Federal Rule of Civil Procedure 65(c)

(c) Security. No restraining order or preliminary injunction shall issue except upon the giving of security by the applicant, in such sum as the court deems proper, for the payment of such costs and damages as may be incurred or suffered by any party who is found to have been wrongfully enjoined or restrained. No such security shall be required of the United States or of an officer or agency therof.

The provisions of Rule 65.1 apply to a surety upon a bond or undertaking under this rule.