CVE-2026-29138 is a medium-severity LDAP Injection vulnerability identified in SEPPmail Secure Email Gateway versions prior to 15.0.3. The root cause is improper neutralization of special characters in LDAP queries (CWE-90), which allows an attacker to manipulate LDAP queries by injecting crafted input through email addresses. This manipulation enables the attacker to claim another user's PGP signature as their own, effectively impersonating the legitimate user’s cryptographic identity. The vulnerability does not require any authentication or user interaction, making it remotely exploitable over the network (AV:N, AC:L, PR:N, UI:N). The CVSS 4.0 vector indicates partial impact on system integrity (SI:L) but no impact on confidentiality, availability, or other security properties. The exploitation could undermine the trust model of PGP signatures by allowing signature spoofing, potentially facilitating phishing, fraud, or unauthorized message acceptance. No public exploits or active exploitation have been reported yet, but the vulnerability is publicly disclosed and should be addressed promptly. The affected product is widely used in secure email environments, especially in organizations requiring strong cryptographic assurances for email authenticity.

The primary impact of this vulnerability is on the integrity and trustworthiness of email communications secured by SEPPmail Secure Email Gateway. Attackers exploiting this flaw can impersonate legitimate users by claiming their PGP signatures, which could lead to acceptance of forged emails as authentic. This undermines the cryptographic assurances that PGP signatures provide, potentially enabling phishing attacks, fraud, and unauthorized access to sensitive information. Organizations relying on SEPPmail for secure email transmission, especially those in regulated industries or handling sensitive communications, face increased risk of reputational damage and operational disruption. While confidentiality and availability are not directly impacted, the erosion of signature integrity can have cascading effects on organizational security policies and compliance. The ease of exploitation without authentication or user interaction increases the threat level, making it accessible to remote attackers. The absence of known exploits in the wild reduces immediate risk but does not diminish the urgency for remediation.

To mitigate CVE-2026-29138, organizations should upgrade SEPPmail Secure Email Gateway to version 15.0.3 or later, where the vulnerability has been addressed. If immediate patching is not feasible, implement strict input validation and sanitization on email addresses processed by the gateway to neutralize special LDAP characters and prevent injection. Employ network-level controls such as firewall rules to restrict access to the SEPPmail management interfaces and LDAP services to trusted sources only. Monitor email logs and PGP signature verification processes for anomalies or unexpected signature claims. Conduct regular security audits and penetration testing focusing on LDAP query handling and email signature validation. Educate users and administrators about the risks of signature spoofing and encourage vigilance in verifying email authenticity. Finally, maintain up-to-date threat intelligence feeds to detect any emerging exploits targeting this vulnerability.