SOGo before 5.12.13 contains an improper cryptographic signature verification vulnerability that allows remote attackers to replace a victim's contact S/MIME certificate by sending a spoofed multipart/signed message with a mismatched signature and an impersonated From address. Attackers can deliver a crafted CMS signedData object containing their own self-signed certificate, which SOGo's UIxMailView stores on the contact card without validating signature authenticity or certificate identity, enabling subsequent encrypted communications to be decrypted with the attacker's private key.