Please upgrade to MikroTik RouterOS 6.40.8 [bugfix] or 6.42.1 [current], the issue was addressed and fixed there,
Versions affected: 6.29 to 6.43rc3 (included). Updated versions in all release chains coming ASAP. Edit: v6.42.1 and v6.43rc4 have been released!
Am I affected? Currently there is no sure way to see if you were affected. If your Winbox port is open to untrusted networks, assume that you are affected and upgrade + change password + add firewall. The log may show unsuccessful login attempt, followed by a succefful login attempt from unknown IP addresses.
What do do: 1) Firewall the Winbox port from the public interface, and from untrusted networks. It is best, if you only allow known IP addresses to connect to your router to any services, not just Winbox. We suggest this to become common practice. As an alternative, possibly easier, use the “IP -> Services” menu to specify “Allowed From” addresses. Include your LAN, and the public IP that you will be accessing the device from. 2) Change your passwords.
What to expect in the coming hours/days: Updated RouterOS versions coming ASAP. RouterOS user database security will be hardened, and deciphering will no longer be possible in the same manner.
EXAMPLE how to protect yourself: