Under what section of cpc should i file an application to dismiss a suit for fraud upon the court?

If an ex-parte order has been passed and you are (defendant is) informed in compliance of Order XXXIX Rule 3 CPC, the defendant shall move under order XXXIX Rule 4 CPC whereinn Order XXXIX Rule 4 of the Code of Civil Procedure (CPC) allows a court to discharge, vary, or set aside an injunction order.

However, second proviso to Order XXXIX Rule 4 of CPC lays down that where an order of injunction has been passed after giving to a party an opportunity of being heard, said order shall not be discharged, varied or set aside except where such discharge, variations or setting aside has been necessitated by a change in circumstances.

File application under Order-39-rule 4, as explained by Dr. Vashista;

4. Order for injunction may be discharged, varied or set aside.-

Any order for an injunction may be discharged, or varied, or set aside by the Court, on application made thereto by any party dissatisfied with such order:

²[Provided that if in an application for temporary injunction or in any affidavit supporting such application, a party has knowingly made a false or misleading statement in relation to a material particular and the injunction was granted without giving notice to the opposite party, the Court shall vacate the injunction unless, for reasons to be recorded, it considers that it is not necessary so to do in the interests of justice:

Provided further that where an order for injunction has been passed after giving to a party an opportunity of being heard, the order shall not be discharged, varied or set aside on the application of that party except where such discharge, variation or setting aside has been necessitated by a change in the circumstances, or unless the Court is satisfied that the order has caused undue hardship to that party.]

2. Ins. by Act 104 of 1976, s. 86 (w.e.f. 1-2-1977).

It is an exparte injunction order granted by court at the time of filing the suit.

Upon receiving notice you enter appearance and file counter/ objection to the interim injunction application and get the exparte injunction order vacated.

The suit can be dismissed only after conduct of trial hence you defend your interest and challenge the pleadings on merits and get the case dismissed on merits.