The vncserver wrapper for vnc before 3.3.3r2-21 uses the rand() function instead of srand(), which causes vncserver to generate weak cookies.

TightVNC before 1.2.4 running on Windows stores unencrypted passwords in the password text control of the WinVNC Properties dialog, which could allow local users to access passwords.

TightVNC before 1.2.6 generates the same challenge string for multiple connections, which allows remote attackers to bypass VNC authentication by sniffing the challenge and response of other users.