with limited impact on availability. Organizations relying on SEPPmail for secure email processing worldwide face a heightened risk until patched.

1. Immediately plan and execute an upgrade to SEPPmail Secure Email Gateway version 15.0.3 or later, where the vulnerability is fixed. 2. Until the upgrade is applied, implement strict email filtering and validation policies to detect and block suspicious or malformed S/MIME-encrypted messages. 3. Monitor email gateway logs for unusual header manipulations or anomalies that may indicate exploitation attempts. 4. Employ additional layers of email security such as DMARC, DKIM, and SPF to reduce the risk of spoofed emails reaching end users. 5. Educate users about the risks of phishing and suspicious emails, even those appearing to be encrypted or from trusted sources. 6. Coordinate with SEPPmail support and security advisories for any interim patches or workarounds. 7. Conduct regular security assessments and penetration testing focused on email infrastructure to identify potential exploitation vectors.