It's haraam since you're actually dealin' with riba:
I have been offered a job at a Bank, and I am not sure whether I should work there or not, because the bank makes money from interest (Riba). Please tell me whether working there is allowed (Jaaiz), or not.
Praise be to Allaah.
You will find the answer in the hadeeth of Jaabir, may Allaah be pleased with him, who said: "The Messenger of Allaah cursed the one who consumes riba (interest or usury), the one who gives it, the one who writes it down and the one who witnesses it. He said: 'All of them are equal in sin
.'" (Reported by Muslim, may Allaah have mercy on him, in his Saheeh, no. 1598)
Commenting on this hadeeth, al-Nawawi, may Allaah have mercy on him, said: This is clearly a prohibition of the writing down or witnessing of a contract between the two parties involved in a riba-based deal. It also includes a prohibition of helping others to commit wrong acts. And Allaah knows best.
It is inevitable that the employee in a riba-based bank will play a part in riba dealings one way or another, even if he is just a security guard. Maybe Allaah will help you, my brother, if you are patient, to find halaal employment. (Interpretation of the meaning) "… And whosoever fears Allaah and keeps his duty to Him, He will make a way for him to get out (from every difficulty)." [al-Talaaq 65:2]
Shaykh al-Barraak (www.islam-qa.com