The twofactor_gauthenticator plugin for Roundcube is vulnerable to brute-forcing of the second-factor code. Due to the lack of rate limiting, an attacker who obtains user's password can submit arbitrary 6-digit codes, resulting in second-factor bypass and unauthorized access to the system.
This issue was fixed in commit 4640889.
Improper Restriction of Excessive Authentication Attempts The product does not implement sufficient measures to prevent multiple failed authentication attempts within a short time frame.