Summary
An SSL stripping vulnerability was discovered in Trojitá, a fast Qt IMAP e-mail client. User's credentials are never leaked, but if a user tries to send an e-mail, the automatic saving into the "sent" or "draft" folders could happen over a plaintext connection even if the user's preferences specify STARTTLS as a requirement.
Background
The IMAP protocol defines the STARTTLS command which is used to transparently upgrade a plaintext connection to an encrypted one using SSL/TLS. The STARTTLS command can only be issued in an unauthenticated state as per the IMAP's state machine.
RFC 3501 also allows for a possibility of the connection jumping immediately into an authenticated state via the PREAUTH initial response. However, as the STARTTLS command cannot be issued once in the authenticated state, an attacker able to intercept and modify the network communication might trick the client into a state where the connection cannot be encrypted anymore.
Affected versions
All versions of Trojitá up to 0.4 are vulnerable. The fix is included in version 0.4.1.
Remedies
Connections which use the SSL/TLS form the very beginning (e.g. the connections using port 993) are secure and not vulnerable.
Possible impact
The user's credentials will never be transmitted over a plaintext connection even in presence of this attack.
Because Trojitá proceeded to use the connection without STARTTLS in face of PREAUTH, certain data might be leaked to the attacker. The only example which we were able to identify is the full content of a message which the user attempts to save to their "Sent" folder while trying to send a mail.
We don't believe that any other data could be leaked. Again, user's credentials will not be leaked.
Acknowledgement
Thanks to Arnt Gulbrandsen on the imap-protocol ML for asking what happens when we're configured to request STARTTLS and a PREAUTH is received, and to Michael M Slusarz for starting that discussion.