In a free society,
unions are run by the worker's elected officials, who are responsible only to the workers. But as always, freedom requires constant vigilance and involvement in their union by the rank and file. If the workers get complacent, or lazy, or are bribed, then there is little government can do to stop that.
The alternative, to abolish unions altogether, is exactly what you would expect management to suggest.
Of course, you can say the same thing about a free market, too. No system, especially a democratic one, can be guaranteed free of corruption. Organized crime cannot exist in a dictatorship.