This was supposedly written for and sung at a U.S. Department of Justice Office of Legal Counsel Christmas party during the Carter administration; please let me know if you have a more precise attribution:

You'd better watch out,
You'd better not cry,
You'd better not pout;
I'm telling you why.
Santa Claus is tapping
Your phone.

He's bugging your room,
He's reading your mail,
He's keeping a file
And running a tail.
Santa Claus is tapping
Your phone.

He hears you in the bedroom,
Surveills you out of doors,
And if that doesn't get the goods,
Then he'll use provocateurs.

So—you mustn't assume
That you are secure.
On Christmas Eve
He'll kick in your door.
Santa Claus is tapping
Your phone.

And a slightly different version: