Is it permissible to work for a company whose income and profits are based on bribery, given that the nature of the work is administrative and unrelated to sales?

As long as your work in this company is limited to permissible activities, such as administrative services that have no connection to bribery, there is no harm to you. However, you must advise them and warn them of Allah, and clarify the danger and gravity of the sin. Your silence, when you are able to denounce it without causing a greater harm, is considered an acceptance of the Munkar (evil).