Adobe has updated its Flash Player to version 18.104.22.168, available from here.
Although there are unspecified bug fixes, this release is primarily intended to fix many critical security vulnerabilities; Adobe lists 13 which it claims have been addressed in this update. These are graded as being critical, and Adobe warns that they could allow an attacker to take control of systems running an earlier version of Flash Player.
All 13 vulnerabilities are reported to be able to lead to code execution. Adobe does not state whether any are yet being exploited, though.
This appears to be an urgent and important update for anyone with Flash Player installed.