GNOME Geary before 3.36.3 mishandles pinned TLS certificate verification for IMAP and SMTP services using invalid TLS certificates (e.g., self-signed certificates) when the client system is not configured to use a system-provided PKCS#11 store. This allows a meddler in the middle to present a different invalid certificate to intercept incoming and outgoing mail.

Geary before 0.6.3 does not present the user with a warning when a TLS certificate error is detected, which makes it easier for remote attackers to conduct man-in-the-middle attacks via a crafted certificate.