Is it forbidden to guarantee someone at the bank?

Guarantying someone at a usurious bank or in a forbidden transaction is considered cooperating in sin and aggression, and it is forbidden based on the Almighty's saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). One must repent from this and not repeat it, and urge the guaranteed party to end their dealings with the bank and withdraw the guarantee if it is ongoing. As for guaranteeing at an Islamic bank and in a non-forbidden transaction, it is permissible and considered a good deed.